3 Metre Kitchen Extension London 2026: Costs, Layouts & PD Guide
Three metres is the magic number for London extensions. It is the maximum depth you can build under permitted development on a terraced or semi-detached house with no planning application, no prior approval, and no council involvement at all. Here is exactly what fits in a 3m extension, what it costs, and the mistakes that catch people out.
Quick Answer
A 3m kitchen extension costs £50k–£108k all-in (inc VAT) depending on width. A 3m×4m costs £50k–£72k, a 3m×5m costs £62k–£90k, a 3m×6m costs £75k–£108k. No planning application is needed for terraced or semi-detached houses under permitted development. You still need building regulations approval and a party wall agreement.
£62k–£90k
3m×5m (most popular)
None
Planning needed
10–14 weeks
Build time
Check your specific property constraints
Free Property CheckWhy 3m Is the Most Common Extension Depth in London
Under permitted development rules (Class A, Part 1 of the GPDO), you can extend a terraced or semi-detached house by up to 3 metres from the original rear wall without any planning application. No prior approval, no neighbour notification, no 42-day wait. You just build, subject to building regulations and the Party Wall Act.
For detached houses, the limit is 4m. But the majority of London housing stock is terraced or semi-detached, which means 3m is the line that matters.
This makes 3m the path of least resistance: no application fees, no risk of refusal, no delay waiting for the council. You can appoint an architect, get building regulations drawings done, serve your party wall notices, and be on site within a few months.
What Actually Fits in a 3m Kitchen Extension
The depth is fixed at 3m but the width depends on your house. Most London terraced houses are 4.5m to 6m wide at the rear. That width is what determines your layout options.
3m × 4.5m (13.5m²)
A narrow but functional kitchen-diner. Enough for a galley kitchen along one wall with a compact island and dining for 4. Common on Victorian mid-terraces with narrower plots. Works well as a single run of units opposite bi-fold doors with a small table between.
3m × 5m (15m²)
The most popular size for a 3m extension. An L-shaped kitchen with an island or peninsula, plus a dining area for 4–6. This is the layout that transforms daily life: cooking, eating, and homework all happen in one room. If you include a side return, this width is comfortably achievable on most terraces.
3m × 6m (18m²)
A full kitchen-diner with room for a proper island, seating for 6, and space to breathe. This width is typical on wider terraces or semi-detached houses. You can fit a U-shaped or L-shaped kitchen with a generous island and a separate dining zone near the glazing.
A key point: when you say “3m deep” that is the external dimension. The new external wall is roughly 300mm thick (brick outer leaf, cavity, block inner leaf, insulation, plaster). Your usable internal depth is closer to 2.7m. This matters for kitchen layout planning. Always work from internal dimensions when choosing units and islands.
2026 Costs by Size
All figures below include VAT at 20%. Construction cost is the build contract only. All-in adds architect fees, structural engineer, building regulations, party wall, and a 10% contingency.
| Size | Floor area | Construction | All-in |
|---|---|---|---|
| 3m × 4m | 12m² | £42k–£60k | £50k–£72k |
| 3m × 5m | 15m² | £52k–£75k | £62k–£90k |
| 3m × 6m | 18m² | £62k–£90k | £75k–£108k |
What adds to cost
Why 3m Extensions Cost More Per Square Metre
A 3m extension typically costs £3,500–£5,000 per m², while a 5m or 6m extension often works out at £3,000–£4,500 per m². The reason is fixed costs.
Every single-storey rear extension needs the same things regardless of depth: foundations along the full width, a structural steel beam to open up the rear wall, a new flat roof with drainage, and bi-fold or sliding doors at the back. Whether you build 3m deep or 6m deep, these elements cost roughly the same. The only things that scale with depth are the side walls, the floor slab, and insulation.
This means a 3m extension gives you less floor area to spread those fixed costs across, pushing the per-m² rate higher. It does not mean 3m is poor value. It is still the cheapest total cost. But if you are choosing between 3m (free under PD) and 4m (needs prior approval), the extra metre adds relatively little to the total bill while giving you substantially more room.
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Kitchen Layout Design at 3m Depth
At 3m deep (roughly 2.7m internally), every centimetre matters. Here are the layout decisions that make or break the space.
The structural opening
The opening between the existing house and the new extension is defined by a steel beam (RSJ) that carries the load from above. The wider the opening, the larger the beam and the more it costs. Most people want the widest possible opening to create a genuinely open-plan feel. A full-width opening (leaving only small piers at each end for the beam to sit on) typically requires a 200–300mm deep steel beam. Your structural engineer sizes this based on the loads above.
The beam depth eats into your ceiling height at the junction. If your existing ceiling is 2.4m, a 250mm beam brings the soffit down to 2.15m at that point. Solutions include boxing the beam into a bulkhead, raising the floor level in the extension to create a flush ceiling, or (where budgets allow) using a flitch beam or multiple smaller beams for a shallower profile.
Island placement
With 2.7m of usable depth, an island is tight but workable. A standard kitchen island is 600mm deep. You need at least 900mm clearance on each side for comfortable access (1,000mm is better). That leaves 300mm for worktop depth along the back wall, which is not enough for a full run of units.
The practical solution: place the island perpendicular to the back wall (running from front to back of the extension), not parallel. Or use a peninsula extending from one wall, which only needs clearance on one side. A peninsula at 3m depth works significantly better than a freestanding island.
Bi-fold door placement
Full-width bi-fold doors across the rear wall are the default choice. They flood the space with light and create an indoor-outdoor feel. At 3m depth, the doors are close to the structural opening, which means the extension feels like one continuous space when both the doors and the internal wall are open.
