Rear Extension London 2026: Costs, Sizes & Planning Guide
The rear extension is the most common home improvement in London. Here are the real 2026 costs by size, exactly what permitted development allows, and what actually makes the difference between a good and a great result.
TL;DR - The Quick Answer
A rear extension in London costs £55,000–£160,000+ including VAT, depending on size. A common 3m×5m single-storey costs £55k–£80k all-in; a 4m×5m runs £72k–£105k. Most single-storey rear extensions qualify for permitted development without full planning permission, though extensions over 3m (terraced) or 4m (detached) deep require prior approval. Two-storey rear extensions almost always need full planning permission. Mayfair Studio provides architectural drawings and planning applications from a fixed fee of £1,295.
What Size Rear Extension Do You Need?
The most useful way to think about size is the room you want to create. Most rear extensions are open-plan kitchen-diners. Here is what common sizes actually give you:
3m depth (12–15m²)
A usable kitchen extension or utility room. Enough for an island and dining for 4. The minimum depth worth building for a kitchen-diner.
4m depth (16–24m²)
The sweet spot for most London houses. Kitchen with island, dining area, and a seating zone. Comfortable for everyday family use. Stays within permitted development limits for most terraced houses under the prior approval scheme.
5–6m depth (25–36m²+)
A genuinely large open-plan living space. Kitchen, dining, and seating all separate and generous. Usually requires prior approval or full planning permission, depending on house type.
Width matters too. Most London terraced houses are 4.5–6m wide at the back. A full-width extension (retaining a side path) uses the whole rear wall and feels much more generous than a narrow one. If you have a side return as well, a wrap-around extension combines rear and side return in one project.
2026 Costs by Size
All figures below include VAT at 20%. Construction cost is the build contract. All-in adds architect, structural engineer, building regulations, and a 10% contingency.
| Size | Floor area | Construction | All-in |
|---|---|---|---|
| 3m × 4m | 12m² | £42k–£60k | £50k–£72k |
| 3m × 5m | 15m² | £52k–£75k | £62k–£90k |
| 4m × 5m | 20m² | £68k–£98k | £82k–£118k |
| 4m × 6m | 24m² | £82k–£118k | £98k–£142k |
| 5m × 6m | 30m² | £102k–£148k | £122k–£178k |
| 6m × 6m | 36m² | £122k–£178k | £146k–£214k |
Cost per m² in London 2026
All figures inc VAT at 20%. Smaller extensions cost more per m² than larger ones due to fixed costs (foundations, structural openings, roofing details). Inner London (zones 1–2) typically runs 10–15% above these figures.
If your property is a terrace, our breakdown of terraced house extension cost compares rear, side return and wrap-around options type by type.
What adds to cost
| Size | Floor Area | Construction | All-in |
|---|---|---|---|
| 3m × 4m | 12m² | £42k–£60k | £50k–£72k |
| 3m × 5m | 15m² | £52k–£75k | £62k–£90k |
| 4m × 5m | 20m² | £68k–£98k | £82k–£118k |
| 4m × 6m | 24m² | £82k–£118k | £98k–£142k |
| 5m × 6m | 30m² | £102k–£148k | £122k–£178k |
| 6m × 6m | 36m² | £122k–£178k | £146k–£214k |
Source: Mayfair Studio project data • Data as of 2026
Permitted Development vs Planning Permission
Whether you need planning permission depends on the depth and height of your extension, your house type, and where you live. There are three routes for a single-storey rear extension.
Route 1: Permitted development (no application needed)
You can build without any application if all of these apply:
- Depth from original rear wall: 3m max for terraced and semi-detached, 4m max for detached
- Maximum height 4m (or 3m if the eaves are within 2m of the boundary)
- No more than half the original garden covered
- Materials similar in appearance to the existing house
- Not a conservation area, listed building, or Article 4 area
House type changes what you can build. If you own a semi, our guide to semi-detached house extension rules covers the side and wrap-around allowances that the exposed flank wall opens up.
Route 2: Neighbour Consultation Scheme (prior approval)
This is a simplified process that allows larger single-storey rear extensions. No design assessment by the council, just a neighbour notification period.
- Terraced and semi-detached houses: up to 6m from original rear wall
- Detached houses: up to 8m from original rear wall
- Submit a prior approval application to the council (£120 fee)
- Council notifies neighbours, who have 21 days to comment
- Council has 42 days to approve or refuse (refusal is rare and limited to impact on neighbours only)
- Can proceed once approved or if no response within 42 days
Important: Prior approval still requires all other permitted development conditions (height limits, coverage, materials). It only extends the depth allowance.
