When a developer intends to build after the design stage its time to make approximate quantities. This is by far the most accurate method of costing a project. This method of costing a project assists the home owner to know not only the total sum of the project but also the sum per item or elements. The sum per item or elements construction shows from foundations normally called substructures,walling above ground called superstructure up to roofing and finishes.
Foundations or substructures works are trench excavations, foundation blinding, concrete, steel, foundation walls, backfillling trenches, hardcore base, blinding the base, damp roof membranes, insecticide treatment, concrete to ground floor slab. On a good site topography average eight dollars per square foot of plan area for the house to be erected. This rate can be used to cost foundation works described in this paragraph to make approximate quantities.
Superstructures or works above ground floor slab is all walls, which include internal and external walls. This can be rated as six dollars per square foot to make approximate quantities per floor area of proposed house. The cost of roofing would include roof covers like tiles, slates, roof trusses, which can average eight dollars. In the case of first or other floors which include concrete and reinforcements can average ten dollars per plan area of house.
The final is finishes cost of floor screeds, plaster to walls or slab soffits at an average of seven dollars per square foot of plan area. Add all the approximate quantities from foundations, superstructures, roofing and finishes to get total rate in construction costing. If the total rate was say thirty dollars multiply this by floor plan area to get construction costing per square foot and Make adjustments to suit the intended house.