Published 2026-08-30 · 2 min read
How to estimate house materials from a 3D model
A '€500 per square metre' estimate is fine for deciding whether you can afford the house. It is useless at the builders' merchant, where you have to say how many cubic metres of concrete you want. The gap between the two is called a take-off, and it comes out of the model.
Why per-square-metre estimates mislead
A price per square metre is an average across very different houses. Two 120 m² houses can differ by 30% on shape alone: one compact, one long and narrow with far more external wall to build, render and insulate.
The 3D model knows exactly how many metres of wall you have, at what height, with which openings. That yields a number, not an average.
What comes straight out of the model
- Volume of external and internal walls, minus door and window openings.
- Area to finish: render, filler, paint — inside and outside separately.
- Volume of slabs and foundations, i.e. the concrete.
- Roof area, larger than the house footprint because of pitch and eaves.
- Door and window counts, by size.
The allowances nobody adds at first
A volume calculation is not an order. Real losses go on top, or you stop halfway through pouring day.
- Concrete: +5% for losses and uneven excavation.
- Brick: +5–7% for cuts and breakage.
- Roof tiles: +10%, because of overlaps and cuts at ridges.
- Tile and flooring: +10% for cuts, +15% if laid diagonally.
- Mortar and adhesive: per the product data sheet, which varies a lot.
A take-off that updates itself
The advantage of calculating from the model is that it does not go stale. Change wall thickness from 25 to 30 cm and volumes recompute — you are not left with an old table where you no longer know what was current.
In GrafitGo the cost sheet walks the objects, sums volumes per material and shows them at the unit prices you set. Export as CSV, open in a spreadsheet, negotiate with the supplier.
Does a model take-off replace a professional bill of quantities?
Not for a contract and not for tendering. Use it to know the order of magnitude, compare variants and check a quote that looks inflated.
How do I estimate labour?
Labour does not come out of geometry. A rough local rule is 40–50% of total cost, but get two or three quotes rather than trusting a percentage.
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