Route 3: Full planning permission
Full planning permission is needed when:
- Building a two-storey rear extension (the 7m rear boundary rule makes this almost always the case in London)
- Your property is in a conservation area or listed
- Subject to an Article 4 direction removing permitted development rights
- Exceeding the prior approval depth limits
Full planning permission takes 8–10 weeks and costs £528 in application fees.
| Scenario | Route | Time added |
|---|---|---|
| Single-storey, 3m depth, terraced | Permitted development | 0 weeks |
| Single-storey, 4–6m depth, terraced | Prior approval | 6–8 weeks |
| Single-storey, 4–8m depth, detached | Prior approval | 6–8 weeks |
| Conservation area (any depth) | Full planning | 10–14 weeks |
| Two-storey rear extension | Full planning | 10–14 weeks |
Key Design Decisions
Flat roof or pitched?
Flat roofs dominate single-storey rear extensions in London for practical reasons: they sit within permitted development height limits (3–4m), allow roof lanterns and skylights to flood the space with light, and are structurally simpler when building against an existing house.
Pitched or lean-to roofs can be required in conservation areas or where the council or neighbours raise concerns about the visual impact from the rear. They add cost (more roof structure, weatherproofing at the junction) and limit overhead glazing options.
Bi-fold or sliding doors?
Bi-fold doors
- Full opening: wall disappears when folded
- Lower cost for same aperture
- Panels stack inside when open (takes floor space)
- More frame sightlines when closed
Typical cost 3m set: £4,000–£7,000
Sliding doors
- Slimmer frames, more glass when closed
- Cleaner look from inside and outside
- Panels cannot fully clear the opening (always 50% glass)
- Higher cost for equivalent quality
Typical cost 3m set: £5,500–£10,000
Roof lantern or skylights?
Both bring light into a flat-roofed extension. The right choice depends on the extension geometry and how much overhead light you want:
- Roof lantern: a raised glass structure, visually striking, good light from multiple angles. Cost £2,500–£6,000. Needs sufficient depth (at least 1.5m from nearest wall).
- Flat skylights (rooflights): sit flush or near-flush with the roof. More discreet. Can be used in multiples across a larger roof. Cost £600–£1,800 each. Velux and FAKRO are the dominant brands.
Full width or retain side access?
A full-width extension (where the house is full-width with no side return gap) uses the whole rear wall and creates the most usable space. Where a side return exists, extending full-width means infilling the side return too (a wrap-around extension). Retaining a narrow side path preserves garden access and may be required by the council if you have no other route to the rear.
Building Regulations Requirements
All rear extensions need building regulations approval regardless of planning status. Building control inspects at key stages and issues a completion certificate on sign-off.
Structural opening and beam
Creating the opening in the existing rear wall requires a structural steel beam (RSJ) to carry the load above. A structural engineer must size the beam, specify the padstones and supports, and submit calculations to building control. This is typically one of the largest single-item costs in a rear extension.
Foundations
Strip foundations are standard for single-storey extensions. Depth varies by soil conditions (typically 900mm–1.2m in London clay). Trial holes are sometimes needed for building control sign-off. In a conservation area or where a build-over agreement is required, Thames Water drainage proximity may require deeper or engineered foundations.
Insulation (2026 standards)
- Walls: U-value 0.18 W/m²K or better
- Roof: U-value 0.15 W/m²K or better (flat roof typically requires 150–200mm PIR insulation)
- Floor: U-value 0.13 W/m²K or better
- Glazing: U-value 1.4 W/m²K or better (standard double glazing is borderline; triple glazing is safer)
Drainage
Flat roofs must drain to a downpipe that connects to the drainage system. If building over or near a public sewer you will need a build-over agreement with Thames Water (required within 3m of a public sewer). Your architect or drainage consultant identifies this at design stage.
Electrics and heating
Any new electrical installation in the extension must be to Part P of building regulations (usually self-certified by a registered electrician). Underfloor heating must connect to the existing heating system and be designed to heat the space adequately. Both are inspected by building control.
Party Wall: When It Applies
Rear extensions on terraced or semi-detached houses almost always trigger the Party Wall Act 1996. Specifically when:
- Excavating foundations within 3m of a neighbouring structure (almost every extension in London)
- The extension wall is built on or along the boundary line
- Any work directly to the party wall (cutting in beams, making good, etc.)
Typical cost: You pay for your surveyor and your neighbour's. Budget £800–£1,400 per neighbour. A terraced house with neighbours on both sides: £1,600–£2,800.
Timing: Serve notice at least 2 months before work starts. If neighbours dissent or do not respond, add 4–8 weeks for the Award. Start the party wall process as soon as you have outline drawings.
Realistic Timeline
Design and drawings
3–5 weeksArchitect site visit, measured survey, planning and building regulations drawings
Prior approval (if 3–6m depth)
6–8 weeksSubmit to council, neighbour notification period, decision
Building regulations
3–5 weeksFull plans submission (runs in parallel with planning or prior approval)
Party wall process
2–4 monthsNotice period plus Award if neighbours dissent. Runs in parallel but must start early
Construction on site
10–18 weeksSmall extension (under 15m²): 10–12 weeks. Medium (15–25m²): 12–16 weeks. Large (25m²+): 14–20 weeks. Includes new kitchen fit-out if applicable.
