Rear extension cost in London, explained

Ask three London builders what a rear extension costs. You will get three numbers that cannot be compared. One includes the kitchen. One excludes the groundworks. One is a guess dressed up as a quote. The useful figure is a build rate per square foot of new space. Once you hold the right one, every quote you receive becomes checkable arithmetic.

Our own rates are published by work type and specification. You will find them in the NU Build Cost Calculator. Use it to get the band for a rear extension before you read on. 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. That includes excavation and foundations, the shell, the roof and glazing. It includes first and second fix. It also includes finishes to a standard consistent with a well-finished London family home. Multiply the footprint you are adding by the band. You then 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. That means professional fees and planning. It means party wall matters where a neighbour’s structure is affected. It also means the specification of what goes inside the new room. A modest bespoke kitchen and a top-end one can be far apart. The gap can be 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. What the dig finds decides how hard the foundations have to work. Structural openings. Wider openings between old house and new room mean more steel carries the load. And glazing. Glass is the most expensive wall you can buy. Large sliding or pocket doors push a project toward the top of the band.

You may be 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 is often the smarter move. That is the narrow strip beside the back addition. It usually costs more per square foot gained. The premium buys complexity. That means working tight to the boundary and more party wall interface. It also means proportionally more structure for each square foot you win. Which one suits your house depends on the plot rather than on preference. 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 matters. It decides who carries the risk. That is when the dig finds something nobody drew.

The honest arithmetic

Take the band from the calculator and multiply it by your footprint. You then have the build cost before specification choices. It will not be a small number. It is not a mysterious one either. A quote far below the band is not a bargain. It is a scope with pieces missing. 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. We will tell you honestly which route suits the plot. We will also tell you what it should cost.