Basement Extension London 2026: Costs, Planning & Complete Guide
A basement extension is the most complex and expensive project you can undertake on a London home. It is also the only option when you cannot go up, out, or sideways. Here are the real 2026 costs, what planning requires, and what makes or breaks a basement project.
Quick Answer
Basement extensions cost £30k–£520k+ inc VAT in London depending on type. Cellar conversion £30k–£90k. New basement under existing house: £180k–£380k for 40–60m². Planning permission is required in almost all cases. Party wall agreements with 3–6 neighbours are typical.
£30k–£90k
Cellar conversion
£180k–£260k
New basement (40m²)
12–20 months
Timeline
Check your specific property constraints
Free Property CheckTypes of Basement Project
The type of project you have determines cost, complexity, and planning requirements more than anything else. There are four distinct types:
1. Cellar conversion (existing basement)
The structure already exists. Work involves waterproofing, lowering the floor slab (if head height is insufficient), insulation, services, and fit-out. Substantially cheaper than a new excavation. Common in Victorian and Edwardian London terraces that have existing cellars.
Cost: £30,000–£90,000 inc VAT depending on floor lowering, waterproofing system, and fit-out spec
2. New basement under existing house
Full excavation beneath the existing building footprint. Requires underpinning the existing foundations, forming new reinforced concrete walls and slab, waterproofing, drainage, light wells, and fit-out. The most common basement project in London. The most complex and expensive.
Cost: £180,000–£520,000+ inc VAT depending on size and spec
3. Basement with rear garden extension
Extends the basement under the rear garden as well as the house footprint. Creates the most floor area. Involves more excavation, additional waterproofing area, and a more complex light well or landscaped roof detail. Common in high-value inner London properties.
Cost: £250,000–£650,000+ inc VAT depending on combined footprint and spec
4. Two-level basement
Extremely rare and restricted by most London borough policies. Requires very deep excavation, complex engineering, and extensive party wall engagement. Only found in high-value prime central London (Belgravia, Knightsbridge, Kensington) where development budgets allow for it. Westminster and RBKC have effectively prohibited new two-level basements.
Cost: £500,000–£1,500,000+ inc VAT. Not achievable in most boroughs.
2026 Costs: New Basement Under Existing House
All figures below include VAT at 20%. These are for a new single-storey basement excavated beneath an existing London house.
| Basement size | Structure only | All-in (basic fit-out) | All-in (high spec) |
|---|---|---|---|
| Small (25–35m²) | £130k–£185k | £160k–£230k | £220k–£340k |
| Medium (40–55m²) | £185k–£265k | £230k–£340k | £320k–£500k |
| Large (60–80m²) | £265k–£380k | £330k–£490k | £460k–£700k+ |
Cost per m² in London 2026
All figures inc VAT. Cost per m² is higher for smaller basements due to fixed costs (underpinning length, drainage, light wells). Inner London (zones 1–2) typically runs 15–25% above outer London rates.
Where the cost goes: itemised breakdown
Cellar conversion costs (2026)
Where an existing cellar or semi-basement already exists, costs are substantially lower because the structure is already in place:
Minimum habitable head height is 2.2m. Many Victorian cellars are only 1.6–1.8m to the underside of the floor joists above. Floor lowering involves breaking up and re-casting the slab at a lower level, which requires mini-piled or concrete underpinning of the existing walls.
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Planning Permission in London 2026
Planning permission is required for almost all basement extensions in London. The only exception is a purely internal conversion of an existing cellar with no external changes whatsoever (no new light wells, no new access). The moment you add a light well or external staircase, planning permission is needed.
Borough policies on basements
After the basement boom of the 2000s and 2010s, most London boroughs introduced specific basement development policies. These go significantly beyond standard planning considerations:
| Borough | Key restrictions |
|---|---|
| Westminster | Single storey only. Maximum 50% under garden. Structural engineer report and monitoring plan required. Strong neighbour consultation. |
| Kensington & Chelsea | Single storey only. Maximum 85% of existing footprint. No extension beyond property boundary. Very strict neighbour impact assessment. |
| Islington | Basement development policy requires independent structural assessment. No two-storey basements. Flooding risk assessment required in some areas. |
| Wandsworth | Detailed construction management plan required. Neighbour monitoring during excavation. Single level maximum in most areas. |
| Camden | Conservation area restrictions apply throughout much of the borough. Structural and drainage impact assessments required. |
| Other boroughs | Most have adopted basement policies. Check your borough's Local Plan and Supplementary Planning Documents before instructing an architect. |
What planning looks at
- Impact on structural stability of neighbouring properties and the street
- Flood risk and surface water drainage
- Visual impact of light wells, external stairs, and garden landscaping changes
- Construction management: traffic, noise, vibration, and working hours
- Archaeological potential (particularly in older parts of inner London)
Planning applications for basements typically take 10–16 weeks. The application fee is £528 for a standard householder application. Many boroughs require additional technical reports (structural, drainage, arboricultural if tree roots are involved) adding £3,000–£8,000 to pre-application costs.
