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Basement Extension in London

Basement extension in London costs £200,000–£500,000+ for typical 50–75m² across house and garden, taking 28–36 weeks on site. Combines full basement excavation under house with sub-garden extension and light wells. Enables specialist fit-outs (cinema, gym, pool, spa). Restricted by borough basement policies — K&C limits sub-garden extension to 50% of rear garden depth.

Add basement floor area beyond the existing house footprint — sub-garden excavation with light wells for cinema, gym, spa.

Typical cost
£200k–£500k
Timeline
28–36 wks
Get fixed-price quoteSee cost table
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Indicative range
£45,000 – £120,000
8–14 weeks on site

Overview

Basement Extension explained.

A basement extension adds new floor area below the existing house AND beyond the original footprint — typically extending under the rear garden, creating substantial new space for cinema, gym, spa, swimming pool, wine cellar or guest suite. It is the most ambitious and expensive basement build type. Planning approval is restrictive in most central London boroughs but achievable with the right design. Builderr delivers basement extensions in London at £200,000–£500,000+ over 28–36 weeks.

  • Combined basement-under-house and sub-garden extension
  • Multiple light wells with glazed roofs
  • Pool, sauna, cinema fit-outs available
  • Full perimeter underpinning to existing structure
  • Sub-garden structural slab and retaining walls
  • Comprehensive Basement Impact Assessment
  • Construction Management Plan and neighbour engagement

Cost table

Basement Extension costs in London 2026.

ConfigurationCost rangeTimeline
Standard basement extension, 50–65m² across house + garden£230,000–£320,00028–32 wks
Premium basement extension with light wells, cinema, gym£320,000–£420,00030–36 wks
Basement extension with pool or spa£380,000–£500,00032–40 wks
Inner London K&C/Westminster ultra-premium spec£450,000–£800,00036–48 wks
Why us

Direct labour, fixed scope, one accountable team.

We employ our carpenters, plumbers, electricians and decorators directly. No subcontracted gangs, no day-rate creep, no finger-pointing when something goes wrong. The same people you meet at survey are on site every week until handover.

10M
Public liability
10yr
Structural warranty
1hr
Callback target
<3
Snags at handover
01

What a basement extension delivers

A basement extension creates floor area in two zones. Zone A: under the existing house. Created by full basement excavation (see our full-excavation page). Zone B: beyond the existing house, typically under the rear garden. Created by new excavation, sub-garden retaining walls and a reinforced concrete slab with light wells breaking through at strategic points. Combined, this gives a typical 50–75m² basement floor on a London terrace — enough for a cinema (20m²), gym (15m²), spa room (10m²), utility (10m²) and circulation. On larger detached or semi-detached properties, basement extensions of 100m²+ are possible if planning allows. Sub-garden extension typically requires the new structure to be backfilled with engineered fill to support garden re-instatement, including any garden landscaping or tree planting overburden.

02

Planning constraints in inner London

Sub-garden basement extension is the most planning-restricted basement type. K&C policy explicitly limits basements to 50% of the rear garden depth (e.g. a 12m garden allows 6m of basement extension) and bans excavation under front gardens. Camden, Westminster and Hammersmith & Fulham have similar restrictions. Islington and Hackney are slightly more permissive. The Basement Impact Assessment is more demanding for extensions: structural impact on adjoining properties, hydrology (water-table changes, drainage), tree retention (TPO and conservation area trees), biodiversity and the construction methodology must all be assessed in detail. Light well design is critical — most boroughs require light wells to be discreet, with glazed roofs flush with garden surface and minimal visual impact. Some boroughs (K&C in particular) restrict light wells in front gardens entirely.

03

Specialist fit-outs: cinema, pool, gym

Basement extensions enable specialist fit-outs that are difficult above ground. Cinema room: typical 20–28m², acoustic-isolated construction (separate stud walls with mineral wool, double plasterboard), blackout-rated lighting, 4K projection, 5.1 or 7.1 surround sound, raised seating tier. £35,000–£90,000 above the core build cost. Pool: typical 3m×8m endless-pool style or 5m×12m lap pool, requires steel-reinforced concrete pool tank, integrated dehumidification, heat recovery, automated cover. £80,000–£220,000 above core. Gym: typical 15–25m², heavy floor reinforcement for plate-load equipment, full mirror wall, acoustic isolation, MVHR with high turnover. £20,000–£50,000 above core. Spa/sauna: 8–15m², waterproofed timber-clad sauna room, plumbing and steam generator, dehumidification. £25,000–£70,000 above core. Wine cellar: 8–15m², independent climate control (12-14°C, 65% humidity), oak racks, security. £18,000–£50,000 above core.

Recent basement extension work

Built across London.

Basement living space below garden
Light well to basement
Basement cinema room
Sub-garden basement extension

FAQ

Basement Extension: common questions.

