Basement Conversion Cost in London (2026)
Last updated: June 2026 · Figures verified against live North London quotes and current council planning records.
Quick answer: A basement conversion in London costs £4,000–£7,000 per m² with underpinning in 2026, or £150,000–£400,000+ for a full project. A new dig-out (lowering plus underpinning) runs £3,000–£5,000 per m². It is the single most expensive way to add space to a home.
If you are researching what a basement costs, the honest headline is: more than any other way of adding space, and it depends heavily on how deep you dig and what the ground beneath your house is doing. A basement is not a bigger version of an extension — it is a structural engineering project that happens to produce a room at the end. This guide gives real 2026 London prices, breaks down where the money actually goes (underpinning and waterproofing dominate), separates the three very different jobs people call "a basement", and helps you decide whether the spend stacks up. For the full service and how we deliver it, see our basement hub.
How much does a basement cost per m²?
Basements are priced per square metre of new lower-ground floor, then loaded with fixed extras. These are realistic London ranges for 2026, excluding VAT:
| Scope | What it is | Cost per m² | Typical total |
|---|---|---|---|
| Cellar conversion (existing space) | Tank, drain and fit out a usable cellar | £1,500 – £3,000 | £40,000 – £90,000 |
| New dig-out (lowering + underpinning) | Lower the floor, underpin foundations | £3,000 – £5,000 | £120,000 – £250,000 |
| Full dig-out + underpinning (complex) | Deeper dig, difficult ground or water | £4,000 – £7,000 | £150,000 – £400,000+ |
| Dig-out + lightwell | Adds external excavation and glazing | + £15,000 – £25,000 | £200,000 – £400,000+ |
Three numbers to anchor your expectations:
- A typical London terrace basement lands between £150,000 and £400,000+ all-in.
- Underpinning alone is 35–50% of a dig-out budget — it is the biggest single line.
- Each extra metre of depth adds roughly 18–25% to the structural cost.
Get an instant ballpark for your property on the Extension Builder.
What drives the cost of a basement?
A basement quote is really a quote on your ground conditions. Six factors move the price more than anything else:
- Depth of dig. The single biggest driver. Every extra metre adds 18–25%, because deeper means more spoil to remove, more underpinning and more retained structure. Head height is the most expensive thing to buy underground.
- Ground type. London clay — under most of North London — is relatively forgiving and predictable to underpin. Thames-edge gravels are not: looser, wetter and harder to work safely, which pushes you toward the top of the range.
- Water table. A high water table means a more serious waterproofing and pumping system, and more ongoing maintenance. Waterproofing and drainage are 15–22% of the total on most projects.
- Structural condition. Shallow Victorian footings or existing movement mean more underpinning and more careful, slower engineering.
- Trees nearby. Mature trees draw moisture from clay soil and affect foundation design — a real consideration on leafy London streets.
- Underpinning extent. How much of the perimeter needs underpinning, and how deep, drives 35–50% of the whole budget.
Cellar conversion vs new dig vs dig-plus-lightwell
The biggest mistake people make is comparing prices for jobs that are not the same. There are three:
- Cellar conversion — you already have a cellar with near-usable head height and sound structure. The work is tanking, drainage, fit-out and bringing it to building regulations. At £1,500–£3,000 per m², this is by far the cheapest route, because you are not paying for the structural engineering that dominates the other two. If your home has a decent existing cellar, this is where the value is.
- New dig-out — lowering the floor and underpinning the foundations to create head height that was not there before. At £3,000–£5,000 per m², this is a true structural project. Most "basement conversions" people imagine are actually this.
- Dig-out plus lightwell — adding an external excavation and glazing to bring in natural light and, often, a separate entrance. The lightwell adds £15,000–£25,000 but is frequently what turns a dark, technically-habitable space into a room people actually want to use.
The extras you must budget for
A headline per-m² figure covers the build of the basement box. It rarely covers everything around it. Budget for:
| Extra | Typical cost | Notes |
|---|---|---|
| Design & engineering (full package) | £15,000 – £25,000 | Architect, structural engineer, drawings |
| Lightwell / separate entrance | £15,000 – £25,000 | External dig plus glazing |
| Waterproofing & drainage | 15–22% of total | Tanking, sump and pump system |
| Party wall surveyors | £1,000 – £2,500+ per neighbour | Almost always required on terraces/semis |
| Basement Impact Assessment | £3,000 – £8,000+ | Where the borough requires one |
| VAT | 20% | Most residential basement work is standard-rated |
The two extras people forget are the Basement Impact Assessment (a borough requirement in much of North London) and VAT, which at 20% on a £200,000 project is £40,000 on its own. Always confirm whether a quote is VAT-inclusive.
Do you need planning permission? (It affects cost and timeline)
A basement built under your existing footprint that does not change the external appearance is often permitted development. But many London boroughs — Camden most strictly, and parts of Barnet, Enfield and Haringey — have specific basement policies that require a full planning application, a Basement Impact Assessment and a structural method statement. Where these apply, expect to add design time and fees to your programme. Party wall agreements almost always apply on terraces and semis, because you are excavating below a shared wall. Treat every basement as a full, planning-heavy job and you will not be caught out.
Is a basement worth it?
The financial case is simple: a basement only makes sense where finished space is worth enough per square metre to justify the highest construction cost of any home improvement. That points to premium London streets — Highgate, Hampstead, Crouch End and period homes — where a £250,000 basement can still add value above what it cost. On those streets, with no garden left to extend into, a basement may be the only way to add a whole new floor.
Where it rarely makes sense: an average-value street with garden space to spare. There, a rear or wrap-around extension gives you far more room for your money. Our honest rule: if you can extend outwards or upwards, price that first. Go down only when down is the right answer.
Frequently asked questions
How much does a basement conversion cost per m² in London? Converting an existing cellar runs £1,500–£3,000 per m². A new dig-out with underpinning is £3,000–£5,000 per m², rising to £4,000–£7,000 per m² for deeper or more difficult jobs. A typical London terrace basement totals £150,000–£400,000+ all-in, excluding VAT.
Why is underpinning such a big part of the cost? Underpinning deepens and strengthens your existing foundations in carefully sequenced sections so the house sits safely on a lower floor. It is highly skilled structural work and accounts for 35–50% of a dig-out budget — the single largest line in the quote.
How much does depth add to the price? Roughly 18–25% per extra metre. Cost does not rise in a straight line with depth — a deeper basement is disproportionately more expensive because of the extra spoil, underpinning and retained structure involved.
Do I have to pay VAT on a basement conversion? Yes, in almost all cases. Most residential basement work is standard-rated at 20%. On a £200,000 project that is £40,000, so always confirm whether a quote includes VAT before you compare numbers.
Is a basement cheaper than an extension? No — it is the most expensive way to add space. An extension almost always wins on cost per square metre. A basement only makes financial sense on premium streets where land values are high and there is no garden left to extend into.
Want a real number for your property, not a range? Get an instant ballpark on the Extension Builder, then book a free site visit for a fixed, itemised quote. See how we deliver basements safely on our basement hub, or call The Extension Company on 020 3051 9430.