Cost guide · 2026
BasementConversionCostinLondon2026
Comprehensive 2026 pricing for every type of London basement conversion — cellar refurbishment, full basement excavation and basement extension. Real costs from completed projects, broken down by depth, footprint and zone.
Quick answer
A basement conversion in London costs £35,000–£500,000+ in 2025–2026 depending on type. A cellar conversion (refurbishing existing under-house space) starts £35,000–£90,000. A full basement excavation under an existing house £150,000–£350,000. A basement extension under the garden adding new footprint £200,000–£500,000+. Cost per m² runs £3,500–£7,500. Underpinning, waterproofing and structural engineering drive the price.
What moves the price
The seven cost factors that matter most.
Cellar refurb £35–90k; full excavation £150–350k; basement extension £200–500k+. Type dominates the budget.
If a cellar already exists at near-full headroom, costs are much lower than excavating from scratch (£35k vs £150k+ starting).
Full perimeter underpin £40,000–£90,000 on typical terrace. Driven by linear metres and depth.
Type A (tanking) £80–150/m²; Type C (cavity drain) £150–250/m²; combined Type A+C £250–400/m² for high-risk groundwater.
Going down under existing house only is mid-range; extending under the garden adds £80,000–£200,000 in structure and excavation.
External light wells (essential for habitable rooms) £15,000–£40,000 each; vehicle-access basement (lift) adds £50,000–£120,000.
Most basements need full planning; some boroughs (K&C, Camden, Westminster) have specific basement policies restricting depth and footprint.
Cost breakdown
Where the money actually goes.
Percentage breakdown of a typical mid-spec project. Add or subtract per your finish tier.
| Line item | % of total | £ on avg project |
|---|---|---|
| Excavation and spoil removal | 15% | £40,125 |
| Underpinning | 18% | £48,150 |
| Structural slab and walls | 12% | £32,100 |
| Waterproofing | 12% | £32,100 |
| Ventilation and MEP | 10% | £26,750 |
| Light wells and glazing | 6% | £16,050 |
| Internal fit-out | 8% | £21,400 |
| Fees and approvals | 9% | £24,075 |
| Site management and plant | 6% | £16,050 |
| Margin and contingency | 4% | £10,700 |
Real projects
Worked examples from completed builds.
London zones
Where your borough sits in the price band.
| Zone | Multiplier | Notes |
|---|---|---|
| Zone 1 (W1, SW1, NW1, EC, WC) | 1.20–1.30× | K&C/Westminster basement policies restrict depth; access surcharges severe |
| Zone 2 (NW3, SW3, SW10, N1) | 1.10–1.18× | Premium areas, conservation-heavy |
| Zone 3 (SW6, W6, NW5, N16) | 1.00–1.08× | Baseline London basement pricing |
| Zone 4–5 (Outer London) | 0.90–1.00× | Easier access; clay sub-soil still demands underpinning |
Builderr vs other London builders.
The construction industry has a wide distribution of operators. Here's what changes between a directly-employed, fixed-scope outfit and the alternatives.
| Criterion | Builderr | Typical London builder | Cowboy outfit |
|---|---|---|---|
| Labour model | Directly employed team (PAYE) | Mixed subcontract gangs | Day-rate cash labour |
| Pricing | Fixed-scope itemised quote | Estimate + provisional sums | Verbal price + variations |
| Design & engineering | In-house architect + SE | Outsourced, separate billing | Builder draws on the back of an envelope |
| Planning + LDC handled | Yes — included in price | Often charged extra | Builder asks you to apply |
| Party wall surveyors | Instructed by us | Your responsibility | Skipped (illegal) |
| Building control | Plans + site inspections booked by us | Building Notice route | Not registered |
| Project management | Dedicated PM, weekly photo updates | Foreman doubles up | Owner-manager juggles 5 jobs |
| Payment schedule | Stage payments against signed-off milestones | Weekly invoices | Cash up front |
| Insurance | £10M PL + 10yr structural warranty | £2–5M PL only | No documented cover |
| Snags at handover | <3 typical | 20–30 typical | Walk-off mid-job common |
| Variation creep | 0% — fixed scope | +15–25% over original quote | +40%+ regularly |
Save £53,500–£120,375 on a full house renovations.
Industry data (FMB, RICS, Which? Trusted Trader 2024) shows the average London construction project overruns by 18–22% on cost and 25–35% on time. Fixed-scope contracts with a single accountable team eliminate that variance. The savings above assume a typical project at £267,500.
Related
See the full full house renovations service page.
FAQ
Basement Conversion Cost in London:
common questions.
Is a basement conversion worth it in London?
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On a London property in zones 1–3 where a basement adds a habitable floor, the ROI is typically positive — a £250,000 basement conversion on a £1.5m property adds £350,000–£500,000 in value, recovering the cost and adding £100,000+. In outer London (zones 4–6) where house prices are lower, basements rarely return their cost. The other reason to build: lifestyle (gym, cinema, music room) when moving is not an option.
How long does a basement conversion take?
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10–36 weeks on site depending on scope. Cellar refurbishment 10–14 weeks. Full basement excavation under existing house 22–28 weeks. Basement extension with light wells 28–36 weeks. Pre-construction adds 16–28 weeks for design, basement impact assessment, planning, building regs and party wall — these projects almost always need full planning.
Do I need planning permission for a basement?
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Almost always yes. Most London boroughs require planning for any new basement creating additional habitable floor area. Some boroughs (K&C, Camden, Westminster, Islington) have specific basement supplementary planning documents (SPDs) restricting depth (typically no deeper than one storey below the existing ground floor), restricting footprint (typically no extension beyond the original house footprint without exceptional justification) and requiring a Basement Impact Assessment (BIA) on every application.
What is the cheapest type of basement conversion?
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Refurbishing an existing cellar with near-full headroom is by far the cheapest route — £35,000–£70,000 for a 15–20m² home office or utility, no excavation needed, just waterproofing, insulation, electrics and finish. Many London Victorian properties have an existing cellar or coal-vault under the front pavement that can be opened up.
What's the difference between Type A, Type B and Type C waterproofing?
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Type A (tanking): a cementitious or membrane barrier applied to walls and floor to physically block water — relies on continuity of the barrier, vulnerable to defects. Type B (integral): the concrete structure itself is the waterproof barrier, achieved via dense concrete and crack control. Used in new-build, rarely retrofit. Type C (cavity drain): an internal cavity behind plasterboard collects any water that penetrates the structure and channels it to a sump and pump system. Most reliable in London's clay/water conditions, often combined with Type A for redundancy.
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