Basement Conversion Cost London 2025

The cost of a house extension in London varies widely depending on location, specification, complexity of the build, site conditions, and the type of extension you are considering. Generally speaking, you can expect the following ballpark figures:

• Basic rear or side extension (single-storey): £2,900 – £3,500 per m²

• High-spec rear or side extension: £3,500 – £5,500+ per m²

• Double-storey extension: Often starts from £3,500 per m² and can exceed £4,500 per m² depending on finishes

• Basement extension: £4,700 – £6,500+ per m², as basements require extensive structural works, waterproofing, and drainage

• Wrap-around extension (rear + side return): Typically £90,000 – £160,000+ depending on size and design

Other costs to factor in:

• Professional fees (architect, structural engineer, party wall surveyor, planning consultant, etc.): typically 10-15% of the build cost.

• Planning application and building regulations fees: £500 – £3,000 depending on the complexity.

• VAT: Most house extensions attract 20% VAT, unless they qualify for reduced rates (such as certain conversions).

It’s also important to budget for contingencies — typically at least 10% of the project cost — as unforeseen issues can arise, especially in older buildings or in dense urban locations.

Whether or not you need planning permission depends on the specifics of your proposed extension and the constraints of your property:

• Permitted Development (PD) rights: Many single-storey rear and side extensions can be carried out without full planning permission under PD rules. These rules set limits on height, depth, and proximity to boundaries. For example, under PD:
  

  • You can extend the rear of a detached house by up to 8 metres (single-storey) or 3 metres (double-storey), subject to neighbour consultation.

  • Side extensions must be single-storey, no more than 4 metres high and no wider than half the width of the original house.

  • Materials must be similar to the existing property.

• Full planning permission: Required if:
  

  • Your extension exceeds PD limits.

  • Your property is in a conservation area or is a listed building (where PD rights are restricted or removed).

  • You want to alter the front of your property or the roof profile significantly.

  • Your proposal impacts protected trees or the local character.

Planning permission applications typically take 8-10 weeks for a decision. Engaging an architect or planning consultant early can help ensure the best chance of success.

Yes, planning policies and the interpretation of national rules can differ from one borough to another. Some key points to note:

• Inner London boroughs (e.g. Islington, Camden, Westminster): Often stricter controls, especially in conservation areas or where heritage is a concern. Boroughs may have detailed design guides that set out what is acceptable.

• Outer London boroughs (e.g. Bromley, Croydon, Barnet): Tend to offer more flexibility, particularly for rear and side extensions.

• Neighbour consultations: Most councils will notify neighbours of your application, and their feedback could influence whether permission is granted.

Each council publishes its local planning policies and design guides. It’s important to check these or seek professional advice to ensure your proposal aligns with local requirements.

Additionally, fees and processing times for applications and approvals may vary slightly between boroughs.

London properties come in a wide range of styles — from Georgian and Victorian terraces to 1930s semis and modern houses — and several types of extensions are popular:

• Rear extensions: Extend out into the garden, often creating open-plan kitchen/diner spaces. This is one of the most common ways London homeowners add space.

• Side return extensions: Fill in the narrow side alley of Victorian and Edwardian terraces, transforming cramped kitchens into spacious living areas.

• Wrap-around extensions: Combine a rear and side return extension, maximising ground floor space.

• Double-storey extensions: Add space on two levels, often for an additional bedroom and a larger kitchen or living area.

• Loft conversions: Convert unused roof space into habitable rooms — commonly combined with a rear extension to reconfigure the whole home.

• Basement extensions: Particularly in prime areas like Chelsea or Hampstead where space is at a premium. These are complex, costly and often require planning permission and specialist contractors.

The type of extension you choose will depend on your property type, your budget, and how much space you want to gain.

Timelines depend on the type and complexity of the extension, as well as on the planning process:

• Design and approvals phase: 2-4 months (including surveys, design development, planning applications, party wall agreements)

• Single-storey rear or side extension: 12–20 weeks of construction

• Double-storey extension: 20–30 weeks

• Basement extension: 30–50 weeks depending on complexity

• Loft conversion: 8–12 weeks (if done independently of other works)

You should also allow extra time for:

• Lead-in period: For contractor procurement, ordering materials, and mobilisation.

