Development of Cost Modelling Prediction Tool for ...

Development of Cost Modelling Prediction Tool for Green Home: A Review Nurul Akmam Naamandadina, Abdul Razak Sapiana, Sharifah Mazlina Syed Khuzzanb a Department of Architecture, Kulliyyah of Architecture and Environmental Design, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysia b Department of Quantity Surveying, Kulliyyah of Architecture and Environmental Design, International Islamic University Malaysia, P.O. Box 10, 50728 Kuala Lumpur, Malaysia

The awareness on sustainable development has increased in Malaysia, leading to the implementation of an energy rating guideline; which serves to assess environmental and energy performance of buildings. Nonetheless, the movement towards sustainability has become insufficient as the economic factor does not devote the housing development during constructing, operating, and equipping the built environment due to the absence of a cost model and standard method of measurement for green home design; hence, causing additional cost towards the construction of green homes as compared to the traditional housing development. Therefore, it causes difficulties for designers in optimising their green design, as well as to prepare cost planning and cost control for green homes. This paper introduces the concepts and issues surrounding the development of a cost model to be used as a predictor tool for designers in order to estimate building cost for their green home design. This prediction tool is also to relieve the burden of “re-designing” the works of the designers as they would be able to keep their green home design within the budget agreed through the cost model. The findings from this paper, leading to determine the gap in developing cost modelling prediction tool for green homes. Keywords: Cost Modelling, Cost Prediction Tool, Green Homes

INTRODUCTION A house always refers to a shelter or building or structure that is a dwelling or place for habitation by human being (Schoenauer and Norbert, 2000). It can be in landed property such as a terrace house in terrace unit, detached or semi-detached and also subdivided building such as condominiums, flats, apartments or townhouses. People generally call any building they routinely occupy as “home”. A home is a place of residence or refuge and comfort (Schoenauer and Norbert, 2000). The word “home” can be used for various types of residential in which people can live; one of it is “Green Home”. Despite of this, there are many jargons have been used with various approaches to the residential building concepts. Before people came out with the ‘Green Home’ concept, there had been terms like ‘energy efficient home’, ‘highperformance home’ and ‘intelligent home’; which appeared to be similar, but most people are still defining and quantifying what these terms really mean. The terms look fashionable and parallel, but the ideas, concepts and objectives can be assumed to

sustain the built environment. The term of green home started when a common definition of sustainable development had been developed. Before introducing the concepts and issues surrounding the development of a cost model to be used as a predictor tool for designers in order for them to estimate building cost for their green home design, first, this paper will introduce the concept of Green Home.

THE CONCEPT OF GREEN HOME The concept of Green Home started when the awareness of sustainable development increased in the construction industry. Then, it led to the implementation of an energy rating guideline. Now, its awareness has become more important because by the year 2050, it is predicted that almost half of the World’s carbon emissions will be from developing countries (Pomeroy, 2011). The first green building guideline was introduced in the UK in 1990 is the Building Research Establishment Environmental Assessment Method (BREEAM). Since then, several countries have developed their own green or environmentally friendly building programs (Potbhare, Syal, Arif, Khalfan, and Egbu, 2009). Some of the programs include the Leadership in Energy and Environmental Design (LEED) of the USA, GreenGlobe launched in 2002 from Canada (Pomeroy, 2011), the Comprehensive Assessment System for Building Environmental Efficiency (CASBEE) of Japan, and the Green Star of Australia (Zimmerman and Kibert, 2007). In Singapore, in the building sector, one of the efforts to promote sustainable development is an implementation of the Green Marks (GMS) (Building and Construction Authority, 2014). In Malaysia, Green Building Index (GBI) was introduced in 2009 to assist the building industry in demonstrating towards sustainable development. The GBI is an environmental rating system for buildings developed by Pertubuhan Arkitek Malaysia (PAM) and Association of Consulting Engineers Malaysia (ACEM). It is the first rating system for evaluating the environmental design and performance of Malaysia buildings. It was developed specifically for the Malaysia tropical weather, environmental and developmental context, cultural and social needs. It is also defined green buildings by establishing a common language and standard of measurement, promote integrated, whole-building design, recognise and reward environmental leadership, transform the built environment to reduce its environmental impact, and to ensure new buildings will remain relevant in the future and existing buildings are refurbished and upgraded properly to remain relevant (GBI, 2011). Before the GBI was released, the Malaysian Standards Code of Practice for Energy Efficiency and Use of Renewable Energy for Non-Residential Building (MS1525) was introduced in 2001 (revision 2007). MS1525 (2007) defines a sustainable design approach as a combination of site planning, architectural, engineering, and landscaping (multidisciplinary) approach to design a building while optimising the energy efficiency of a building especially when employing combined passive and active devices. Each of the above green residential assessments focuses on a different type of residence. Table 1 shows the summary of the selected Green Residential Assessment. From Table 1, it can be seen that LEED, BREEAM and GBI are generalised in categorising the residential building type. Meanwhile, CASBEE focuses on detached houses. This is due to the fact that in Japan, detached house contributed up to 50% of residential type. Green Star focuses on multi-unit residential.

