Rear extension cost in London, what the number covers

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. The useful figure is a build rate per square foot of new space, and once you hold the right one, every quote you receive becomes checkable arithmetic.

Our own rates are published by work type and specification in the NU Build Cost Calculator. Use it to get the band for a rear extension before you read on, because the rest of this only makes sense against a number.

What the band covers

A rear extension build rate 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 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. The band describes the build. The specification conversation happens with your eyes open, in writing, before anyone signs anything.

What moves you inside the band

Three things, mostly. Ground conditions, because what the dig finds decides how hard the foundations have to work. Structural openings, because the wider the opening between old house and new room, the more steel carries the load. And glazing, because glass is the most expensive wall you can buy, and large sliding or pocket doors push 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. It usually costs more per square foot gained. The premium buys complexity: working tight to the boundary, more party wall interface, and proportionally more structure for each square foot you win. Which one suits your house depends on the plot rather than on preference, and it is the question a site visit answers in twenty minutes.

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.

The honest arithmetic

Take the band from the calculator, multiply it by your footprint, and you have the build cost before specification choices. It will not be a small number and it is 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. Architecture, interior design, project management and construction sit under one contract, so one accountable team stands behind the number you agree.

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