Basements2026 figures
What a basement costs in London.
What a London basement costs, from our own rates and our 2026 cost guide. Two kinds of figure, each labelled.
The figures.
A new London basement runs £450 to £500 a sq ft, construction only. Our 2026 cost guide puts a new dig at £500k to £900k and more. That is a project total. Converting an existing basement runs £180k to £300k. Fees, party wall costs, contingency and VAT sit outside these.
£450 to £500a sq ft, construction only. Our basement band, one of six scoped rates.
£180k to £300kConverting an existing basement. A project total, from our 2026 cost guide.
£500k to £900k+Digging a new basement. A project total, from our 2026 cost guide.
Every figure on this page is construction cost. Fees, statutory costs, party wall awards, contingency and VAT sit outside it.
Three kinds of basement.
- Fit out a cellar
- The smallest job. The cellar is already there, so the work is waterproofing, services and finishes.
- Lower and widen
- The cellar is dug deeper or wider, usually by underpinning. Party wall work follows.
- Dig a new floor
- A new storey under the house or the garden. This is the cost guide’s new excavation figure.
What moves the figure.
- The borough
- Each council limits how far a basement can go. Less floor area means a smaller total, not a lower rate.
- The ground
- Clay, gravel and groundwater decide the waterproofing, the pumps and the temporary works.
- Party walls
- A terrace shares its walls. Underpinning them adds structure, surveyors’ fees and time.
- Access
- Every tonne of spoil leaves through the house or the street. Tight access slows the dig.
- What goes in it
- A pool, a cinema or a plant room adds depth, services and cost.
- The finish
- A super prime finish has its own band, £500 to £800 a sq ft.
The rules, borough by borough.
- Kensington and Chelsea
- Policy CL7. One storey, under no more than half the garden. Basements in Chelsea
- Kensington
- The same CL7 rule, across all of Kensington. Basements in Kensington
- Hammersmith and Fulham
- Policy DC11. One storey, half the garden’s depth and half the house’s depth at most. Basements in Fulham
- Westminster
- City Plan Policy 50. One storey below the lowest original floor, under half the garden. Belgravia, under Westminster
Basement costs, answered.
How much does a basement cost in London?
Our basement band is £450 to £500 a sq ft, construction only.
As a project total, a new dig runs £500k to £900k and more.
Converting an existing basement runs £180k to £300k.
What is the cost per square foot for a basement?
£450 to £500 a sq ft, construction only, at 2026 prime London rates.
A super prime finish runs £500 to £800 a sq ft.
How much does a basement conversion cost in London?
Our 2026 cost guide puts it at £180k to £300k, as a project total.
That assumes the cellar is already there.
Lowering or widening it costs more, because of the underpinning.
How much does a basement extension under the garden cost?
It is a new dig, so it runs £500k to £900k and more.
The council decides how much of the garden it can go under.
Why do basement quotes vary so much?
Because the ground, the party walls and the access vary house by house.
A quote that ignores them will move later.
We price the house itself, from a survey.
What is not included in a basement price?
Design and planning fees, party wall awards, contingency and VAT.
Our cost guide lists them, so the budget is whole from the start.
Basement cost
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