According to GBCA (2013), multi residential is meant the building should comprise two or more residential units. While Green Marks has specified on landed houses although Singapore has limited land. Table 1: Summary of the selected Green Residential Assessment GREEN ASSESSMENT

BUILDING TYPE OF ASSESSMENT

OBJECTIVE

RATING

LEED

LEED For Home

Promotes the design and construction of highperformance homes – energy efficient, resource efficient and healthy for occupants.

   

90+ points– Platinum 75 – 89 Points – Gold 60 to 74 points – Silver 45 to 59 points – Certified

BREEAM

Code for Sustainable Homes (ECO HOMES)

Rates and certifies the performance of new homes based on BRE Global's EcoHomes scheme. It aims to encourage continuous improvement in sustainable home building.

     

90 Points – Level 6 (******) 84 Points – Level 5 (*****) 68 Points – Level 1 (****) 57 Points – Level 1 (***) 48 Points – Level 1 (**) 36 Points – Level 1 (*)

CASBEE

CASBEE for Home (Detached House)

Evaluates the general environmental performance of detached houses from viewpoints: (i) environmental quality and (ii) environmental load imposed by the house on the external environment

BEEH = 3.00 or higher – Excellent BEEH = 1.5 < 3.0 – Very Good BEEH = 1.0 < 1.5 – Good BEEH = 0.5 < 1.0 – Fairly Poor BEEH = less than 0.5 – Poor

GREEN STAR

Multi Unit Residential

Evaluates the environmental design and performance of Australian buildings.

 4 Star Green Star –Best Practice  5 Star Green Star – Australian Excellence  6 Star Green Star – World Leadership

GREEN MARKS

Landed Houses

Evaluates a building for its environmental impact and performance.

 Green Mark Platinum – Scored 95 and above  Green Mark GoldPlus – Scored 85 to < 95  Green Mark Gold – Scored 75 to < 85  Green Mark Certified – Scored 50 to < 75

Green Building Index (GBI)

Residential for New Construction

An environmental rating system to evaluate the environmental design and performance of Malaysian buildings

   

86+ points– Platinum 76 – 85 Points – Gold 66 to 75 points – Silver 50 to 65 points – Certified

Sources: (BRE, 2006; GBI, 2011; BCA, 2013; GBCA, 2011; U.S. Green Building Council, 2010; JaGBC, 2008)

Each of the above green assessments serves similar objective as to enhance a better built environment and encourage improvement in sustainable residential building. The rating points have been categorised accordingly to level their practice. However, there is still lacking of a clear definition of Green Home. In order to get a

clear definition of Green Home and to find out the components of Green Home, a comparative analysis of the selected green assessment criteria has been carried out. Table 2 shows a summary of a comparative analysis of the selected green assessment which have been drawn into nine (9) main categories which are similar. Based on the finding which has been summarised in Table 2, it can be generalised that a green home is a house structure which is using the processes that are environmentally responsible has to be resource-efficient and cost-effective throughout a house's life-cycle from the determination of site to the design, construction, operation, maintenance, renovation and demolition by avoiding pollution, emissions, effluents and other impact to the environment and, during the occupancy, green home has to be durable, provide good indoor quality, health and comfort to the users (BRE, 2006; GBI, 2011; BCA, 2013; GBCA, 2011; U.S. Green Building Council, 2010; JaGBC, 2008). Table 2: A summary of overall assessment criteria for each of selected green home assessments

