The superficial floor area also known as the square metre method of estimating is one of the quickest methods of calculating the cost of a building/construction project. It is the first estimate that will be prepared by a Quantity Surveyor when initial sketch drawings are produced by the architect. In this early planning and design stage, an architect may produce design alternatives that can be priced by a Quantity Surveyor for cost comparison purposes. Alternatively, the architect may design to a cost taking into account the client’s budget.
The square metre method involves measuring the internal gross floor area of a building and multiplying by the cost per m2, to get the total cost of building. The internal gross floor area is measured within the external walls (i.e. floor area enclosed by external walls) without adjustments for partition walls, staircases or lifts. For example when measuring a simple rectangular single storey plan (20m x 5m) with 230mm external brickwalls and no open space, in a location where the building rate is R8,000 per m2, you have to calculate the cost of building as follows:
First Step: Calculate the inside length of external walls (measured from inside face):
20m – (0.23 x 2) m = 19.54m
5m – (0.23 x2) m = 4.54m
Total Estimated Cost of Building Project = Internal Gross Floor Area x Cost per m2
Total Cost = (19.54 x 4.54) m2 x 8000
Total Cost = 88.71m2 x 8000
Total Cost = R709,680
Building with Open Spaces
For a building with open spaces (e.g. enclosed courtyards, areas under roof lights, veranda, porch, patio etc), you will need to adjust the internal gross floor area by deducting the area of the open space.
In this case, if the same single storey plan (20m x 5m) has a corner veranda measuring (3 x 1.5) m, you will need to adjust the gross floor area of the building:
19.54 x 4.54 = 88.71m2
3m – 0.23m = 2.77m
1.5m – 0.23m = 1.27m
Area = (2.77 x 1.27) m = 3.52m2
Adjusted Gross Floor Area:
88.71 – 3.52 =85.19m2
Total Estimated Cost of Building Project = 85.19 x 8000
Total Cost = R681,520
When estimating or measuring buildings with multiple storeys or floors, the concept is the same. You have to adjust the internal gross floor area of each floor for open spaces such as balconies and decks.
Whereas you can use a single rate when pricing a single-storey building, an experienced Estimator will analyze the floor specifications in each room, so that an appropriate rate can be applied to that room. In a luxury or exclusive house designed by an architect, the types of floor finishes may vary across rooms – for example, such a house might have a tinted grano floor in the garage, terrazzo floor in the kitchen, ceramic tiles in the bathroom, marble floor in the living room and carpets in the bedroom. In this case, the Estimator has to measure the rooms separately so that the floor finishes can be appropriately priced.
The same applies to a multi-storey building. The type of floor finish on each floor has to be accounted for and priced accordingly. This approach will allow the Estimator to come with an accurate estimate. The layout of such an estimate will show the different areas of a floor by the type of finish:
|Proposed Three Storey Office Block on No.12, Acardia Street, Cape Town|
|Total Estimated Project Cost|
After measuring the building floors, you have to add allowances and provisional sums to your project so as to come with an estimate which is as close as possible to a complete Bills of Quantities.
The Estimator must add:
- External Works ( Site Pavings, Boundary Walls, Drainage and Water Reticulation)
- Provisional Sums (Specialist Sums by Nominated Subcontractors e.g. Electrical Installation, Fire Detection, Security Alarm etc.)
When To Use Square Metre Method of Estimating
The Superficial Floor Area method of estimating is used when the first sketch designs are produced by the architect. It is a fast method of estimating and it relies on building rates of similar projects done in the past in a specific location. These rates can be obtained from municipality plan approval office, property developers, quantity surveyors, bank housing loan department and experienced building contractors with a rich portfolio of finished works.
Any type of building project from a low-income house to a large commercial project can be estimated using the cost per gross floor area method.
Advantages of Square Metre Method
- It relies on architect’s sketch drawings
- It relies on the gross floor area of the building plan
- It is much faster to produce than a Bill of Quantities
- The current building index can be incorporated in the estimate
- Estimates are based on past similar projects
Disadvantages of Square Metre Method
- It does not take into account the aerial configuration of the building. Although two buildings may have the same gross floor area, the aerial plan might be different.
- It doesn’t take into account the total perimeter of the walls.
- It doesn’t take into account the floor-to-ceiling height of the building.
- It doesn’t take into account the area and type of wall finishes.
These disadvantages are considerations that a Quantity Surveyor should assess when estimating the cost of a building. Otherwise, it is a fairly accurate estimate if the rates are obtained from a similar project.