How much does a basement extension cost in London?+

A standard 50–65m² basement extension across house and garden costs £230,000–£320,000 in 2025. Premium spec with light wells, cinema and gym £320,000–£420,000. Adding a pool pushes total to £380,000–£500,000+. Inner London K&C/Westminster ultra-premium spec can exceed £800,000.

Can I extend a basement under my garden in London?+

In most London boroughs, yes — though restrictions apply. K&C limits extension to 50% of rear garden depth; other boroughs have similar policies. Front-garden basements are heavily restricted everywhere. Full planning permission and Basement Impact Assessment are mandatory. The viability of sub-garden extension is borough-specific and we assess at the initial consultation.

Do basement extensions add value?+

On zone 1–2 properties, yes — a £350,000 basement extension typically adds £500,000–£800,000 in value, often more than recovering cost. The largest value uplift comes from specialist amenities (pool, cinema, gym) that buyers in the £3m+ bracket expect. Outer London zone 4+ basement extensions rarely return their cost.

How long does a basement extension take?+

28–36 weeks on site for a standard basement extension; longer for pool installations (32–40 weeks). Add 28–40 weeks pre-construction for design, BIA, planning, building regs, party wall — basement extensions are the longest-running London residential builds. Total project: 60–80 weeks from first consultation to handover.

What's the difference between a basement and a basement extension?+

A 'basement' (full excavation) creates new floor space under the existing house only. A 'basement extension' goes further — it adds floor space beyond the original house footprint, typically extending under the rear garden, with light wells at the sub-garden zone. Basement extensions are more ambitious, more planning-restricted and more expensive than basements.

Basement Extension by borough

Local basement extension across all 33 London boroughs.

Basement Extension in Barking and DagenhamBasement Extension in BarnetBasement Extension in BexleyBasement Extension in BrentBasement Extension in BromleyBasement Extension in CamdenBasement Extension in City of LondonBasement Extension in CroydonBasement Extension in EalingBasement Extension in EnfieldBasement Extension in GreenwichBasement Extension in HackneyBasement Extension in Hammersmith and FulhamBasement Extension in HaringeyBasement Extension in HarrowBasement Extension in HaveringBasement Extension in HillingdonBasement Extension in HounslowBasement Extension in IslingtonBasement Extension in Kensington and ChelseaBasement Extension in Kingston upon ThamesBasement Extension in LambethBasement Extension in LewishamBasement Extension in MertonBasement Extension in NewhamBasement Extension in RedbridgeBasement Extension in Richmond upon ThamesBasement Extension in SouthwarkBasement Extension in SuttonBasement Extension in Tower HamletsBasement Extension in Waltham ForestBasement Extension in WandsworthBasement Extension in Westminster
Compare

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.

Builderr fixed price
£350,000
a basement extension · no provisional sums
Typical builder + variations
£420,000
+£70,000 vs Builderr (≈20% overrun)
Cowboy outfit + cost creep
£507,500
+£157,500 vs Builderr (≈45% overrun)
CriterionBuilderrTypical London builderCowboy outfit
Labour modelDirectly employed team (PAYE)Mixed subcontract gangsDay-rate cash labour
PricingFixed-scope itemised quoteEstimate + provisional sumsVerbal price + variations
Design & engineeringIn-house architect + SEOutsourced, separate billingBuilder draws on the back of an envelope
Planning + LDC handledYes — included in priceOften charged extraBuilder asks you to apply
Party wall surveyorsInstructed by usYour responsibilitySkipped (illegal)
Building controlPlans + site inspections booked by usBuilding Notice routeNot registered
Project managementDedicated PM, weekly photo updatesForeman doubles upOwner-manager juggles 5 jobs
Payment scheduleStage payments against signed-off milestonesWeekly invoicesCash up front
Insurance£10M PL + 10yr structural warranty£2–5M PL onlyNo documented cover
Snags at handover<3 typical20–30 typicalWalk-off mid-job common
Variation creep0% — fixed scope+15–25% over original quote+40%+ regularly
Bottom line

Save £70,000–£157,500 on a basement extension.

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 £350,000.

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Other types

Explore other full house renovations.

Load-Bearing Wall Removal

Open up your ground floor — steel beam install, structural calcs, building regs and party wall, fixed-scope.

£7k–£18k · 2–4 wks
RSJ Installation

Steel beam supply and install for openings, extensions and structural alterations — engineer-signed, building-regs certified.

£5k–£14k · 1–3 wks
Underpinning

Strengthen existing foundations — for subsidence repair, basement excavation or load increase. Engineer-led, building-regs compliant.

£1k–£3k · 3–10 wks
Cellar Conversion

Convert your existing under-house cellar into a habitable home office, utility, gym or guest room — no excavation needed.

£35k–£90k · 8–14 wks
Full Basement Excavation

Create a new basement under your existing house — full excavation, underpinning, waterproofing and habitable fit-out.

£150k–£350k · 22–28 wks