Free tool · England & Wales

What could your extension cost?

Put in your postcode and the change you're thinking about. You'll see whether it's likely to be permitted development, a build-cost range from BCIS data, the value it could add, and whether the numbers stack up, before you pay anyone for drawings.

How a single-storey rear extension is measured

Postcode UK postcode — we'll detect the authority and the right rules. Property type Permitted-development limits differ by type.

What do you want to do?

Rear projection (m) How far it sticks out beyond the existing rear wall.

New floor area

Approximate added floor space (m²).

What will the new space be? A bedroom adds more value than the same floor area as kitchen or living space.

Doing more than one job, like a rear extension and a garden room? Add each one and we'll check how they affect each other and price them together.

How nicely finished?

Pick the closest. It sets which end of the BCIS cost range we use (lower quartile, median or upper quartile).

Current property value (£) Optional. Leave blank and we'll use recent local sales. Current floor area (m²) Optional. It's on your EPC certificate. Makes the value figure more accurate.

— Confirm anything that applies to the property:

In a conservation area A listed building Under an Article 4 direction In a flood-risk zone

Indicative only. This is a screening tool, not planning advice or a formal valuation. Permitted-development rules carry conditions a screen can't see (height, boundaries, prior extensions, Article 2(3) land). Confirm with the local planning authority and a RICS surveyor before committing money.