Read this first
Why the ranges are wide
Two identical-looking extensions on the same street can differ in price by a factor of two. That isn't builders being inconsistent — it's foundations, ground conditions, access, glazing specification and what the extension forces you to change in the rest of the house.
The figures below are broad market brackets for London and the surrounding areas as at 2026, covering construction only. They are indicative and are not a quote. Anyone who gives you a firm price before seeing the property and a drawing is guessing, and the guess will be revised.
Brackets
Roughly what to expect
- Single-storey rear
- The most common extension. Cost scales with footprint, glazing and how much of the existing ground floor has to change. Expect the middle of the market to sit well above the national average figures you'll find quoted, simply because London labour, access and waste disposal all cost more.
- Side return
- Less floor area than a rear extension but often similar money — it's a narrow, awkward site with a party wall on one side and frequently more roof and glazing per square metre of gain.
- Wrap-around
- Rear and side combined. More than the sum of the two individually is rare, but expect close to it, plus party wall awards on both sides and a full planning application.
- Double storey
- Adds roughly 50–70% to a comparable single-storey scheme rather than doubling it — the foundations and roof are largely shared. The best value per pound of the common options.
- Professional fees
- Architect, structural engineer, planning fees, Building Control and party wall surveyors. Budget 10–15% of construction cost on top, and more on a constrained or listed site.
Drivers
The five things that actually move the number
Foundation depth
The least visible and least predictable cost. Trees, clay, made ground, a high water table or a neighbour's deeper foundation all add depth — and depth is concrete, labour and weeks before anything above ground starts. This is the single most common reason a groundworks provisional sum moves.
Glazing
A standard set of bifolds and a structural glazed corner with a slim-framed sliding run are different products with different structural requirements. This is usually the biggest single specification decision in the project.
What the extension does to the rest of the house
Opening the back of a house into a new space almost always means relocating the kitchen, extending heating, adding underfloor, and re-running power and lighting across the whole ground floor. That work is part of your project whether or not it's on the extension drawing.
Access
A terrace with no rear access and controlled parking means everything comes through the house in small loads, and waste goes out the same way. It's labour, and it's real.
Specification
Kitchens, stone, sanitaryware, ironmongery and joinery span an enormous range. This is where budgets are most often won or lost, and where you have the most control.
Watch for
Costs that get left out of a headline figure
- Party wall surveyor fees — you generally pay for your neighbour's surveyor as well as your own
- Skip and scaffold licences from the council where there's no off-street space
- Building Control application and inspection fees
- Structural engineer's calculations, and a second visit if the ground turns out differently
- Making good the existing house where the new opening meets it — floors rarely align
- Extending the heating and electrical circuits across the affected floor
- Decoration of the rooms adjoining the works, which always need it afterwards
- Temporary kitchen provision if you're staying in the house
Comparing quotes
How to read three quotes that aren't the same
The cheapest quote is frequently the one with the least in it. Before comparing totals, check that all three price the same scope: the same foundation depth assumption, the same glazing, the same amount of work to the existing house, and the same treatment of provisional sums.
Look specifically at how each handles the unknowns. A quote with no provisional sum for groundworks isn't cheaper — it's a quote that will change. One that flags the assumption openly is more honest and usually more accurate.
And check what's excluded. Exclusions are where a low headline number usually comes from, and they're rarely on the first page.
Common questions
In London, extending usually compares well once stamp duty, agent fees, legal costs and moving are added to the price difference between properties — particularly on a double-storey extension, which adds the most value per pound spent. It stops being true when the extension you need requires substantial structural work or a basement.
It varies enormously by area and by what the extension does. Adding a bedroom and bathroom generally returns more than adding floor area to an existing reception. What's more reliably true is that the ceiling price for a street is real — spending beyond what the road supports rarely returns.
For a defined extension with a full set of drawings, a fixed price against a clear scope is normally the right structure. Cost-plus makes more sense on refurbishment of an old building where the extent of the work genuinely can't be known until things are opened up. What matters more than the structure is that the unknowns are named rather than hidden.