Total project
4–8 monthsStraight PD with no party wall issues: 4–5 months. Prior approval and party wall: 6–7 months. Full planning with party wall: 7–9 months.
Common Mistakes
Measuring from the wrong point
Permitted development depth is measured from the original rear wall of the house, not from any existing single-storey outrigger, toilet extension, or previous extension. If a previous extension exists, your council's planning portal shows the original building footprint. Measure wrong and you may not have PD rights you thought you had.
Not checking for a public sewer in the garden
Many London back gardens have public sewers running through them. Building within 3m requires a build-over agreement with Thames Water, which adds cost and time. Check the Thames Water sewer map before finalising your extension footprint.
Choosing glazing that breaches permitted development height
Eaves height must not exceed 3m if the extension is within 2m of a boundary, and the overall height must not exceed 4m. A roof lantern sitting on a 3m wall can push the overall height over these limits. Check with your architect before specifying glazing heights.
Leaving kitchen specification too late
If you are fitting a new kitchen, the kitchen designer needs to be involved during building regulations stage, not after construction starts. Drainage, water supply, gas, and electrical positions must be built into the floor and walls. Changing these after the slab is poured is expensive.
Underestimating disruption during the structural opening
Creating the opening in the existing rear wall involves propping the structure, cutting through brick, and installing a steel beam. This takes 2–5 days and makes the house temporarily uninhabitable. Plan temporary kitchen arrangements and somewhere for pets and children to be for this phase.
A single-storey rear extension in London costs £40,000–£65,000 for a typical 20m² project. Most single-storey rear extensions under 3m deep (terraced/semi) or 4m deep (detached) qualify as permitted development - no planning permission needed. The Prior Approval scheme allows up to 6m or 8m deep with neighbour notification. Mayfair Studio handles rear extensions from £1,295 including planning drawings, structural calculations, and planning application.
Frequently Asked Questions
How much does a rear extension cost in London 2026?
A single-storey rear extension in London costs £40,000–£65,000 for a typical 20m² project (£2,000–£2,600/m² inc VAT). Size-by-size: a 3m×5m extension costs £55,000–£80,000; a 4m×5m costs £72,000–£105,000; a 5m×6m costs £110,000–£160,000. Architect fees from £1,295 (Mayfair Studio, includes planning and structural), structural engineer £1,500–£2,500, and planning fee £258 if required are additional.
What is the average cost of a rear extension in London?
The average cost of a single-storey rear extension in London is £52,000–£70,000 for a typical 20–25m² extension. Build costs average £2,100–£2,400/m² across London, rising to £2,600–£3,200/m² in conservation areas. Two-storey rear extensions average £90,000–£125,000. Architect fees from £1,295 (Mayfair Studio) are additional. Total all-in for a standard 20m² project: £45,000–£75,000.
Do I need planning permission for a rear extension in London?
Most single-storey rear extensions in London do not need planning permission if they qualify as permitted development: maximum 3m deep for terraced and semi-detached houses, 4m deep for detached houses, up to 4m high, single-storey only. However, conservation areas, Article 4 directions (common in Hackney, Islington, Camden, and other inner London boroughs), and listed buildings remove permitted development rights - requiring full planning permission.
Can I build a rear extension without planning permission in London?
Yes, if your rear extension qualifies as permitted development. The rules: single-storey only, maximum 3m depth (terraced/semi-detached) or 4m depth (detached), maximum 4m height. These rights are removed in conservation areas, Article 4 direction zones, and for listed buildings. The Prior Approval scheme extends limits to 6m (semi/terraced) or 8m (detached) with neighbour consultation - free to apply and takes 42 days.
How long does a rear extension take to build in London?
A typical single-storey rear extension takes 8–12 weeks to build once construction starts. Add 4–8 weeks for design and planning if permitted development applies, or 8–12 weeks if full planning permission is required. Total timeline from decision to move-in: 4–6 months for PD projects, 6–10 months for planning permission projects. Mayfair Studio's design and planning phase typically takes 4–8 weeks.
Summary
A rear extension is the most straightforward way to significantly increase living space in a London house. The planning route (PD, prior approval, or full planning) depends mainly on depth and whether you are in a conservation area. Start with that question before commissioning a design.
The 4m × 5m size is the most popular in London for good reason: it is large enough to transform everyday life, usually achievable under prior approval, and the cost-to-value ratio is strong.
Budget 15–20% above the base construction cost for professional fees and contingency. Start the party wall process as soon as you have an outline scheme.