Our AI checks planning constraints for your specific address including basement policies, conservation area status, and flood risk zone.
Check my property — it is free →Party Wall: The Most Critical Process
Party wall is the single most common cause of basement project delays in London. It is also the area where disputes are most likely. Get this right from the start.
What triggers it for a basement
- Section 6 notice: any excavation within 6m of a neighbouring structure that goes deeper than the bottom of that structure's foundations. This captures almost every basement excavation in a terraced or semi-detached London house.
- Section 2 notice: underpinning a party wall, cutting into it, or placing beams into it. Required when underpinning the shared wall between your property and your neighbour's.
- Section 1 notice: building a new wall on or at the boundary. Relevant when forming the new external basement wall along a shared boundary.
How many neighbours in a London terrace
A typical London mid-terrace has two immediate neighbours (left and right). But due to the 6m Section 6 radius, neighbours at the rear and even diagonal neighbours may also be affected. Most basement projects in terraced houses serve party wall notices on 3–5 neighbours.
At £1,500–£2,500 per neighbour (you pay your surveyor and their surveyor), total party wall costs of £6,000–£15,000 are typical. In contentious cases with disputes, this can reach £20,000–£40,000.
Party wall timeline for a basement
- 1Serve all notices at least 2 months before work starts (Section 2 and 6 notices)
- 2Neighbours have 14 days to consent or dissent. Most dissent for basements (it is a major disruption)
- 3Surveyors are appointed for each dissenting neighbour. A Schedule of Condition is prepared (documents the state of neighbouring properties before work starts)
- 4Party Wall Awards are agreed. For basements these are detailed documents specifying underpinning methodology, monitoring requirements, and working restrictions
- 5Monitoring during construction: most basement Awards require structural monitoring of neighbouring properties throughout excavation. Survey pins are fixed and readings taken regularly.
Realistic timeline from notice to Award: 3–5 months. Start the party wall process immediately after planning permission is granted. Do not wait until you have a start date.
Structure: Excavation and Underpinning
Ground conditions in London
London's ground conditions are not uniform. What your excavation encounters determines cost and method:
Rubble, Victorian fill, old foundations. Variable. Can contain contamination on former industrial sites.
Stiff, over-consolidated clay. Expensive to excavate and remove. Sensitive to moisture change (swells when wet, shrinks when dry). This shrinkage and swelling causes cracking in neighbouring properties and is the main reason for monitoring requirements.
Found in the Thames flood plain (south London, east London, riverside areas). Easier to excavate but water-bearing. Requires dewatering and more complex waterproofing.
Underpinning methods
Traditional mass concrete underpinning (sequential bays)
The existing foundations are undercut in short sections (bays), concrete is poured, and the next bay started after the concrete has cured. The most common method. Cheapest but most disruptive and time-consuming.
Typical cost: included within RC structure costs
Mini-piled underpinning
Small diameter piles (150–300mm) are drilled through the existing foundations and into load-bearing strata below. Less disruptive to neighbouring structures. Used where traditional underpinning is not viable due to space constraints or where engineers require a more controlled approach.
Additional cost: £15,000–£40,000 over traditional underpinning
Top-down construction
The ground floor slab is cast first, then excavation proceeds beneath it. The slab acts as a structural prop during digging. Used for very deep or complex basements. Rare in domestic projects. Most common in large commercial basements or very deep residential projects.
Significantly more expensive. Only specified where required by engineer.
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Waterproofing: Getting It Right
Waterproofing is the most critical element of a basement. A failure costs more to fix than it did to install correctly. The standard that governs all UK basement waterproofing is BS 8102:2022.
BS 8102 specifies three types of protection:
Type A (barrier protection / tanking)
Waterproof membrane applied to the outside of the structure (external tanking) or inside (internal tanking). Prevents water entering the structure. If the membrane is breached, water infiltrates. Suitable for low water table conditions. External tanking is preferred but can only be applied before backfilling.