ASSESSMENT CRITERIA 1

Management / Project Management

2 3 4

Energy Transportation Water Materials & Resources / Ensuring a Long Service Life Land Use / Ecology / Site / Environmental Protection / Townscape Pollution, Emissions, Effluents & Other Impacts / Waste Indoor Environmental Quality / Health and Comfort Innovation / Design / Construction Process

5 6 7 8 9

LEED (USA)

BREEAM CASBEE (UK)

(JAPAN)

  



 

GREEN STAR

GREEN MARK

(AUSTRALIA) (SINGAPORE)



  

 

  



















GBI (MALAYSIA)

 























Sources: (BRE, 2006; GBI, 2011; BCA, 2013; GBCA, 2011; U.S. Green Building Council, 2010; JaGBC, 2008)

The term of Green Home required a holistic approach compared to ‘energy efficient home, ‘high performance home’ and ‘intelligent home’. In Malaysia, the term of ‘energy efficient home’ was started when the electricity price rose in the early third millennium and its idea was to save the energy used from being wasted during the occupancy period. Accordingly, Pusat Tenaga Malaysia (2006) define that ‘energy efficient home’ means a home using less energy for heating, cooling and lighting. It also means buying energy-saving appliances and equipment for home use. It also emphasised an important criterion for ‘energy efficient home’ is the building envelope, which is everything that separates the interior of the building from the outdoor environment: the doors, windows, walls, foundation, roof and insulation. As to improve the building envelope, for example, windows with special glazing can let in sunlight without heat gain and also can reduce

heat loss when temperatures drop. Currently, energy efficient becomes a constitutive component of Green Home. The term 'high-performance home' was common in United State. Under Energy Independence and Security Act of 2007 Title IV - Energy Savings in Buildings and Industry Sec. 401 for example, it defines 'high-performance building' as a building that integrates and optimises on a life cycle basis all major high performance attributes, including energy conservation, environment, safety, security, durability, accessibility, cost-benefit, productivity, sustainability, functionality, and operational considerations (Grant & Moy, 2014). This definition is not much difference as compared to Green Home. Moreover, NYC DDC (1999) claimed 'highperformance home' as an old term for Green Home. While ‘intelligent home’ derives from the word intelligent which means incorporates computer programs to coordinate home subsystems and to regulate interior temperature, HVAC, and power (Kubba, 2010)c. The goal is usually to reduce the operating cost of the building while maintaining the desired environment for the occupants. The building-automation systems (BAS) which normally installed in the ‘intelligent home’ can ensure that equipment operates at the minimum needed capacity by automatically resetting operating parameters to match the current weather conditions, scheduling turns equipment on or off and monitor information such as temperatures, flows, and pressures inside the house. BAS even has a capability to detect and report faults in the mechanical and electrical systems to the building owner (Kubba, 2010)c. Intelligent home is also known as a ‘smart home’ and through the definition; an ‘intelligent home’ is actually a constitutive component of energy efficient home. As to achieve energy efficiency, BAS helps to optimise the energy use of being wasted.

COST AND GREEN DESIGN Generally, in every stage of any development or construction process including green home, there is a correlation to the buildings’ cost. As it comprises: i) preliminaries cost, ii) sub-structure cost, iii) super-structure cost, iv) mechanical and electrical services, v) external works, vi) site development, vii) furniture and equipment, viii) consultants fees and ix) contingency cost. When the Quantity Surveyors prepare the estimating for the development cost, the cost itself does not only involve the materials cost, but also the method of implementations. However, improving occupant health, comfort, and productivity or pollution reduction are not easy to quantify. When considering green design and its relationship to the construction costs, the costs and benefits have to be analysed by using a holistic approach. According to Kubba (2010)a and Kubba (2010)b, a holistic approach to design means strategic integration of mechanical, electrical, and materials systems, user productivity and health, and other financial measurements. Plus, empirical experience has constantly shown that the construction-cost estimations have the greatest impact to drive decisions about sustainable design (Kubba, 2010)a. Langdon (2004) suggests that, to succeed in building green and to keep the costs of its design under control, three critical factors must be understood and implemented: (i) clear goals should be determined and articulated at the very beginning in the design process, (ii) once the sustainability goals have been defined, it is necessary to integrate them into the design, and, (iii) it is very important to integrate the construction team into the project team as to avoid poor construction practices.

