How much does a rear extension cost in London?

A rear extension in London runs £280 to £320 per square foot of new space. That is the build rate for constructing the extension itself; professional fees, planning and the specification of what goes inside the room sit outside it.

Is a side return extension more expensive than a rear extension?

Per square foot, yes. A side return runs £310 to £340 per square foot against £280 to £320 for a rear extension, because working tight to the boundary carries more complexity and proportionally more structure per square foot gained.

#NU BLOG

Rear Extension Cost in London: £280 to £320 per Square Foot

5 hours ago
3 min read

Ask three London builders what a rear extension costs and you will get three numbers that cannot be compared: one includes the kitchen, one excludes the groundworks, and one is a guess dressed up as a quote. So here is the figure the way we actually price it: in London, a rear extension runs £280 to £320 per square foot of new space. That is a build rate — the cost of constructing the extension itself — and once you hold it, every quote you receive becomes checkable arithmetic.

What the band covers

The £280 to £320 per square foot band covers the construction of the new structure: excavation and foundations, the shell, the roof, glazing, first and second fix, and finishes to a standard consistent with a well-finished London family home. Multiply the footprint of the space you are adding by the band and you have a realistic bracket before a single drawing exists.

What sits outside any per-square-foot rate is the scope that varies house by house: professional fees, planning, party wall matters where a neighbour's structure is affected, and the specification of what goes inside the new room. A kitchen at the modest end of bespoke and a kitchen at the top of it can be separated by more than the extension shell itself — which is why the band describes the build, and the specification conversation happens with your eyes open, in writing, before anyone signs anything.

Open-plan kitchen, dining and living space on Tite Street, Chelsea SW3, lit by a full-length rooflight, with a marble island and glass balustrade

What moves you inside the band

Three things, mostly. Ground conditions: what the dig finds decides how hard the foundations have to work. Structural openings: the wider the opening between old house and new room, the more steel carries the load. And glazing: glass is the most expensive wall you can buy, and large sliding or pocket doors move a project toward the top of the band.

If you are weighing this against other ways of adding space, our guide to loft conversion costs uses the same per-square-foot discipline, so the two can be compared honestly.

Rear extension or side return?

On a Victorian terrace the side return — the narrow strip beside the back addition — is often the smarter move, but it is the more expensive rate: £310 to £340 per square foot, against £280 to £320 for a rear extension. The premium buys complexity: working tight to the boundary, more party wall interface, and proportionally more structure per square foot gained. Which one suits your house depends on the plot, not on preference, and it is exactly the question a site visit answers in twenty minutes. If you would like that visit, speak to us at nuprojects.co.uk.

The route you buy the work through matters as much as the rate. Whether an architect-led route or design and build suits your project decides who carries the risk when the dig finds something nobody drew.

Kitchen with a dark grey-veined stone waterfall island, integrated hob, brass extract hood and marble splashback, Abingdon Road, Kensington W8

The honest arithmetic

A 400 square foot rear extension at £280 to £320 per square foot is a £112,000 to £128,000 build before specification choices — not a small number, and not a mysterious one either. A quote far below the band is not a bargain; it is a scope with pieces missing, and the missing pieces arrive later as variations.

NU Projects is a design and build practice established in 2010, with over 100 prime central London projects behind us. Architecture, interior design, project management and construction sit under one contract, so the number you agree is the number one accountable team delivers. Talk to us before you commit to a figure: nuprojects.co.uk · 020 7731 6841.

Ready to find out what your extension would really cost? Book a consultation or call 020 7731 6841. Send us the address and the footprint you have in mind, and we will tell you honestly which route suits the plot and what it should cost.