A basement costs more per square metre than any other way of adding space to a house, and the reason is simple: most of the money goes on work you never see. Before a single room is fitted out, the house has to be held up, the ground taken away, a concrete structure built beneath it and the water kept out for good. This guide sets out the typical London range, what it does and does not include, what moves a price within it, and the extras that sit on top. For how the work is done and what each North London council asks for, see our page on basement conversions in London.
How much does a basement cost per m²?
Basements are priced per square metre of new floor below ground. These are typical London ranges for 2026:
| Scope | What it is | Typical London cost per m² |
|---|---|---|
| Cellar conversion, no dig | Waterproof, ventilate and fit out a cellar that already has usable head height | £1,500 – £3,000 |
| Cellar with the floor lowered | Excavate inside an existing cellar and underpin the walls | £4,500 – £7,500+ |
| New basement under the house | Excavate a new storey and underpin the whole footprint | £4,500 – £7,500+ |
| Rear basement under a garden or extension | Excavate at the back of the house, usually beneath a new extension | £4,500 – £7,500+ |
The £4,500–£7,500+ figure is a typical market range for basement work in London. It is a guide, not a quote for your house: a real price comes after a ground investigation and a structural design. It covers the build, meaning the excavation, the underpinning, the waterproofing and the fit-out. It does not cover VAT, design and engineering fees, the planning application or party wall costs, which are set out below.
The lower band is a different job. It applies only where the space already exists at a usable height and nothing needs to be dug or underpinned. As soon as the floor has to come down, the work goes below the existing foundations, the walls have to be underpinned, and the project is priced as an excavation.
What that means for a whole basement
Multiply the floor area by the range and you have the build budget to plan around:
| New basement floor area | At £4,500 per m² | At £7,500 per m² |
|---|---|---|
| 30 m² | £135,000 | £225,000 |
| 40 m² | £180,000 | £300,000 |
| 50 m² | £225,000 | £375,000 |
| 60 m² | £270,000 | £450,000 |
For comparison, a 30 m² cellar conversion with no dig, at £1,500–£3,000 per m², is £45,000–£90,000.
The plus sign on the upper figure matters. Deep basements, wet ground, poor access and high-specification fit-outs all go past £7,500 per m², and no honest range has a ceiling for those.
What moves the price within the range?
A basement quote is mostly a quote on your ground and your house. These are the things that push a project up or down:
- Depth. Head height is the most expensive thing to buy below ground. A deeper dig means more spoil to remove, deeper underpinning and taller retaining walls, and the cost rises faster than the depth does.
- Ground. London clay, which lies under much of North London, is relatively predictable to underpin. Gravels, made ground and mixed soils are harder to hold back and slower to work in.
- Water. A high water table means a heavier structure, more waterproofing and a bigger pumping system, with more to maintain afterwards.
- Access. Every cubic metre of soil leaves through the front of the property. A terrace with no side access and a narrow street costs more to dig than a detached house with a drive.
- The existing structure. Shallow footings, earlier alterations or signs of movement mean more temporary works and slower, more careful underpinning.
- Neighbours. The more of the basement's edge that is a shared wall, the more underpinning is party wall work, with the notices and surveyors that go with it.
- Trees. Mature trees affect clay soils and foundation design, and councils protect them.
- What goes in it. A utility room and a gym cost less to fit out than a cinema, a bathroom or a pool.
The extras to budget for
The per m² range covers the build. These sit on top of it:
| Extra | Typical cost | Notes |
|---|---|---|
| Lightwell | £15,000 – £25,000 | External excavation and glazing, for daylight and escape |
| Design and engineering | £15,000 – £25,000 | Architectural and structural design for a full dig |
| Basement Impact Assessment | £3,000 – £8,000+ | Where the council requires one. Camden also charges the applicant for an independent audit |
| Party wall surveyors | £1,000 – £2,500 per neighbour | More for a complex basement award |
| Householder planning application | £548 | Needed for every basement or lightwell excavation at a house in Camden, and for most new digs elsewhere |
| Lawful development certificate | £274 | Where a basement stays within permitted development |
| VAT | 20% | Most work to an existing home is standard-rated |
VAT is the one people forget. At 20%, it adds £40,000 to a £200,000 build, so always check whether a figure you are comparing includes it.
Does planning change the cost?
It changes the fees and the programme more than the build. In Camden every basement excavation needs a planning application, a Basement Impact Assessment and, in most cases, an independent audit of that assessment paid for by the applicant. Haringey asks for an impact assessment under its basement policy. Barnet looks at ground conditions and flood risk, and Enfield at groundwater flooding. The engineering has to be done before the application goes in, so a share of the design cost is spent before you know the outcome. Each council's rules are set out under basement planning by council on the hub.
Party wall costs are close to certain on a terrace or a semi. Excavating near a neighbour's foundations and underpinning a shared wall both need notices under the Party Wall Act, and the owner doing the work pays the surveyors. Our Party Wall Act guide explains how it works.
Is a basement worth the money?
The test is whether finished floor space on your street sells for more per square metre than it costs to create. At £4,500–£7,500+ per m² before VAT and fees, that points to high-value streets where there is no room left to extend, which is why basements cluster in places like Highgate and Hampstead.
Where a house can still go out or up, a rear extension or a loft conversion adds space for less, and the North London extension cost guide has those figures. The exception is a house that already has a cellar. Converting space that exists costs far less than digging space that does not, and it can make sense on streets where a full dig never would.
Who does the work?
In North London, Pine Grove's own team carries out the excavation, the underpinning, the waterproofing and the fit-out. The structural work is not handed to a subcontractor. You get a fixed, itemised price once the ground investigation and the structural design are done, because that is the first point at which a basement can be priced properly.