ESTIMATING AND COSTING
Preliminary estimates
Approximate estimating: This is an estimate of the total cost of a construction project even
before final drawings have been done. The main objective of preparing an approximate estimate
is to forecast the anticipated total cost of a project for the following purposes.
1. To give the client an idea of the financial outlay necessary to complete the project.
2. To provide the architect with information which will influence his forms of construction,
finishes, fitments etcetera
3. To assist in cost planning of the project
The method adopted relies on realistic brief by the client on his intended requirements as well as
access to accurately recorded cost data of previously completed buildings of similar type
There are various methods of preparing approximate estimates. Some are more suited than
others to certain types of buildings.
The main choice of which method to adopt for any project depends on;
1. Time available for the project.
2. How quickly the estimate is required.
The following are the main methods used for approximating. These are:
(i). the functional unit method
(ii). the cubic method
(iii). the superficial or floor area method
(iv). the story enclosure method
(v). the use of approximate quantities.
(vi). the use of bills of quantities.
i) The functional unit method This is a simple method in which the client expresses
his requirements in simple terms such as, ”a building to house Y number of people, a
50 bed hospital, a 40,000 seater stadium or a 3000 people conference room”. The
total cost of the project is then obtained by extending the total number of units by a
unit rate of a recently completed similar building taking into account the necessary
allowances for such factors as increases in wage rates, material costs, location, size
and shape of building.
, ii) The cubic content method In this method, the cubic content of the proposed building
is determined by taking the external dimensions, i.e. the length, width and height with
no allowance for wall thickness .The height varies depending on the type of roof e.g.
the height of a building with a pitched roof is taken from the top of the foundation
concrete to one third of the pitched roof; the height of a flat roof is taken from the top
of the foundation concrete to 600mm above the flat roof etcetera. The total cubic
content is then multiplied by a cubic rate of a recently completed similar building
taking into account the necessary allowances for various cost factors. The method
makes no allowance for design variations or construction techniques. It does not
produce an accurate rate.
iii) The superficial or floor area method. This involves measurement of total floor area
within external walls but making no deductions for the thickness of internal walls or
openings such as stairwells. The area obtained is expressed as a total in metres
squared and multiplied by a unit rate obtained from past data of similar buildings. The
method has simplified calculations and being related to the accommodation floor
area, it is easy to understand by the client. However it is not flexible and does not take
into account design variables.
iv) Story enclosure method. This involves considering a building as a box frame or egg-
crate type of construction and involves calculating areas of all surfaces within each
compartment i.e. floors, walls, ceiling finishes. The type of structure and finishes are
left to be assessed in building up unit rates and makes allowances for design variables
to improve the accuracy of the estimated figure. Areas of Floors/Roofs are measured
between faces of external walls and also vertical positioning of floors, ground floor
area is multiplied by a factor of 2.0 to allow for cost of normal foundations, floors
below ground level are multiplied by a further factor of 1.0 in addition to 2.0. All
other floors above ground level are multiplied by 2.0 to allow for finishes plus an
additional factor of 0.15 to each floor as height increases. The roof area is multiplied
once to allow for finishes. The total area is then calculated by a unit rate obtained
from past data. Additional items such as external works, services, extensive
foundation works are priced separately and added together to give the total cost. The
method produces a more accurate estimate.
Preliminary estimates
Approximate estimating: This is an estimate of the total cost of a construction project even
before final drawings have been done. The main objective of preparing an approximate estimate
is to forecast the anticipated total cost of a project for the following purposes.
1. To give the client an idea of the financial outlay necessary to complete the project.
2. To provide the architect with information which will influence his forms of construction,
finishes, fitments etcetera
3. To assist in cost planning of the project
The method adopted relies on realistic brief by the client on his intended requirements as well as
access to accurately recorded cost data of previously completed buildings of similar type
There are various methods of preparing approximate estimates. Some are more suited than
others to certain types of buildings.
The main choice of which method to adopt for any project depends on;
1. Time available for the project.
2. How quickly the estimate is required.
The following are the main methods used for approximating. These are:
(i). the functional unit method
(ii). the cubic method
(iii). the superficial or floor area method
(iv). the story enclosure method
(v). the use of approximate quantities.
(vi). the use of bills of quantities.
i) The functional unit method This is a simple method in which the client expresses
his requirements in simple terms such as, ”a building to house Y number of people, a
50 bed hospital, a 40,000 seater stadium or a 3000 people conference room”. The
total cost of the project is then obtained by extending the total number of units by a
unit rate of a recently completed similar building taking into account the necessary
allowances for such factors as increases in wage rates, material costs, location, size
and shape of building.
, ii) The cubic content method In this method, the cubic content of the proposed building
is determined by taking the external dimensions, i.e. the length, width and height with
no allowance for wall thickness .The height varies depending on the type of roof e.g.
the height of a building with a pitched roof is taken from the top of the foundation
concrete to one third of the pitched roof; the height of a flat roof is taken from the top
of the foundation concrete to 600mm above the flat roof etcetera. The total cubic
content is then multiplied by a cubic rate of a recently completed similar building
taking into account the necessary allowances for various cost factors. The method
makes no allowance for design variations or construction techniques. It does not
produce an accurate rate.
iii) The superficial or floor area method. This involves measurement of total floor area
within external walls but making no deductions for the thickness of internal walls or
openings such as stairwells. The area obtained is expressed as a total in metres
squared and multiplied by a unit rate obtained from past data of similar buildings. The
method has simplified calculations and being related to the accommodation floor
area, it is easy to understand by the client. However it is not flexible and does not take
into account design variables.
iv) Story enclosure method. This involves considering a building as a box frame or egg-
crate type of construction and involves calculating areas of all surfaces within each
compartment i.e. floors, walls, ceiling finishes. The type of structure and finishes are
left to be assessed in building up unit rates and makes allowances for design variables
to improve the accuracy of the estimated figure. Areas of Floors/Roofs are measured
between faces of external walls and also vertical positioning of floors, ground floor
area is multiplied by a factor of 2.0 to allow for cost of normal foundations, floors
below ground level are multiplied by a further factor of 1.0 in addition to 2.0. All
other floors above ground level are multiplied by 2.0 to allow for finishes plus an
additional factor of 0.15 to each floor as height increases. The roof area is multiplied
once to allow for finishes. The total area is then calculated by a unit rate obtained
from past data. Additional items such as external works, services, extensive
foundation works are priced separately and added together to give the total cost. The
method produces a more accurate estimate.