Type B (structurally integral waterproofing)
Watertight reinforced concrete using low water/cement ratio and crystalline admixtures. The structure itself resists water. Used in most modern London basements as the primary waterproofing system. Self-healing in some formulations (Xypex, Kryton etc.). Relies on good concrete pour and vibration.
Type C (drained cavity system)
A dimple membrane on the internal walls and floor creates a cavity that collects any water and channels it to a sump pump. The most resilient system: even if the structure cracks and water enters, it is collected and pumped away before it reaches the habitable surface. The sump pump requires electricity and regular maintenance. Most London basement specialists recommend Type B + Type C in combination.
What most London basements use
Type B (waterproof RC) as the primary structure, plus Type C (cavity drain with sump) as the secondary system. This redundancy means if the concrete ever cracks (from ground movement, settlement, or shrinkage) the cavity drain catches any ingress. The sump pump runs automatically and most systems have an alarm. Cost: £12,000–£25,000 for the combined system on a 40–60m² basement.
Natural Light and Ventilation
A basement without natural light feels oppressive and adds less value than one that feels like a proper room. Natural light is the most important design decision in a basement.
Rear light well
An excavated area in the rear garden with a glazed screen facing the basement. Brings in south or west light for rear basements. Must be waterproofed, drained, and guarded. Popular in inner London. Cost: £4,000–£9,000 each. Needs planning permission.
Front light well / pavement vault
Light well at the front of the property under the front garden or pavement. In terraces where the footway is council-owned, a licence from the local highway authority is needed. Common in central London Victorian terraces. Brings north light but light nonetheless. Cost: £5,000–£12,000 including highway licence.
Sunken courtyard
Where the rear extension extends under the garden, a sunken courtyard can be formed in the garden above the basement roof. The most generous source of natural light and gives a direct outdoor connection. Requires more garden loss above ground. Adds significantly to the value and usability of the basement. Cost: included in the RC structure; additional landscaping £8,000–£25,000.
Rooflights over basement stairs / void
A rooflight at ground floor level over the basement staircase or a double-height void can bring light down from above without a light well. Less effective than a direct opening but useful where external excavation is restricted.
Building regulations (Part F) require mechanical ventilation in basement spaces regardless of natural light provision. A heat recovery ventilation (MVHR) system is the standard solution and is included in most basement fit-outs. Cost: £3,000–£8,000.
Realistic Timeline
Design and planning submission
3–5 monthsArchitect, structural engineer, technical reports (structural, drainage, arboricultural). Planning application preparation and submission.
Planning determination
10–16 weeksStatutory determination period. Many basement applications receive objections from neighbours, which extends the process. Some go to committee.
Party wall process
3–6 monthsServe notices immediately after planning approval. Surveys of neighbouring properties, Award negotiations. Start this in parallel with planning if possible.
Building regulations and contractor procurement
6–10 weeksFull plans submission and approval. Tendering to specialist basement contractors. Typically 3–5 contractors for a London basement.
Construction (new basement)
5–9 months on siteEnabling works, excavation, underpinning (done in bays over many weeks), RC structure, waterproofing, drainage, light wells, first fix services, fit-out.
Total project
14–22 monthsCellar conversion with no planning: 4–6 months. New basement with planning and party wall: 14–18 months. Complex project with disputes or planning delays: up to 22 months.
Common Mistakes
Using a general builder instead of a basement specialist
Basement excavation is specialist work. Underpinning must be done in the correct sequence. Waterproofing requires specific product knowledge and application skills. The consequences of errors (flooding, structural movement, neighbours suing for damage) are severe. Always use a contractor with a portfolio of completed London basement projects and check their professional indemnity insurance.
Underestimating party wall costs and time
Many homeowners budget £2,000–£3,000 for party wall based on extension experience. Basement party wall costs are typically 3–5x higher due to the number of neighbours affected, the complexity of the Award (underpinning methodology, monitoring plan), and the likelihood of disputes. Budget at minimum £8,000–£15,000 and expect 3–5 months for the process.
Not installing adequate natural light
A basement without natural light is used less and adds less value. The cost difference between a basement with a good light well and one without is £5,000–£10,000. The value difference at resale is far larger. Plan light wells into the design from day one, not as an afterthought.
Single waterproofing system only
Relying on Type A tanking alone is not current best practice for London basements. London clay moves with seasonal moisture changes and can crack even good waterproof renders over time. A combination of Type B (waterproof concrete) and Type C (cavity drain) provides redundancy. The additional cost is modest relative to the total project budget.
