Question 1

How much does a house extension cost per square metre in London?

Accepted Answer

In Greater London and the South East, a single-storey extension costs approximately £2,200-£2,800/m2 for the shell (structure, roof, windows, external walls) and £3,000-£3,800/m2 fully finished including internal fit-out. A double-storey extension costs £1,800-£2,400/m2 because the roof and foundations are shared across two floors. These are 2026 prices and include preliminaries, scaffolding and building control but exclude kitchen/bathroom units (client-supplied second-fix items).

Question 2

Do I need planning permission for a house extension?

Accepted Answer

Many single-storey rear extensions are covered by Permitted Development (PD) under the Neighbour Consultation Scheme (Prior Approval): up to 6m depth on terraced/semi or 8m on detached properties. PD does not apply to: flats, maisonettes, listed buildings, conservation areas (without Article 4 Direction review), or properties where PD rights have been removed. Two-storey extensions always need full planning permission. Side extensions under PD must be single-storey, no wider than half the house, and set back from the front elevation. A Lawful Development Certificate (LDC) costs about £103 and is strongly recommended.

Question 3

How long does a house extension take to build?

Accepted Answer

A single-storey rear extension (20-30m2) takes 10-14 weeks on site. A wrap-around extension takes 12-16 weeks. A double-storey extension takes 14-20 weeks. Add 6-12 weeks for pre-construction: architectural drawings (2-4 weeks), structural engineering (1-2 weeks), planning application if needed (8-13 weeks for a decision), Building Control application (1-2 weeks) and party wall notices (2 months if triggered). The total programme from first consultation to handover is typically 6-9 months.

Question 4

What is a side return extension?

Accepted Answer

A side return extension fills in the narrow alley (the "side return") that runs along the side of many Victorian and Edwardian terraced houses in London. It is typically 1-2m wide and extends the kitchen or living area sideways, creating an open-plan space. Side return extensions are very cost-effective (£30,000-£60,000 depending on length and finish) because you are building between existing walls. Most are covered by Permitted Development. They often feature a glazed roof lantern or long skylight to bring natural light into the centre of the plan.

Question 5

What foundations does an extension need?

Accepted Answer

Standard strip foundations (typically 600mm wide, 1m deep in London clay) are the most common. If trees are nearby (especially oaks, willows or poplars), the Building Control officer may require deeper trenchfill foundations (up to 2.5m on London clay) or piled foundations. On shrinkable clay soils, NHBC guidelines Table 4.2 determines depth based on tree species, distance and soil plasticity index. Piled foundations (mini-piles) typically cost £1,500-£2,500 per pile and are required when standard depths are impractical. A ground investigation (trial pit or bore hole) costs £500-£1,000 and may be required by Building Control.

Question 6

How much does a wrap-around extension cost?

Accepted Answer

A wrap-around extension (combining rear and side return into one L-shaped or U-shaped structure) typically costs £70,000-£130,000 in Greater London, depending on size, ground conditions and internal specification. It usually adds 25-40m2 of floor space. Wrap-arounds are popular on semi-detached and end-of-terrace properties where there is access down both sides. Most require full planning permission because the side element exceeds PD limits.

Question 7

What is the maximum size of an extension under Permitted Development?

Accepted Answer

Under the current PD rules (2024 update, still in effect 2026): single-storey rear extension max depth is 6m (terraced/semi) or 8m (detached) under the Neighbour Consultation Scheme. Maximum eaves height 3m, maximum overall height 4m (if within 2m of boundary) or same as the existing house. Single-storey side extension max width is half the original house width, max eaves height 3m. Total additions (including any previous extensions) must not exceed 50% of the curtilage of the original house. No extension is permitted forward of the principal elevation.

Question 8

Do I need Building Regulations approval for an extension?

Accepted Answer

Yes - every extension requires Building Regulations approval regardless of whether planning permission is needed. Building Regulations cover structure (Part A), fire safety (Part B), energy efficiency (Part L), ventilation (Part F), drainage (Part H), electrical safety (Part P) and accessibility (Part M). RCB submits a Full Plans application at the start of every project. The Building Control officer inspects at key stages: foundations, damp-proof course, drainage, structural steels, first-fix, insulation and completion. Fees are typically £600-£1,500.

Question 9

How does an extension affect my council tax band?

Accepted Answer

Adding an extension does not automatically trigger a council tax revaluation. The Valuation Office Agency (VOA) only reassesses your band when there is a change of ownership (sale) or if you request it. In practice, most homeowners are not revalued after extending. If reassessed, a single-storey rear extension on a mid-band property rarely causes a band increase. A large double-storey extension that significantly increases the property value could push the band up. You can check your band and challenge it on the GOV.UK website.

Question 10

What is the Party Wall Act and does it apply to extensions?

Accepted Answer

The Party Wall etc. Act 1996 applies if you are: (a) building on or at the boundary line, (b) cutting into a party wall or party fence wall, or (c) excavating within 3m of a neighbouring structure (or 6m if the excavation goes below the bottom of their foundations). Most rear and side extensions trigger at least one of these conditions. You must serve a Party Wall Notice at least 2 months before starting work. If your neighbour does not consent within 14 days, surveyors are appointed. Allow £900-£2,000 per neighbour in surveyor fees, paid by the building owner.

Question 11

Can I extend my house if I live in a conservation area?

Accepted Answer

Yes, but your Permitted Development rights are more restricted in a conservation area. Rear extensions may still be allowed under PD but side extensions, cladding changes and some roof alterations are typically removed. You will likely need full planning permission, which considers the impact on the character and appearance of the conservation area. Materials must usually match the existing building. Pre-application advice from the council (£50-£300) is strongly recommended before committing to design fees.

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