London Extension Costs 2026: The Complete Price Guide
Real extension prices for 2026 based on current builder quotes and completed projects. From single-storey rears to basement conversions, here is what you will actually pay in London.
TL;DR - The Quick Answer
According to Mayfair Studio's analysis of 1.2 million UK planning applications and current builder pricing, London home extensions in 2026 cost £3,200-£4,600 per m², up roughly 3-4% from 2025 due to labour inflation. A single-storey rear extension runs £60,000-£135,000; a loft conversion £52,000-£100,000; a two-storey extension £110,000-£290,000. These figures cover construction only. Add 10-15% for architect and engineer fees, plus a 10% contingency. Reputable builders are typically booked 3-6 months ahead.
2026 Construction Market Overview
Construction costs in London have risen approximately 3-4% from 2025 to 2026. This follows the pattern of steady increases since 2022, driven primarily by labour costs rather than materials.
Key 2026 trend: Labour costs are up 7% year-on-year, with skilled tradespeople in high demand. For a detailed breakdown of what individual trades charge, see our extension trade costs guide. Material costs have stabilised but remain elevated. Builder capacity is still constrained, meaning lead times of 3-6 months are common for reputable contractors.
What is Driving 2026 Prices?
- Labour inflation: Skilled tradespeople wages continue rising above general inflation
- Building Regs 2025: Updated Part L insulation requirements add 2-3% to build costs
- Capacity constraints: Good builders are booked 3-6 months ahead
- Materials stable: Most materials flat, but metals 6-10% above 2024 lows
Cost Per Square Metre in 2026
London extension costs in 2026 range from £3,200 to £4,600 per m² depending on specification and complexity. Our detailed guide to extension costs by size breaks down how the per-m² rate changes with project scale.
A rear extension in London costs £55,000–£110,000 all-in (2026), depending on size and location.
| Specification Level | Cost/m² (2026) | What is Included |
|---|---|---|
| Budget | £3,200 - £3,600 | Standard materials, basic finishes, painted walls |
| Mid-Range | £3,600 - £4,200 | Quality finishes, engineered flooring, good lighting |
| High-End | £4,200 - £4,600 | Premium materials, underfloor heating, smart home features |
| Premium | £4,600+ | Architect-designed, bespoke joinery, high-spec glazing |
Source: Mayfair Studio project data • Data as of 2026
Important: These m² rates cover construction only. Add 10-15% for professional fees (architect, structural engineer), plus your kitchen/bathroom fit-out, plus 10% contingency. See our guide to hidden costs of London extensions for a full breakdown.
Extension Costs by Type (2026)
Single-Storey Rear Extension
The most popular extension type in London. A rear extension creates open-plan kitchen-diners or larger living spaces.
Two-Storey Rear Extension
A two-storey extension adds both ground floor space and an extra bedroom or bathroom above. Best value per m² but requires planning permission.
Loft Conversion
No foundation costs make a loft conversion excellent value. Converts unused roof space into bedroom(s) with en-suite.
Side Return Extension
A side return extension fills the narrow alley beside terraced houses. Widens the kitchen significantly for a modern open-plan layout.
Wrap-Around (Side + Rear)
A wrap-around extension combines side return with rear extension for maximum ground floor space. Popular with Victorian terraces.
Basement Extension
A basement extension is the most expensive per m² but creates significant space without reducing garden. Complex engineering required.
What is Included (and What is Not)
Typically Included
- Foundations and groundworks
- Walls, roof, and structure
- Windows and external doors
- First-fix electrics and plumbing
- Plastering and decoration
- Basic floor finish
Usually Extra
- Kitchen units and appliances
- Architect fees (8-12%)
- Structural engineer (£1.5k-3k)
- Party wall agreements (£1k-3k)
- Planning fees (£258-500)
- Landscaping and paving
Costs by London Area (2026)
Extension costs vary significantly by area due to local builder rates, access constraints, and parking zone requirements. For a full breakdown, see our guide to extension costs by London borough.
| Area | Cost Variation | Notes |
|---|---|---|
| Prime Central | +15-25% | Kensington, Chelsea, Westminster - premium builder rates |
| Inner London | +5-15% | Islington, Hackney, Camden - access challenges, parking |
| South London | Baseline | Lewisham, Greenwich, Southwark - average costs |
| Outer London | -5-10% | Bromley, Croydon, Barnet - better access, lower rates |
| Conservation Areas | +5-10% | Higher design standards, matching materials required |
Source: Mayfair Studio project data • Data as of 2026
How to Budget Your Extension
The Real Total Cost Formula
Here is how to calculate a realistic budget for your 2026 London extension. If you need help funding the project, our guide to extension financing options covers remortgaging, bridging loans, and other routes.
Example: 25m² Single-Storey Extension (2026)
Frequently Asked Questions
How much does a house extension cost in London in 2026?
In 2026, London house extensions typically cost £3,200-£4,600 per m². A single-storey rear extension costs £60,000-£135,000, a loft conversion £52,000-£100,000, and a two-storey extension £110,000-£290,000. Prices increased approximately 3-4% from 2025 due to labour cost inflation.
What is the cost per square metre for an extension in London 2026?
London extension costs per m² in 2026 range from £3,200 for basic single-storey builds to £4,600+ for high-specification finishes. Two-storey extensions cost £3,400-£4,200/m² due to additional structural requirements.
How much has construction cost increased in London for 2026?
Construction costs in London have risen approximately 3-4% from 2025 to 2026. This is primarily driven by labour cost inflation (up 7% year-on-year) and capacity constraints in the construction sector.
What is the cheapest type of extension in London?
The most cost-effective extension per m² is a single-storey rear extension at £3,200-£3,800/m². However, for total project cost, a small side return (8-12m²) or a loft conversion can offer better value for the space gained.
Should I get multiple quotes for my London extension?
Yes, always get 3-4 quotes from reputable builders. In London's 2026 market, quotes can vary by 20-30% for the same project. Compare like-for-like specifications and check what is included.
Summary: 2026 Extension Costs
London extension costs in 2026 have increased 3-4% from 2025, driven primarily by labour cost inflation. For a typical mid-range project, budget:
- Single-storey rear: £95,000-£130,000 all-in
- Loft conversion: £65,000-£95,000 all-in
- Two-storey: £165,000-£250,000 all-in
Start with accurate pricing before commissioning designs. Use our free cost calculator to get a personalised estimate based on your property type, location, and project requirements.