How much does a house extension cost?

What does a house extension cost in London?

Extension costs are usually quoted per square metre of finished space. The rates below are the starting points this calculator uses for London and Surrey — the tool then adjusts them for your house type, the size you want and the features you choose. All figures exclude VAT.

Single storey rear extension

£1,800 – £2,700 / m²

Single storey side extension

£2,100 – £2,900 / m²

Single storey wrap around extension

£2,100 – £3,000 / m²

Two storey rear extension

£2,400 – £3,400 / m²

Garden room or annex

£1,700 – £2,500 / m²

As a rule of thumb, a 25 m² single-storey rear extension lands around £50,000–£70,000 before extras such as glazed doors, rooflights or a new kitchen.

Loft conversion and new build costs

A loft conversion adds a floor without losing garden, and the cost depends far more on the roof type than on the finish. New builds are priced across the whole footprint.

Roof lights / Velux only

£18,000 – £28,000 total

Dormer loft

£1,800 – £2,600 / m²

Hip-to-gable

£2,000 – £2,900 / m²

Mansard

£2,200 – £3,200 / m²

New build (turn-key)

£2,400 – £3,400 / m²

What changes the price most

Two extensions of identical size can differ by tens of thousands. These are the factors the calculator asks about, and what each one typically adds:

Conservation area or listed

+15–20% on the build cost

No drawings yet

£8,000 – £14,000 for design, planning and technical drawings

Party wall agreements

£1,500 – £4,000 where notices are needed

Removing structural walls

£3,000 – £6,000 per opening, including steels

Glazed doors to the garden

£2,500 – £4,500 French, £3,500 – £6,000 sliding, £3,500 – £6,000 bi-fold

Rooflights

£600 – £1,500 for one, £1,200 – £3,000 for two

Access matters too. A mid-terrace with no side route means materials and waste pass through the house, which adds time and labour compared with a detached property.

How the calculator works

1. Answer a few questions about your home, the project type and the size you have in mind.
2. Add the details that move the price — planning status, structural openings, doors and rooflights.
3. See your range immediately on screen. No email gate, no callback, no obligation.

For a fixed, itemised price we follow up with a free 1–2 hour site visit. Read more about house extensions, loft conversions and how the five-stage process works, or see completed projects across South West London and Surrey.

Extension cost calculator — frequently asked questions

Want to understand where the money actually goes? Our guide to house extension costs and what to budget for breaks down professional fees, site costs, structure and finishes.