House extensions cost in London (2026)

Prices reviewed November 2026

The typical price in London is £2,800 to £4,000 per m² (single-storey rear extension). Extension costs are usually quoted per square metre, and the biggest swings come from the design, the ground and the finish.

💷 Typical house extensions prices in London

Typical London prices in 2026. A guide only; your quotes will depend on the job.
Single-storey rear extension	£2,800 to £4,000 per m²
Double-storey extension	£2,200 to £3,500 per m²
Side return extension	£3,000 to £4,200 per m²

What changes the price

The floor area, which sets most of the cost.
Single or double storey, and the height of the structure.
Ground conditions and foundations.
Glazing, kitchens and bathrooms in the new space.
Planning fees, architect drawings and building control.

How long it takes

A single-storey extension usually takes three to six months from the start of building work, after design and approvals.

How to pay less

Get a detailed design before asking for quotes so builders price the same job.
Choose standard sizes for windows and doors to reduce costs.
Keep a contingency fund for ground and drainage surprises.

Before you hire

Ask whether the project needs planning permission or can be built under permitted development.
Ask who handles building regulations approval and the completion certificate.
If you share a wall with a neighbour, ask who will serve the party wall notice.
Ask for a fixed price based on written drawings and a specification.
Ask for public liability insurance and a structural warranty.

💬 Questions about house extensions costs

Do I need planning permission in London?

Many rear extensions fall under permitted development, so they don't need full planning permission. Rules are stricter in conservation areas and for flats. Check with the council or ask builders when they quote.

How long does an extension take?

A single-storey extension is often 3 to 5 months from first dig to final snag, plus time for plans and approvals. Your builder will give you a timeline.

What should the quote include?

A written breakdown of labour and materials, a start date, a payment schedule tied to stages of work, and who handles skips and waste.

Is the per square metre figure the full cost?

Not always. It usually covers the build, but fees, kitchens, bathrooms and fittings can be extra. Ask each builder what their quote includes.

Get house extensions quotes

Use these figures to plan your budget, then compare written quotes for the same work. Your price depends on the job, the size of your home and how soon you need it. Tell us what you need to request up to 3 local quotes; responses depend on availability.