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Techniques of Estimating During the

Design Stages

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Techniques of Estimating During the Design Stages

Functional unit methodSuperficial methodCube methodFunctional area methodStorey enclosure methodApproximate quantities methodElemental method

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Functional Unit Method - 1

Examples of cost per functional unit:(Used at Stage A - Brief)

Schools, universities, colleges - per studentHotels, hospitals - per bedRestaurants, cinemas - per seatCar parks - per car

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Functional Unit Method - 2

Example:Hospital of 640 bedsCost per bed = $16,000

Total estimated cost = 640 x $16,000 = $10,240,000

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Superficial Method - 1

Cost per square metre of floor area:

(Used at Stage A - Brief and Stage B - Outline Proposal)

Example:Project of 852 m2

Cost per m2 = $1,106

Total estimated cost = 852 x $1,106 = $942,312

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Superficial Method - 2

Advantages:

Unit is meaningful to all partiesMost cost data is published in $/m2

Estimates are quickly and easily prepared

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Superficial Method - 3

Disadvantages:Difficult to allow for effects of design decisions such as plan shape, storey height, quality, etcSeparate allowance must be made for site works and external services

(Note: These also apply to functional unit and functional area methods)

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Cube Method

Cost per cubic metre of building volume:

(Could be used at Stage A - Brief and Stage B - Outline Proposal, but obsolete!)

Example:Project of 16,000 m3

Cost per m3 = $325

Total estimated cost = 16,000 x $325 = $5,200,000

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Functional Area Method - 1

Examples of cost per square metre of functional area:

(Used at Stage A - Brief and Stage B - Outline Proposal)

Primary schools and secondary collegesTAFE colleges and universitiesHospitals and medical/community centres

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Functional Area Method - 2

More accurate than simple superficial methodNo published cost dataNot useful for “one-off” projects Useful only for client body involved in continuous and consistent program of similar projects

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Storey Enclosure Method

Cost per square metre of building envelope:

(Could be used at Stage A - Brief and Stage B - Outline Proposal, but obsolete!)

Uses single price rate similarly to functional unit and superficial methodsIncorporates cost effect of design features such as height and shape

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Approximate Quantities Method - 1

Cost per square metre of building structure:

(Used at Stage C - Sketch Design and Stage D - Tender Document)

Not single price rate methodMost accurate methodUses actual construction rates for each group of related items

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Approximate Quantities Method - 2

Figure 5.1Ground Slab and Foundation Detail

Trowelled surface for CarpetGalvanised Iron Flashing

Ground Level

400 x 500Reinforced Concrete

F42 Fabric

Polythene Sheet150 Concrete Slab

150 Crushed rock50 Sand Bed underPolythene Sheet

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Approximate Quantities Method - 3

Example per m2: $100mm hardcore 9.5050mm sand 2.50Polythene 2.80150mm concrete @ $125/m3 18.75F42 steel fabric 4.2550mm screed 4.00

Total cost per square metre $41.80

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Approximate Quantities Method - 4

Simple, reliable and accurate methodCan clearly reflect adjustments of quantity and/or qualityCost data readily availableTime-consumingApplicable only to building workApplicable only in later stages of cost planning

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Elemental Method - 1

Cost per m2 of NPWC elements:Element groups used in Stage B - Outline ProposalIndividual elements used in Stage C - Sketch Design and Stage D - Tender Document

(Note: For format of element codes and makeup of element groups, refer to Topic 3)

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Elemental Method - 2

Element costs are expressed in:Percentage of total building costCost per m2 of gross floor areaUnit rate per m2 of elementTotal element cost

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Elemental Method - 3

Advantages:Flexible, adaptable method, suited to Stages B, C and DEasily understood by all partiesFacilitates comparison between projectsFacilitates analysis of cost implications of design decisions

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Elemental Method - 4

Disadvantages:

Time-consuming

High level of expertise required

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Sources of Cost Data

Rawlinsons Australian Construction Handbook (published annually, index updated quarterly)

Cordell’s Building Cost Guide (published quarterly)

The Building Economist (published quarterly)

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Selection of Estimating Technique

Recognise that design does not proceed uniformly across all elementsUse both rough and sophisticated techniques in the one estimate if necessaryUse an appropriate technique for each elementTechnique must reflect amount and quality of information available