Search “basement conversion cost” and the figures you'll find range from around £20,000 to well over £150,000 for what looks, on the surface, like the same job. It isn't sloppy research — “basement conversion” genuinely covers at least three different projects, and which one you're pricing typically comes down to £1,200–£4,650 per square metre depending on whether you're refurbishing a cellar that already exists, lowering its floor for full headroom, or excavating a basement that isn't there today.
Most of the eye-watering six-figure quotes belong to the third category. A lot of people typing this search have the first: an unconverted or half-used cellar under a Victorian terrace, and what they actually need is a damp-proofing and headroom assessment, not a dig. Knowing which job you're really costing, before a contractor's estimate arrives, changes the number by a factor of two to four.
I'm a building surveyor, not a basement contractor or a waterproofing specialist — I diagnose what's there and refer on for the specialist design work, so I've no reason to talk you into the bigger job. What follows is 2026 UK pricing cross-checked across several trade cost guides, what actually drives the price, and the two things — water and next door's foundations — that cause most of the expensive problems after the room's been plastered and carpeted.
The short answer
Figures below were checked on 9 September 2026 against MyJobQuote, HouseUp and BookABuilder's 2026 cost guides, cross-referenced against each other rather than taken from a single source. As with most trades in this series, London and the South East — Dom's own patch included — commonly run 20–25% above the national figures quoted here.
| Scope of works | Typical cost (2026) | What's included |
|---|---|---|
| Existing cellar, refurbished only | £1,200–£2,250 per m² | Damp-proofing, insulation and finishing a space that already exists, no floor-lowering |
| Floor-lowering for full headroom | £2,300–£3,150 per m² | Underpinning, structural engineer's design and extensive waterproofing |
| New excavation (under house or garden) | £3,000–£4,650 per m² | Full underpinning and tanking; regularly £100,000+ on an ordinary house |
| Waterproofing retrofit alone | £7,000–£14,000 all-in | Cavity drain membrane, sump and pump for a typical 20–30m² cellar |
What drives the price
Which of the three jobs it actually is. Refurbishing existing space, lowering the floor, and digging entirely new ground are priced worlds apart because the structural risk is worlds apart — the first disturbs almost nothing, the last requires underpinning the whole house while it's still standing on it.
Underpinning method and length. Where floor-lowering or new excavation needs it, mass concrete underpinning runs roughly £800–£1,500 per linear metre, and mini-piled underpinning — needed on poor ground or tight access — runs £1,500–£3,000 per linear metre.
Ground conditions. London's clay, and a lot of the South East's, expands and contracts with the seasons, which is exactly the soil structural engineers worry about most when a basement changes how a building's weight is carried into the ground.
Access. A detached house with garden access for machinery is a different quote from a mid-terrace where every barrow-load of spoil comes out through the hallway. Restricted access is one of the main reasons London figures run so far above the national ones.
Waterproofing grade and structural engineer input. A structural engineer's calculations for the underpinning and floor design typically cost £1,500–£3,500 (see my dedicated structural engineer cost guide for the fuller breakdown), and the waterproofing specification below is its own significant line item, not an afterthought.
Region. London and the South East commonly run 20–25% above the national figures quoted here, in line with most trades in this series.
Which conversion are you actually costing?
This is the question worth answering before requesting a single quote, because it's the one that separates a £30,000 project from a £150,000 one. If there's already a cellar under the house — even a damp, low-headroom one currently used for nothing but the boiler and old paint tins — refurbishing it is refurbishment, not excavation. Damp-proofing, insulating, adding light and finishing it as a snug, home office or utility room sits at the bottom of the table above, often without needing planning permission at all, only Building Regulations sign-off.
Full headroom under a low Victorian cellar, or a basement that doesn't exist yet, is a different order of project. Both need underpinning — supporting the existing walls on new, deeper foundations while the ground beneath them is dug out — which is what pushes the price past £2,300 per m² and turns a straightforward refurbishment into a structural engineering job with a Building Regulations application, and very often planning permission, attached. Be specific with anyone quoting: ask them to state in writing which of the three scopes in the table they're actually pricing, because “basement conversion” on its own doesn't tell you.
