Floor Area

Calculating Floor Area

Q - We are putting in a bid for an old barn and the external walls are one metre thick. We are receiving preliminary estimates for the work based on square metre prices but are uncertain whether the rates they are quoting are for the internal or external floor area. How should the floor area be measured and how can we compare these prices if they have a different base?

A - The calculation of floor areas in the construction industry generally follow the guidelines set out in The Standard Form of Cost Analysis published by The Royal Institution of Chartered Surveyors and the definition that applies to domestic buildings is called The Gross Floor Area.

This is defined as the ‘area of all enclosed spaces fulfilling the functional requirements of the buildings measured to the internal structure face of the enclosing walls'. This means that the total internal area between the external walls and including the space taken up by internal walls, chimney breasts and stair well openings.

Your case is special and any quantity surveyor preparing an order of costs for a new building with external walls one metre thick (highly unlikely), would need to take special note of this feature separately in his calculations. Let us assume that the internal area of your barn is 20 metres by 10 metres, that is 200 square metres. Measuring from the outside of the walls the calculation is 22 x 12 metres, that is 264 square metres - an increase of 32%!

If you receive two identical quotes of £400 per square metre but based on the internal and external areas, the sums being quoted are £80,000 and £105,600! Some of the timber-framed firms also work on figures based on the external wall measurements and this makes the quoted square metre prices attractively low.

There is nothing wrong in using this method provided that all parties understand the basis of how the calculation has been reached. You should check on any cost information you receive in quotations based on square metre costs to ensure that there are no misunderstandings on the basis of the calculation.

A further point in connection with this is the relationship between costs based on square foot and square metre prices. A lot of people are still more comfortable dealing in square foot costs, and if quoted a square metre price, will tend to divide it by ten to change it a square foot price.

This should only be done when dealing with costs in the most general terms. A square metre is 10.76 square feet so if a cost was mentioned at £600 per square metre, taking a short cut and dividing it by ten would covert it to £60 a square foot, but dividing it by 10.76 would produce £55.76 per square foot.

In a house with a floor area of 2,500 square feet, the difference in the two ways of making the calculation would be £10,600. This difference is too large to be ignored and would have a serious effect on any budget.

Cost per unit of floor area

Q - To provide the same number of rooms and the same floor area, is it cheaper to build a house or a bungalow?

A - This question is often asked and the answer is not straightforward! Imagine a small bungalow with a floor area of 100 square metres. In broad terms, to provide the same accommodation in a two-storey house would require two floors of 50 square metres each.

So theoretically, the house roof and foundations would be half the size and cost of those for the bungalow but extra costs would be incurred in the provision of a staircase and the first floor ceiling and floor construction. Other items, such as external walls, windows, doors and plumbing should be approximately the same.

So overall it is cheaper to build a house than a bungalow but the difference is not significant and a decision on how to proceed should be based upon other criteria not on a marginal cost saving.

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