House Extension Cost London: The Complete 2026 Guide

If you own a home in London and you're planning an extension, the first question everyone asks is: how much will this cost? The honest answer is that it depends — but that doesn't mean you have to go in blind. This guide gives you real 2026 figures, broken down by extension type, specification level, and London area, so you can plan with confidence.

How Much Does a House Extension Cost in London in 2026?

Cost per square metre by specification level

Architect-designed extensions in London cost between £3,000 and £5,500+ per square metre for the build alone, before professional fees, fit-out, or VAT. That range is wide because specification choices have a huge impact on cost. A straightforward brick-built rear extension with a flat roof sits at the lower end. A full-width glazed extension with a roof lantern, underfloor heating, and premium bi-fold doors sits firmly at the top.

• Basic specification: £3,000–£3,500 per m²

• Mid-range specification: £3,500–£4,500 per m²

• High specification: £4,500–£5,500+ per m²

Why London costs 25–40% more than the UK average

London extensions consistently cost more than equivalent projects elsewhere in the UK. Higher labour rates are the single biggest factor — skilled trades in London command a premium because demand is high and good contractors are busy. Add logistical challenges on terraced properties, the prevalence of Victorian-era homes with unpredictable foundations, and a planning environment that demands more professional input, and the London premium makes complete sense.

What the figures include — and what they don't

The cost per square metre figures cover the structural shell: foundations, frame, walls, roof structure, windows, and external doors, plus basic internal finishes. They do not include fit-out — the kitchen, bathroom, flooring, and decoration that make a space liveable. These additional costs can add £20,000 to £60,000 or more depending on your choices.

Cost by Extension Type

Single storey rear extension

The most common extension in London. A typical 20–30m² single storey rear extension costs £80,000–£150,000 all-in, including professional fees and VAT at mid-range specification. Smaller basic-spec projects can come in below this; larger or highly specified projects regularly exceed it.

Side return extension

Side return extensions fill the narrow gap beside Victorian and Edwardian terraces to create a wider kitchen-diner. They cost similarly to rear extensions — £75,000–£140,000 — but often involve more structural work, including removal of an external wall and infilling an existing void.

Double storey extension

A double storey adds space across both ground and first floor simultaneously, making it considerably better value per square metre than two separate projects. Expect £120,000–£220,000+ for a typical London project. Almost all double storey extensions require full planning permission.

Wraparound extension

Combining a rear and side return in an L-shape, wraparound extensions create the largest single-storey additions possible on a standard terraced plot. They typically cost £130,000–£250,000+ and nearly always need planning permission due to their scale.

Loft conversion

Loft conversions add a bedroom and bathroom without extending your footprint. In London, costs range from £60,000 for a basic Velux conversion to £120,000+ for a full Mansard.

Basement extension

The most expensive form of residential development — typically £6,000–£8,000 per square metre. A modest London basement project rarely comes in below £100,000, and complex projects regularly reach £200,000 or more.

Cost by London Borough

Inner London (Zones 1–2)

Boroughs including Westminster, Kensington & Chelsea, Islington, and Hackney see the highest costs: £4,000–£5,500/m² or more. Conservation area density is high, parking restrictions add delivery cost, and contractor rates reflect the premium market.

Mid London (Zones 3–4)

Boroughs like Lambeth, Lewisham, Haringey, and Ealing sit in the middle: £3,200–£4,500/m². Many properties here are Victorian terraces with their own planning sensitivities, but logistics are more manageable.

Outer London (Zones 5–6)

Outer boroughs — Bromley, Havering, Hillingdon, Kingston upon Thames — offer the most competitive rates: £2,800–£3,800/m². Detached and semi-detached properties are more common, making access easier and structural solutions more straightforward.

Professional Fees: What You Pay Beyond the Build

Professional fees are consistently underestimated. Budget 15–20% on top of your construction cost for all professional services. Here's what that typically includes:

• Architect / design and planning consultant: 8–15% of build cost

• Structural engineer: £1,500–£4,000 for a standard extension

• Planning application fee: £548 (from April 2026 in England)

• Party wall surveyor: £800–£2,000 per adjoining neighbour

• Building regulations: £800–£1,500 for a typical extension

At Studio : MASS, we provide end-to-end design and planning services — covering feasibility, design, planning, and building regulations — so you have one team managing every stage rather than coordinating multiple separate consultants.

Hidden Costs Most London Homeowners Miss

Fit-out costs

The most commonly underestimated category. A new kitchen costs £15,000–£50,000+ for units, appliances, and installation. A bathroom adds £8,000–£20,000. Flooring, lighting, and decoration throughout add further cost that many homeowners forget to factor in at the outset.

Contingency

Every experienced project manager recommends a contingency of 10–15% of total project cost. London's period property stock makes unforeseen structural issues a real risk — unexpected drainage, underpinning requirements, or poor existing foundations can all add cost mid-project.

Landscaping

Once the extension is built, the rear garden often needs reinstating. Expect to budget £3,000–£15,000 depending on size and your chosen specification.

How Much Value Does a Home Extension Add in London?

London property values are high enough that a well-designed extension almost always delivers a positive return. A single storey rear extension typically adds 10–15% to a property's value. A double storey can add 20% or more. Adding a bedroom to a 2-bed London property delivers disproportionate value because it moves the property into a completely different buyer demographic.

How to Control Your Extension Budget Without Cutting Corners

• Define your specification clearly before going out to tender — vague specs produce vague, incomparable quotes

• Get at least three competitive quotes based on detailed drawings and a schedule of works

• Reserve a 10–15% contingency for unforeseen structural issues

• Prioritise budget on structure, insulation, and glazing — these are expensive to fix later

• Agree a payment schedule tied to build milestones, not arbitrary dates

How to Get Accurate Quotes — and Spot the Red Flags

The most reliable quotes come after your design is fully developed — at planning stage minimum, with detailed drawings and a schedule of works. Early ballpark figures based on square meterage alone should be treated with caution. Red flags include quotes significantly below all others (something has almost certainly been excluded), no written specification attached, and requests for large upfront payments before work begins.

Frequently Asked Questions

How much does a 20m² extension cost in London?

A 20m² single storey rear extension in London typically costs £80,000–£120,000 all-in at mid-range specification, including professional fees, planning, and VAT. High-spec projects with premium glazing and kitchen fit-out can exceed £150,000.

Does planning permission add significantly to the cost?

Yes — planning adds the application fee (£548 from April 2026), plus the cost of detailed architectural drawings and any specialist reports required by your council. For a typical householder application, expect to add £3,000–£8,000 in professional fees. Conservation area applications can cost more.

Can I build an extension in London for under £50,000?

For a complete, build-ready extension in London, £50,000 is very unlikely to be sufficient. The minimum realistic budget for a small single storey addition at basic specification is around £60,000–£80,000 once professional fees and VAT are included.

Want a clear, honest picture of what your extension could cost? Book a free architectural advice call with Studio : MASS and we'll guide you through feasibility, planning requirements, and realistic budget from day one.