Estimation in General
Estimation is a technical process of predicting the 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 the design team of cost commitments
- To establish a budget and to exercise cost control
Used for:
- Budgeting – Whether the project should proceed
- Controlling – Throughout the design process
- Comparing – Evaluate the 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:
- The first estimate is the one client remember the most
- Accurate – Better a range than a single amount, if a single sum is required then with a confidence limit
- More used to quantify rather than be accurate alone
- Do not overestimate or underestimate
- 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
The benefits of your having a preliminary estimate made include:
In order to achieve early financial commitment, a growing understanding of potential costs
- To assess the budget management costs
- To advise the architects and engineers of the project costs and the obligations you need.
In the approximate estimation, the details would include:
- The date of the calculation and the estimated date for the tender
- The details of the sketches and short specifications
- The anticipated difference from the forecast to the real cost of construction
- The more details presented, the smaller the variance.
Methods of Estimating
Unit Method
Estimate = Std. units of accommodation X Cost/Unit
· Based on the relationship between the cost of the project and the number of functional units
§ Cost per unit
§ Used at very early stages of design
§ Can be used when the purpose or function of the building is known
§ No drawings or Specifications
§ Use published information or experience
· Unit rate is obtained from fairly recently completed buildings of the 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
Read More: quantity-surveying-practices.com
Cube Method
Estimate = Volume X Unit Cost (Cost/m3)
· Uses the volume of the building and restricted to the 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 the calculated overall cost
Advantages & Disadvantages:
· Quick and simple
· Needs a higher level of skill to assess the unit rate
· Costs do not express the actual cost of different parts of the building
· Difficult to adjust unit cost due to a large number of variables
· Do not assist the Architect in designing
· In large buildings a small variation in cost/m3 can be overemphasized
· Does not provide the client any indication of the amount of usable space
· Meaningless since relates more to the enclosed void than the envelope
· Use is restricted to calculate heating requirements of buildings & for estimating fire insurance premium
If the unit rate is Rs. 22,200/m3 Estimate the total cost of the building.
Superficial Area Method
Estimate = Area X Cost per m2
· Measured to the internal face of external walls
· Measured over stairwells, internal partitions, columns and the like
· Special items should be excluded & added to the 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 the 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 areas 1 (ground floor to ceiling of the top floor with no deduction for openings)