Published 2026-10-29 · 2 min read
Gross, net and usable floor area: the differences
A “120 square metre house” can mean three different things, and the gap between them easily passes 20%. When you compare builders' quotes or read planning rules, you need to know exactly which area is meant. Names vary by country; the ideas do not.
The three areas
- Footprint / gross external area: measured to the outside face of the external walls. For a single-storey house it is the footprint on the plot.
- Gross area over all floors: the sum of every floor's gross area. A two-storey 10 × 12 m house has a 120 m² footprint and 240 m² gross.
- Net usable area: the rooms measured inside the walls — no walls, columns or stairwell void. Balconies and terraces are usually listed separately.
An example with numbers
Single-storey house, 10 × 12 m, external walls 35 cm thick.
- Gross: 10 × 12 = 120 m².
- Inside the envelope: (10 − 0.7) × (12 − 0.7) = 9.3 × 11.3 = 105 m².
- Internal walls take another 6–9 m², depending on how many rooms.
- Net usable ≈ 97 m², about 80% of gross. With thicker walls or many small rooms it drops towards 75%.
Plot coverage and floor area ratio
Planning rules usually set two limits. Plot coverage = footprint / plot area × 100. Floor area ratio (FAR) = gross area of all floors / plot area.
On a 500 m² plot with 35% coverage and a FAR of 0.9: footprint up to 175 m², total gross up to 450 m². A two-storey house at 175 m² per floor would be 350 m² gross — it fits. How basements, attics and lofts count varies by authority, so read your local rules.
Price per square metre
A builder quoting “€1,500 per m²” may mean gross, footprint or net. On the same two-storey house of 240 m² gross, €1,500 on gross is €360,000; on net (≈ 190 m²) it is €285,000. Always ask which area the price is on and get it in writing.
In GrafitGo the Analysis panel shows floor area and volumes, and the bill of materials adds up what is in the model — you have your own numbers before you talk to anyone.
Does a terrace count in the footprint?
An open terrace does not. A covered one enclosed on several sides may — it depends on how your local rules define it.
Which area do estate agents use?
It varies by market; many quote gross internal or net area. Ask, and compare listings on the same basis.
New articles by email
We write rarely — only when an article is worth reading. Unsubscribe in one click.