The two things that go wrong later
Water. BS 8102, the British Standard for waterproofing below ground, sets out grades of protection matched to how the space will be used. A habitable room — a bedroom, a home office, a gym, anywhere you'll spend real time — needs Grade 3 protection, which in practice means a proper Type C cavity drainage system (a membrane with a sump and pump, sometimes with battery backup) or a combined Type A and C build-up, not just a tanking slurry painted on the walls. A cheaper Grade 1 or 2 specification, sold as “waterproofed” without saying which grade, is fine for storage but fails once the room is plastered, carpeted and actually lived in — and by then the fix means ripping out finishes that are already paid for. Ask explicitly which BS 8102 grade the quote delivers, and for an insurance-backed guarantee, typically 10–20 years, that survives the installer going out of business.
Next door's foundations. Any excavation deeper than a neighbouring building's foundations, within 3 metres of it, requires an Adjacent Excavation Notice under section 6 of the Party Wall Act 1996, and underpinning a shared party wall needs its own Party Structure Notice on top of that — both covered in more detail in my party wall surveyor cost guide. Beyond the legal notice, there's a genuine structural risk: excavating next to an older property on shifting clay can cause differential settlement, where part of a building moves on newly underpinned ground while the rest doesn't, showing up as cracking that has nothing to do with the neighbour's own foundations being at fault. If there's any doubt about whether existing movement is old and settled or new and ongoing, that's exactly the question an independent structural engineer's assessment — not the contractor proposing the dig — should answer first; see my underpinning cost guide for how that assessment typically runs.
What I'd watch for in a quote
- The BS 8102 grade stated explicitly — "waterproofed" or "tanked" on its own doesn't tell you whether it's fit for a habitable room.
- An insurance-backed guarantee on the waterproofing, typically 10–20 years, that survives the installer going out of business.
- A chartered structural engineer's calculations behind any underpinning or floor-lowering, not just a builder's assurance that it'll be fine.
- Party wall notices served, and any agreed surveyor appointed, before excavation starts — not arranged retrospectively once digging is under way.
- A ground investigation (trial pit or borehole) carried out before a fixed price is given, rather than after the dig has started and conditions turn out worse than assumed.
- The exact scope stated in writing — refurbishment of existing space, floor-lowering, or new excavation are different jobs, and the quote should say which one you're buying.
Questions I get asked
How much does a basement conversion cost in 2026? It depends heavily on which of three jobs you're actually doing. Refurbishing an existing cellar with no floor-lowering typically costs £1,200–£2,250 per m². Lowering the floor for full headroom, which needs underpinning, runs £2,300–£3,150 per m². Excavating a brand new basement from scratch is £3,000–£4,650 per m², regularly exceeding £100,000 on an ordinary house. London and the South East commonly run 20–25% above these national figures.
Do I need planning permission for a basement conversion? Not always. Refurbishing an existing cellar with no external changes and no structural alterations can often be done under permitted development, though Building Regulations approval is still required regardless. A new excavation, digging under a garden, or adding a lightwell or window that changes the front or rear elevation will usually need planning permission, and permitted development rights are typically restricted or removed for flats, listed buildings and conservation areas.
Will a basement conversion affect my neighbours, or do I need a party wall agreement? Very often, yes, if the property is terraced or semi-detached. The Party Wall Act 1996 requires an Adjacent Excavation Notice if you're digging within 3 metres of a neighbouring building and to a greater depth than their foundations, and a Party Structure Notice if the party wall itself needs underpinning. Notices need serving before work starts, not after.
What's the difference between a cellar conversion and a new basement excavation? A cellar conversion refurbishes space that already exists below the house — damp-proofing, insulating and finishing it, without moving the floor or digging further down. A new excavation creates space that isn't there today, which needs underpinning, full tanking and usually planning permission. The excavation route costs two to four times more per square metre and carries meaningfully more structural risk.
If you're weighing up a basement conversion on an older or terraced property, the useful first step is usually an independent read on what's actually down there — the existing structure, any signs of movement, and how the current cellar or floor slab is built — before an architect or waterproofing specialist starts drawing up a scheme. That's exactly what my Building Survey is for, and if it turns up cracking or a wall that needs opening up, I'll say so plainly rather than downplaying it. Call 07946 618203 or get in touch before you commission drawings, so the survey and the basement scheme are working from the same facts.
Small print. This guide is general information, not advice on a specific property, and it doesn’t replace a proper survey, structural engineer's assessment and written quotes for your own home. Prices were researched and correct to the best of my knowledge on 9 September 2026; costs move and vary by region, ground conditions and property. Always get at least three written quotes, and confirm the exact scope and BS 8102 waterproofing grade before you instruct anyone.