COST MODELLING Cost is one of the measures of function and performance of a building (Kirkham, Greenhalgh, & Waterman, 2007). Therefore, it shall be capable of being modelled in order that a green home design can be evaluated before the construction phase took place. According to Ferry and Brandon (1999); Kirkham, Greenhalgh, and Waterman (2007); Ng, Zakaria, and Hamzah (2000), cost modelling is defined as a systematic representation of a procedure that delivers adequately acceptable output for an established series of input data. The procedure is determined through extensive analysis of data of a building that has an influence on its cost and after being satisfied with the ability of the procedure to give a good representation of the costs (Ng, Zakaria, and Hamzah, 2000). While Ashworth (2010) defined cost models are the techniques used for forecasting the estimated cost of proposed construction projects. Therefore, the main aim of cost models is to simulate a current or future scenario in such a way that decision makers can make use of the results to decide their investment decision (Kirkham, Greenhalgh, and Waterman, 2007) and the designers can optimise their design as well as cost planning and cost control (Ng, Zakaria, and Hamzah, 2000).

THE NEED OF COST MODELLING PREDICTION TOOL According to Kubba (2010)a, during the energy crises of the 1970s and 1980s, federal government of the United State of America was faced with increasing initial construction costs and on-going operational and maintenance expenses. As a result, facility planners and designers decided to use economic analysis to evaluate alternative construction materials, assemblies, and building services with the objective of lowering costs. In a difficult economic climate, building owners who wish to reduce expenses or increase profits will employ economic analysis as to improve their decision making during the course of planning, designing, and constructing a building. Since the society is becoming increasingly concern with the sustainable development and green design, economic analysis is a good practice to minimise construction costs within the criteria set for design, quality and to maximise the social benefits. Refer to Ashworth (2010), the objective of economic analysis is actually to secure cost-effectiveness (Ashworth, 2010). Furthermore, an efficient cost advice from the quantity surveyor also depends on the understanding of the design method used by architects or engineers (Ashworth, 2010). In accordance with above matters, cost modelling is the best tool to analyse and evaluate the economics of construction materials, their associated costs (including machineries and labour costs), and building services with the objective of reducing costs and enhancing the quality. Moreover, with green home as it was claimed to require extra cost (Kubba, 2010), cost modelling appears to be the best tool to analyse and evaluate the economics of green construction and also to prove whether green home required extra cost or not. Ashworth (2010) summarised eight (8) conventional stages of cost modelling. There are as below: i) Formulate the problem ii) Collection of data iii) Analysis of data iv) Model building v) Optimum model