• Weather or unforeseen delays: Especially for complex builds or where there are supply chain issues.

A well-planned project with experienced professionals will help reduce risks of delay.

For most house extensions in London:

• An architect or architectural designer will be key to developing a design that meets your needs, maximises space, and secures planning permission where required. They can also help manage approvals and oversee works.

• A structural engineer is essential for structural calculations, particularly where steelwork is required (e.g. opening up walls, supporting upper floors, or creating large glazed openings).

• Other consultants may include party wall surveyors (if your works affect shared walls or boundaries) and drainage consultants (especially for basements).

Some design-and-build firms offer these services in-house, which can help streamline the process.

In addition to planning permission, you may need: Building Regulations approval for structural work, drainage changes, and extensions. A Party Wall Agreement if your work affects a shared wall. Freeholder consent for leasehold properties. Highways licences for skips or scaffolding on the street. Listed building consent for listed properties. Securing these approvals in advance is crucial to avoid enforcement action or costly remedial work.

Most house extension works in London are subject to 20% VAT, including materials and labour. However, a reduced rate of 5% VAT may apply if: Converting a building from commercial to residential. Working on empty properties (vacant for 2+ years). New-build residential houses are generally zero-rated for VAT. Confirm VAT rates with your contractor or a tax specialist, especially for complex projects.

A well-designed and well-executed house extension can significantly increase the value of your home, particularly in high-demand areas of London. Typical benefits include:

• Increased living space, making the home more attractive to buyers.

• Modernised layouts that suit contemporary living.

• Enhanced energy efficiency if combined with fabric upgrades (e.g. better insulation, glazing).

However, the value added depends on local market conditions, the quality of the work, and whether the space meets buyer demand in your area. Over-extending relative to plot size or the local market can lead to diminishing returns.

• Experience: Ensure your contractor has experience with house extensions in London, as local challenges (tight sites, planning rules, heritage issues) require expertise.

• References and reviews: Ask to see completed projects and speak to past clients.

• Contracts: Use formal contracts (e.g. JCT) and ensure the contractor understands CDM regulations (health and safety management).

• Insurance: Verify that the contractor has appropriate insurance (public liability, employer’s liability, contractor’s all-risk).

Choosing the right team is key to a smooth, successful extension project.

If you are creating a new basement or extending below your property, planning permission is normally required — especially in conservation areas or for listed buildings.

If you are only refurbishing an existing cellar with no structural changes, it may fall under permitted development — but always check with your borough.

Costs vary by location and complexity:

• Light refurbishment of an existing cellar: from £1,900 per sqm

• New retrofit basement under existing house: £3,700–£7,000 per sqm

• High-spec basement with full fit-out: £5,000–£9,500+ per sqm
  

Typical timelines:

• Small cellar conversion: 3–4 months

• Full retrofit basement: 6–9 months

• Large / complex basements: 9–12 months Each project is different — party wall process and planning permissions can also affect timelines.

Underpinning extends the foundations of the house downwards — making it structurally safe to excavate a basement.

In London, most Victorian and Edwardian properties will require mass concrete underpinning or piled underpinning to support the structure.

Most basements use a Type C drained cavity system (Delta Membranes or similar) with sump pumps. This is the most reliable system in London’s clay soils and high water table zones. The waterproofing system should include a guarantee — and maintenance is essential.

Yes — if your works affect the foundations of a shared wall or go deeper than the neighbour’s footings, a Party Wall Award is legally required.

Party wall surveyor fees (2025) are approx £1,500–£3,000 per neighbour.

Yes — common extras include:

• Spoil removal (can be expensive in central London)

• Parking suspensions

• Party wall awards

• Thames Water build-over agreement

• Temporary works design

Typically £200,000 to £800,000+ — depending on area, finish quality, and how well it integrates with the main house. Basements in prime boroughs (Chelsea, Islington, Hampstead) can add even more.