House materials calculator
A per-square-metre estimate tells you whether you can afford the house. It is useless at the merchant, where you must say how many cubic metres of concrete you want. This starts from geometry instead.
What goes into the calculation
- Masonry: perimeter x height x thickness, minus the percentage of openings for doors and windows.
- Block count, at roughly 60 pieces per cubic metre, with 6% waste from cutting.
- Concrete: perimeter foundation and slabs, with 5% waste.
- Roof covering: floor area x 1.35, because pitch and eaves make it larger.
- Render: wall area counted on both the inside and the outside.
What is not included
The calculation covers structure, not a finished house. It leaves out labour — locally around 40-50% of the total — electrical and plumbing, joinery, insulation, interior finishes and site utilities.
For a number to hand a bank, take the plan to a quantity surveyor. This one is for orders of magnitude and for comparing options.
Where does the 1.35 roof factor come from?
A 100 m2 house with a 30-degree pitch and 60 cm eaves has roughly 135 m2 of roof covering. Steeper pitches raise it further.
Can I use it for an extension?
Yes. Enter the extension as if it were a small house, then subtract shared walls by hand.
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