vi) Evaluation model vii) Testing viii) Application In addition, according to Ashworth (2010), the new data would be added to the collected data – in the database. It can be used for calibrating the model in the next updating. Raftery (1991) suggested four criteria for assessing the performance of the models when discussing the internal cognitive processes of human decision making. i) Data ii) Data/model interface iii) Model technique iv) Interpretation of output Ashworth (1995) summarised that a good model should incorporate five criteria. First, the data required for the model should be freely available in the form and amount necessary. Second, the model should allow for continuous updating by incorporating new data that become available. Third, the model should account for the changing situation in the construction industry. Forth, the model should provide for quick, cheap, and efficient processing. Fifth, the model should be accurate and reliable. While Seeley (1996) suggested that a good model should be simple enough for manipulation and understanding by those who use it. It should be representative enough in the total range of the implications it may have, it should be complex enough to accurately represent the system. To make the development of cost modelling prediction tool for green home coordination become easier, it was suggested by Khairani (2009), Quantity Surveyor had to follow Architect’s work stages. The most prominent Architect’s work stage is a RIBA Plan of Work. According to Brady (2011), the RIBA Plan of Work is the most widely recognised and used framework for building design and construction. It can help to review the green criteria because it offers an appropriate and accessible vehicle for mapping the ways in which sustainable design activities can be integrated into the design and construction process. It is also can organise the process of briefing, designing, constructing, maintaining, operating and using building projects into a number of key stages. According to Sinclair (2013), there are eight stages of the RIBA Plan of Work 2013. There are derived as follows; i) Stage 0 – Strategic Definition is a new stage in which a project is strategically appraised and defined before a detailed brief is created. This is particularly relevant in the context of sustainability, when a refurbishment or extension, or indeed a rationalised space plan, may be more appropriate than a new building. Certain activities in Stage 0 are derived from the former (RIBA Outline Plan of Work 2007) Stage A – Appraisal. ii) Stage 1 – Preparation and Brief merges the residual tasks from the former Stage A – Appraisal – with the Stage B – Design Brief – tasks that relate to carrying out preparation activities and briefing in tandem. iii) Stage 2 – Concept Design maps exactly to the former Stage C – Concept. iv) Stage 3 – Developed Design maps broadly to the former Stage D – Design Development – and part of Stage E – Technical Design. The strategic difference is that in the RIBA Plan of Work 2013 the

Developed Design will be coordinated and aligned with the Cost Information by the end of Stage 3. This may not increase the amount of design work required, but extra time will be needed to review information and implement any changes that arise from comments made before all the outputs are coordinated prior to the Information Exchange at the end of Stage 3. v) Stage 4 – Technical Design comprises the residual technical work of the core design team members. At the end of Stage 4, the design work of these designers will be completed, although they may have to respond to Design Queries that arise from work undertaken on site during Stage 5. This stage also includes and recognises the importance of design work undertaken by specialist subcontractors and/or suppliers employed by the contractor and the need to define this work early in the process in the Design Responsibility Matrix. vi) Stage 5 – Construction maps to the former Stage K – Construction to Practical Completion – but also includes Stage J – Mobilisation. vii) Stage 6 – Handover and Close Out maps broadly to the former Stage L – Post Practical Completion – services. viii) Stage 7 – In Use is a new stage which includes Post-occupancy Evaluation and review of Project Performance as well as new duties that can be undertaken during the In Use period of a building. Cost estimation is one of the most critical tasks concerned by all participants in the architecture, engineering, construction, and facilities management (FM) industry throughout the lifecycle of a building project, especially in the tendering phase after design is completed (Ma, Wei and Zhang, 2013). In a normal practice, either traditional or non-traditional procurement method, during Outline Proposal Stage, Quantity Surveyors will prepare Outline Cost Plan according to the group elements. Only when the detail design is ready, a Quantity Surveyor will carry out a comparative Cost Study. At this stage, a Quantity Surveyor will also carry out Cost Check by comparing the Detailed Cost Plan with the design proposal (Ashworth, 2010; Khairani, 2009; BRE Global, 2011). The crucial part is when this process has been repeated until the desire budget or Cost Limit (maximum budget) is achieved. When the above processes have to be undertaken together with the green assessment and achieve the green requirement it become more complex.

TYPES OF COST MODELS There are many types of cost models those had been proposed, from simple cube method to complex intelligent systems. Ashworth (1995) classified them into two main types: (i) deterministic; and (ii) probabilistic models. In the deterministic model, the outputs are assumed to be predicted exactly as they are attributed by the known input variables. The probabilistic models recognise the uncertainty of some variables which can only be estimated using concepts based on the probability theory. In construction industry practice, the majority of models fall into the second category. In addition of the above two main type, cost models can be at least classified into three main category: (i) empirical; (ii) regression or factor; and (iii) probabilistic or simulation models (Ashworth, 1995; Raftery, 1991). While Boehm (1981) categorised them into; (i) the expert judgement method, (ii) the analogy method, and (iii) the use of software cost estimation models.

