Preliminary Estimating

Estimation in General

Estimation is a technical process of predicting cost of a building.

Preliminary Estimate

An attempt to forecast the cost of a project before detailed information is available.

Prepared before:

· BOQ is available or prepared

· Contract or Tender figure is obtained

Purpose:

· Let the client be aware of cost commitments

· Inform design team of cost commitments

· To establish budget and to exercise cost control

Used for:

· Budgeting - Whether project should proceed

· Controlling - Throughout the design process

· Comparing - Evaluate design solution

Contents of an Estimate:

· Includes

· Excludes

· Date of estimate

· Assumed tender date

· Drawings

· Brief details of Specification

· Expected variation of the estimate from the tender

Characteristics:

· First estimate is the one client remember the most

· Accurate – Better a range than a single amount, if single sum is required then with confidence limit

· More used to quantify rather than be accurate alone

· Do not over estimate or under estimate

· Do not alter just to please the design team or client

· Depends on ease of application, familiarity, speed Experience of the QS/ Estimator and availability of data/ information

Methods of Estimating

Unit Method

Estimate = Std. units of accommodation X Cost/Unit

· Based on relationship between cost of the project and the number of functional units

§ Cost per unit

§ Used at very early stages of design

§ Can be used when purpose or function of building is known

§ No drawings or Specifications

§ Use published information or experience

· Unit rate is obtained from fairly recently completed buildings of same type, size and construction

· Adjustments are made for

§ Site condition

§ Specification

§ Market condition

§ Regional changes

§ Inflation

· Used mainly for budgetary purposes

§ E.g. National Building Programme

Advantages & Disadvantages:

· Very quick and mathematical process is very simple

· Easy to understand and remember

· Useful method for estimating the cost limit or budget

· A cost yardstick

· Difficult to adjust costs for factors such as shape, size, construction method, material etc.

· Lack of precision

· Not accurate enough for individual buildings

Example

Cube Method

Estimate = Volume X Unit Cost (Cost/m³)

· Uses the volume of the building and restricted to initial stage of the design

· Calculation of volume is subject to Rules of Measurement (RIBA 1954)

§ Measured from external faces of external walls

§ Height of the building is taken from the top of the foundation to

· For Pitched roof

§ A point midway between the ceiling and the apex of the roof where roof space is un-occupied

§ A point three quarters from the ceiling to the apex of the roof where roof space is occupied

· For Flat roof

§ A point 610mm (2 Feet) above the roof structure

· Special items should be excluded and added to calculated overall cost

Advantages & Disadvantages:

· Quick and simple

· Needs higher level of skill to assess unit rate

· Costs do not express the actual cost of different parts of the building

· Difficult to adjust unit cost due to large number of variables

· Do not assist Architect in designing

· In large buildings a small variation in cost/m3 can be over emphasised

· Does not provide client any indication of the amount of usable space

· Meaningless since relates more to the enclosed void than the envelop

· Use is restricted to calculate heating requirements of buildings & for estimating fire insurance premium

Superficial Area Method

Estimate = Area X Cost per m²

· Measured to internal face of external walls

· Measured over stair wells, internal partitions, columns and the like

· Special items should be excluded & added to overall cost calculated

Advantages and Disadvantages:

· Rapid

· Directly relates to accommodation

· Allows comparison of different designs

· Easy to understand and remember

· Allows comparison between buildings that differ in size and shape

· A cost yardstick - can be used by an architect

· Adjustments to costs are fairly difficult

Example

Storey Enclosure Method

· Not much used in practice and involves greater calculation

· But, better than all the methods discussed before

· Takes into account of differences in: plan shape, floor area, overall height, storey height etc.

· Historical data are not readily available

· Less helpful when considering client and architects requirements

· Difficult to assess the effect of changes to spec.

· It uses weightings to various cost elements

· Provides a single rate

· Cost of external works etc. can be added

· No standard method of measurement

The rules for calculation are as follows:

1. Floor areas are measured from the internal face of external walls and are subject to the following weightings:

Basement x 3

Ground floor x 2

First floor x 2.15 and thereafter add 0.15 for each successive floor

2. Roof areas are measured to the extremities of the eaves and are measured as the area on plan whether they are pitched or flat and applied weighting factor of 1 to the roof area.

3. External wall areas are measured on the external face of the walls and are subject to the following weightings:

Basement wall area x 2 (basement floor to ground floor level)

Above ground wall area x 1 (ground floor to ceiling of top floor with no deduction for openings)