Not checking Thames Water sewer positions
Many London properties have public sewers running beneath the garden or under the house. Building within 3m of a public sewer requires a build-over or build-near agreement with Thames Water. This can take 8–12 weeks and may require redesigning the structure. Check the Thames Water sewer map as the first step of any basement feasibility assessment.
Frequently Asked Questions
How much does a basement extension cost in London in 2026?
All costs inc VAT. Cellar conversion (existing basement): £30k–£90k. New basement under existing house: small (25–35m²) £160k–£340k all-in, medium (40–55m²) £230k–£500k all-in, large (60–80m²) £330k–£700k+ all-in. Cost per m² for the structural work alone is £3,800–£5,500. These figures include excavation, underpinning, RC structure, waterproofing, drainage, and professional fees but not fit-out to high spec.
Do I need planning permission for a basement extension in London?
Almost always yes. The only exception is a purely internal conversion of an existing cellar with no external changes (no light wells, no new access stairs). Any light well or external opening requires planning permission. Most London boroughs have specific basement development policies with restrictions on depth, footprint, and construction method. Planning takes 10–16 weeks and requires technical reports (structural, drainage) costing £3,000–£8,000.
How much do party wall surveyors cost for a basement?
More than for a standard extension. A basement typically triggers Section 6 notices (excavation within 6m) and Section 2 notices (underpinning party walls). This affects 3–6 neighbours in a typical London terrace. At £1,500–£2,500 per neighbour (you pay both surveyors), total costs of £6,000–£15,000 are typical. In contentious cases with disputes, this can reach £20,000–£40,000. Start the party wall process immediately after planning approval.
How long does a basement extension take in London?
Total project from starting design to moving in: 14–18 months for a straightforward new basement. Add 2–4 months for planning disputes, archaeological investigations, or complex party wall negotiations. Cellar conversions with no planning requirement: 4–6 months. The longest phase is usually getting planning permission (3–5 months including design) and party wall Awards (3–5 months, runs in parallel).
What waterproofing should a London basement have?
The current best practice for London is a combination of Type B (waterproof reinforced concrete) and Type C (cavity drain membrane with sump pump). Type B prevents water entering the structure. Type C collects any water that does get through and pumps it away. This redundancy is important in London clay conditions where seasonal ground movement can crack even good concrete over time. The waterproofing must comply with BS 8102:2022. Budget £12,000–£25,000 for the combined system on a 40–60m² basement.
Will a basement extension add value to my London home?
In inner London prime areas (Kensington, Chelsea, Islington, Hackney, Wandsworth), a well-executed basement with natural light typically adds 15–25% to property value. In outer London, the return is lower and the cost is sometimes not recoverable. The single biggest factor is natural light: a basement with a proper light well or sunken courtyard is worth substantially more than a windowless one. Best uses for value: open-plan kitchen-diner connecting to rear garden, master bedroom suite, home cinema/gym.
Can I convert an existing cellar instead of excavating?
Yes, if the cellar is large enough and has sufficient head height (minimum 2.2m after floor and ceiling treatment). Many Victorian and Edwardian London terraces have partial cellars under the front of the house. If head height is less than 2.2m, floor lowering is required (breaking up the slab and re-casting at a lower level), which adds £15,000–£35,000. Cellar conversions still require building regulations approval and usually waterproofing, but cost substantially less than a new excavation.
What are the risks of a basement extension in London?
Key risks: ground movement in London clay causing cracking in neighbouring properties (manageable with monitoring and proper underpinning); waterproofing failure if single system used or poorly installed; party wall disputes delaying the project; planning refusal (less common with a well-prepared application and experienced architect); unforeseen ground conditions (contamination, archaeology, water-bearing gravels) adding cost. A good structural engineer, experienced basement contractor, and proper party wall process mitigate most of these.
Summary
A basement extension is the most complex and expensive domestic project in London. It is also the only route to significant additional space when planning or site constraints prevent going up, out, or sideways.
The keys to a successful project: a specialist basement contractor with a London track record, a combination waterproofing system (Type B + Type C), good natural light through a properly designed light well, and a realistic party wall budget and timeline (typically £8,000–£15,000 and 3–5 months).
Budget 15–20% contingency on top of the construction estimate. In prime inner London, a well-executed basement consistently adds more value than it costs. In outer London, check comparable sales before committing.
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