Empirical models are based upon observation, experience, and intuition (Ashworth, 2010) and required the expert judgements. It is also known as a traditional model and widely applied as an estimation method (Genuchten and Koolen, 1991). Empirical models are symbolic models which are based on relationships between the design variables and cost (Ashworth, 1995). It involves consulting with one or more experts. The expert is usually an experienced project leader who uses experience on past projects and his understanding of the proposed project to arrive at an estimate of its cost and development time. It derived from conference, financial method, functional unit, superficial, superficial-perimeter, cube, storey enclosure, approximate quantity, elemental cost analysis and bill of quantity (Ashworth, 2010; Ogunlana, 1991; Raftery, 1991). Application of these models can be conducted manually or by using computer aided system and easy to understand (Ashworth, 1995). Regression or factor model relies on costing from only a portion of the scheme and then multiplying this by a factor to obtain the total cost (Ashworth, 2010). Zimmerman (1965) has called these ratio-cost factors (Ashworth, 2010). In extension, Ashworth (2010) explained that regression analysis as a technique that uses the best fitted mathematical equation to express the relationship between the variables studied. In any statistical analysis of relationship, exact relationships are not generally observed. The simplest form of regression analysis involve only one independent variable and one dependent variable, this is called linear regression analysis. In the actual practice, one dependent variable is affected by more than one independent variables. Therefore in describing their relationship, a multiple regression analysis would be applied. The equations developed are used for the purpose of estimating (Ashworth, 1995). Estimation by analogy method involves reasoning by analogy. It is based on experience and record facts from one or more completed projects to relate the actual ‘cost and development time’ to the ‘cost and development time’ of the new project (Genuchten and Koolen, 1991). A simulation model seeks to duplicate the behaviour of the system under investigation by studying the interactions among its components. Simulation is done to avoid direct experiment error and it contains more variability if compared to other research methods. The advantages using the simulation model is when problems occur it can be resolved quickly. It is also not possible if it is done analytically. Secondly, it is easier to understand, and the assumptions to be made are fewer (Ashworth, 2010). The use of software cost estimation models embody human expertise (Mishkoff, 1986). It can acts as intelligent assistants to human expert, the expert’s rule-of-thumb are stored in the computer to help others to solve problems (Nuzul Azam, Salihuddin, and Mohd. Razali, 1994). Due to the advent of computer technologies and mathematical programming techniques, developed approaches tend to use more complex methods and a large volume of data (Mittas, Athanasiades, and Angelis, 2008). As an example, the use of software cost estimation models involves the development of statistical models based on historical data or completed project. Historical records were used to derive mathematical equations that define the relationship between the independent variables and construction costs (Skitmore and Wilcock, 1994). Moreover, the use of software cost estimation models can provide reasonably accurate, easily computed, and inexpensive prediction method to the key people, i.e. The owners, designers and consultants and the contractors who had to be involved in construction (Peurifoy and Oberlender, 2013; Schuette and Liska, 1994). They also serve decision making, economic feasibility, financing, and budget

controlling of the project in the absence of complete design (Kouskoulas and Koehn, 1974; Barrie and Paulson, 1991).

TOOLS AS AN AIDTO ESTIMATE COST There were some arguments that said expert judgement method inferred of lacking in accuracy. As referred to Dikmen, Birgonul, and Han (2007) and Kangari and Riggs (1989), cited from Azari, MuhdFadhil and Rohman (2011), in the construction field, expert knowledge, experience, intuitive judgment and rule of thumbs are commonly ill-defined and vague. It caused due to inadequate statistical data. It impreciseness and vagueness are usually characterised by using the linguistic terms such as low, medium and high. These views of points were also supported by Noel, Cuauhtemoc, and Arturo (2013) that the expert judgement technique implied a lack of analytical argumentation and aimed at deriving estimates based on the experience of experts on similar projects; this technique is based on a tacit (intuitive) quantification step. To accommodate the linguistic terms which are based on subjective judgment (Kasabov, 1996), mathematical tools; model-based technique (Noel, Cuauhtemoc, and Arturo, 2013) was introduced. According to Noel, Cuauhtemoc, and Arturo (2013), model-based technique can be divided into two subcategories: a) Models based on statistics: Its general form is a statistical regression model. b) Models based on computational intelligence: These techniques include fuzzy logic, artificial neural networks, genetic programming, and genetic algorithms. In a sense, all the cost estimating methods are cost models. Figure 1 shows types of construction cost modelling. It has drawn into three categories; traditional model, non-traditional model and new wave model.

