The estimation of the cost of a new building involves a computation of the cubic contents of the building or the number of square feet of floor surface, and the use of the correct unit cost factor. These factors are determined by experience and of course vary with material and labor prices.

The cubic contents of a building are obtained by multiplying together the figures representing its width, depth and height. The number of square feet of floor surface in the building is the product of the width by the depth by the number of floors. The computations are easy in the case of rectangular buildings but involve some complications in the case of those of irregular shape. The figures may be obtained from the architect's plans for the building or from actual measurements.

Experience with actual costs of buildings of certain types and standards lead architects, builders and appraisers to become familiar with the average costs of such buildings per cubic foot or square foot. These units are the factors which are applied to the building under consideration. If the correct type has been determined it is merely a matter of detail to figure the total number of units, apply the factor and arrive at the estimated cost.

For illustration let us assume an apartment building erected on a lot 50 feet in width by 100 feet in depth. Let us assume the law provides only 70 per cent of an inside lot may be built upon so that even if not regular the ground covered will be equal to 50 X 70 or 3,500 square feet. If the building is four stories and cellar it will be about 50 feet in height so that the cubic contents will be 50 X 70 X 50 = 175,000 cubic feet. If the factor of unit cost for such a building is 30 cents, we will obtain the correct result: 175,000 X 30 == $52,500 - cost of building.

In making our computation on the basis of square feet of floor surface, the figures will be: 50 X 70 = 3,500 square feet on one floor; 3,500 X 5 = 17,500 square feet on 4 floors and cellar. The cost of this building will be approximately $3.00 per square foot, so that - 17,500 X $3.00 = $52,500 - cost of building.

As an estimate, either of the two methods would give fairly accurate results assuming of course that we had classified our building correctly as being of a grade costing 30 cents per cubic foot or $3.00 per square foot of floor surface.

The following tables are statements of unit costs of some standard buildings, comparing pre-war costs with those of the summer of 1921:

Cost per unit of cubic feet

Cost per square foot of floor space