Figure 1 : Type of Construction Cost Modelling

Study done by Nor Azmi (2008) showed that, the traditional type of construction cost modelling continued to be in widespread use irrespective organisational type and size. The finding also comparable to the finding from other similar researches conducted in the UK, Hong Kong, Australia and Nigeria.

Table 4: Summary of the traditional cost models, their function, advantages and disadvantages/limitation from various sources.

PREDICTION TOOLS Conference Estimate

FUNCTION

ADVANTAGES

DISADVANTAGES/ LIMITATION

– It is based on a collective – It can be used for the – It is used in circumstances where view of a group of preparation of the earliest historical cost data may not be individuals. price estimate given to the appropriate, as in the case of a client. prototype project. It also offers a qualitative viewpoint to reinforce or otherwise a measured estimate. – The group members must have relevant experience of estimating the costs of similar projects.

Financial Method

Unit method

– This method can fix a cost – This method is used to – Although the architect is able to limit on the building avoid or reduce the risk or enjoy the freedom of expression design, based on either unit embarking on a profitless during the design process, a final of accommodation or rental venture. scheme must be prepared within values. the designated cost limit. – The architect must then ensure that the design can be constructed within such a cost limit. In the private sector, projects are often evaluated in terms of their selling price or rental value.

– The financial decision may limit the quality standards.

– The unit method of – The quickest and the approximate estimating simplest among all the consists of choosing a methods. standard unit of accommodation and – Can be prepared at the very multiplying this by an early stage. approximate cost per unit. – Can be prepared before the – The standard units may design process starts. represent, for example:  Schools – costs per pupil – The recording of data is space very easy.  Hospitals – costs per bed space – It serves as a guideline for  Car parks – cost per car very large projects where space any other method of estimating could take time – It's based on a single price in preparation. rate method upon the cost per functional unit through – Can be used for resource analysing the recently allocation and as a completed projects of guideline for finance similar nature and function. organisation, that is, prior to the commencement of – Allowances should be the project, in a rather made for various aspects advanced time the client such as inflation, market can allocate necessary conditions, site and design resources for a project, and variations, materials, etc. also he can seek for ways and all the other factors and means of organising which affect the prices. the finance.

– It suffers from the major disadvantage of lack of precision, and should only be used for establishing general guideline value. – Should be restricted to the inception and feasibility stages. – This method is often used to know the project cost very roughly and to make comparisons between similar buildings. – Prepared at a rather early stage where information is insufficient, therefore the accuracy is very less. – Only used as a guideline for budgetary purposes, but cannot be used for cost planning purposes. – Necessitate further estimating while the project developer. – Difficult to make allowance for any variation in design, shape, construction methods, materials etc.

– In the national level, the – Desiring at the unit cost is a Government can use it as a difficult task. budgetary tool for fund allocation. – The estimate based on this method can put the client at risk at times where the actual cost of – Stand as a tool in deciding the project remarkably exceeds the feasibility of the the budget due to some reason. project. Cube method

Is based on the cost per cubic meter of the building. Can be a guide in deciding the services cost, such as heating, air conditioning and fire fighting

Simple to work out. A quick estimating.

method

of

Useful where mandate estimates are required. It’s a useful and reliable measure where capacity or weight factors are concerned, such as heating, air conditioning and steel works. May be useful for standard type of buildings such as factories, warehouses, store etc.

Superficial Method

It does not take into account any variations such as the shape, height etc. of the design. Needs provisions for different types of foundations, finishes lifts etc. Adjustment for design variables is a tedious process. A difference is the wall height, which may not have a significant cost, indicate a misleading unit rate. Same building having the total area on the ground floor and in another building in four stories would indicate the same cost but obviously cannot be the same.

– This is still the most – Quick and simple to – It cannot take the design common method in use for employ. variables into account. early price estimating purposes. – It’s readily to understand – Special items such as lifts, by the client and gives a special foundations, etc., which – Is based on cost per square clear idea about the do not depend on the floor area, metre basis. building as its size. need to be allowed for in the unit rate. – Is restricted to outline – The majority of the proposal stage. published cost data is available in this form. – Adjustments are easier compared to other methods because the cost effective items are generally proportional to the floor area than to the cubic content.

Superficial Perimeter Method

– This method of – The formula combined – Due to the reluctance of approximate estimating is a floor area with the length of surveyors to change to this variation on the superficial the building’s perimeter method of approach and of cost floor area method. It was and gave more accurate data sources to publish devised by John Southwell result. appropriate rates, this method has and published in the RICS not been used in practice. paper Building Cost Forecasting (1971).

Storey enclosure method

– It was developed by the – It takes into account the – A tedious calculation process. RICS in order to overcome design variables such as the failures of the other shape, floor area, vertical – Takes longer time to prepare the methods, but still remains heights of floor areas, estimate. as an unedited method due storey heights, extra cost to many disadvantages for basement construction – Data is not readily available in which shall be covered etc. this form.

later under this heading – The figures achieved are – It does not import any meaning to more accurate, almost the client or architect. similar to the actual cost. – Difficult to adjust for any design – The past data need not be or specification variations. on a very similar project in design. – Suitable to be adopted when a very accurate estimate in necessary. Approximate quantities

– It is relatively an accurate – This method does provide a and most reliable form of more detailed and reliable estimating, provided that method of approximate there is sufficient estimating. information. – Suited to a more advanced – The significant items are design stage. measured along with the sub-elements going into it – More accurate and reliable and then a composite item price is applied to get the – Easy to make adjustments figures. The measurement of design or quality rules are similar to the Standard Method of – Takes the design variations Measurement rules. into account – It would be used for cost planning purposes as well.

Elemental Cost Analysis / Elemental Estimating

– Takes time – May prove difficult due to lack of information. – Information becomes available when the estimates are needed no more. – No particular rules of measurement exist, and the composite items resulted from the experience of each individual surveyor. – More information is required from the designer.

– This method analyses the – Quite accurate – It cannot be done without cost of the project on an adequate information elemental basis, attempting – Easy calculation to make use of the cost – Cannot be done without a detail analyses from other similar – Facilitate comparison of analysis of past data. projects. costs between elements – Needs adjustments for price. – Provides a way to calculate – It can be used for cost the elemental costs planning purposes. separately is a building – It’s possible to make any – The past data is processed adjustment in any element in an elemental basis and easily. then the new project is based on those elemental – This is a very effective costs. method of estimating. – It provides cost advice – It keeps the designer fully during the design process, informed of all the cost offering the client better implications of the design value for money. in relation to an approved approximate estimate and is likely to be accepted as the tender sum.

Source: (Nuzul Azam, Salihuddin, and Mohd. Razali, 1994; and Southwell, 1971).

Table 4 summaries the traditional cost models, their function, advantages and disadvantages/limitation from various sources. Each of the cost models has its strengths and limitations and it is more appropriate for some situations than others. Based on the above summary, the appropriate model to be used as a predictive tool to

estimate Green Home shall be Approximate Quantities. The Approximate Quantity model represents composite items which are measured by combining or grouping together a typical bill-measured item. The significant items are measured along with the sub-elements going into it and then a composite item price is applied to get the figures. In this case, it is easy to determine green materials, techniques or procedure to be implemented in the development of a green home. Furthermore, it is easy to make adjustments of the design or quality. However, there is no particular rule of measurement for green home exist. The most similar standard method of measurement exist is a standard method of measurement 2 (SMM2).

CONCLUSION As of the above review, it can conclude and determine the gap in developing cost modelling prediction tool for green homes.

Figure 2: Determination of the gap in developing cost modelling prediction tool for green homes

Based on Figure 2, it can lead to the new the research objective for developing a new wave cost prediction tool to estimate green home as it is to relieve the burden of the designers to “re-design” works and to keep their green home design within the agreed budget. It is also expected to provide a systematic approach for identifying the most economic and environmentally friendly strategy applicable to the green home by utilising an interactive technique. In another word, the appropriate green criteria have to be achieved at the acceptable price and in a reasonable timescale. So, it requires the process throughout evaluating and comparing the economic and environmental impacts. In concurrent result, it will also introduce the new method of measurement for estimating the green home.

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