Consider where the doors stack when folded. Bi-folds fold to one side and the stacked panels take up 600–900mm of width. Do not put your fridge or a tall unit where the doors stack. Sliding doors avoid this issue entirely since the panels slide behind each other, but they only ever open 50% of the aperture.
Not sure what planning route your property needs? Our AI checks your address, depth, and conservation area status instantly.
Check now — it's free →Permitted Development Rules for 3m
A 3m single-storey rear extension on a terraced or semi-detached house falls within permitted development with no application of any kind, provided all of these conditions are met:
- Maximum eaves height 3m (if within 2m of a boundary, which it almost always is in London)
- Maximum overall height 4m
- No more than half the garden covered by extensions and outbuildings combined
- Materials similar in appearance to the existing house
- Not in a conservation area, not listed, and no Article 4 direction in place
- Measured from the original rear wall (not any previous extension)
Important: No planning application does not mean no approvals. You still need building regulations approval (always required), and you almost certainly need a party wall agreement with your neighbours (almost every terraced or semi-detached house in London).
Lawful Development Certificate: You can apply for an LDC (optional, £132) to get written confirmation from the council that your extension is permitted development. Useful when you sell the property, as buyers' solicitors will ask for proof.
Building Regulations Requirements
Every extension in England needs building regulations approval regardless of whether it has planning permission or not. Your architect submits full plans to the local authority or an approved inspector, who then inspects at key stages.
Foundations
Strip foundations are standard for a 3m extension. In London clay, expect 900mm–1.2m depth. If there are trees nearby, the foundation depth may increase to account for clay shrinkage. Building control will want to inspect the trenches before concrete is poured.
Structural beam
A structural engineer must design the steel beam for the opening into the existing house. The engineer produces calculations showing the beam size, padstone details, and temporary propping method. These are submitted to building control as part of the full plans application.
Insulation (2026 standards)
- Walls: U-value 0.18 W/m²K or better
- Flat roof: U-value 0.15 W/m²K or better
- Floor: U-value 0.13 W/m²K or better
- Glazing: U-value 1.4 W/m²K or better
Drainage
The flat roof drains to a downpipe connecting to the existing drainage. If building over or within 3m of a public sewer, you need a build-over agreement from Thames Water. Check the Thames Water sewer map before finalising your extension footprint.
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The Structural Opening: The Most Disruptive Part
The moment that turns a building site into an open-plan kitchen is when the builder knocks through the existing rear wall and installs the steel beam. This is the single most disruptive part of the project.
How it works
Plan for this phase. You will likely need to be out of the kitchen for a week. Temporary cooking arrangements (microwave, electric hob in another room) and somewhere for children and pets to be during the noisiest days make a real difference.
When to Go Beyond 3m
Three metres is the free line, but it is not always the right line. The Neighbour Consultation Scheme (prior approval) lets you build up to 6m deep on a terraced or semi-detached house, and up to 8m on a detached house. It costs £120, takes 6–8 weeks, and the council can only refuse based on impact on neighbours.
| Depth | Route | Time added | Cost added |
|---|---|---|---|
| Up to 3m (terraced/semi) | Permitted development | 0 weeks | £0 |
| 3–6m (terraced/semi) | Prior approval | 6–8 weeks | £120 |
| Up to 4m (detached) | Permitted development | 0 weeks | £0 |
| 4–8m (detached) | Prior approval | 6–8 weeks | £120 |
The extra metre from 3m to 4m adds roughly £8k–£15k to the construction cost but gives you 4–6m² more floor area. That is a much better cost-per-m² deal than the first 3m. If your layout would genuinely benefit from the extra depth (a proper island with clearance on all sides, a separate dining area), the prior approval process is straightforward and rarely refused.
The main reasons to stay at 3m: you want to start immediately with no council delays, you are in a conservation area where prior approval is not available, or you want to keep the total budget as low as possible.
Common Mistakes
Forgetting the wall thickness
The 3m is an external measurement. Your new rear wall is approximately 300mm thick (outer brick, cavity, insulation, inner block, plaster). Your internal usable depth is closer to 2.7m. If you have designed a kitchen layout assuming 3m internally, everything will be 300mm too tight. Always work from internal dimensions.
Not designing the kitchen before the build starts
Drainage positions, water supply, gas pipe routes, and electrical socket locations are all set into the floor slab and walls during construction. If you decide on your kitchen layout after the slab is poured, moving a sink or an island means digging up the floor. Get a kitchen designer involved at the same time as your architect, not after construction finishes.
Measuring from the wrong point
Permitted development depth is measured from the original rear wall of the house, not from a previous single-storey extension, outrigger, or toilet block. If the house already has a rear addition built after 1948, you measure from the wall behind it. Get this wrong and your 3m extension could exceed permitted development limits without you realising.
Ignoring the party wall timeline
Even though no planning application is needed, you must serve party wall notices at least 2 months before construction starts. If a neighbour dissents, add another 4–8 weeks for the Party Wall Award. People who assume “no planning needed” means they can start immediately get caught out by party wall delays. Budget £800–£1,400 per neighbour.
Not checking for a public sewer
Many London gardens have Thames Water public sewers running through them. Building within 3m of one requires a build-over agreement, which adds cost and time. Check the Thames Water sewer map before you finalise your extension footprint. Moving the extension 300mm to one side can sometimes avoid the issue entirely.
Frequently Asked Questions
How much does a 3m kitchen extension cost in London in 2026?
All costs inc VAT: 3m×4m (12m²) £50k–£72k all-in, 3m×5m (15m²) £62k–£90k all-in, 3m×6m (18m²) £75k–£108k all-in. Add £12k–£35k for a new kitchen if replacing as part of the project. Cost per m² is typically £3,500–£5,000 depending on spec and finishes.
Do I need planning permission for a 3m extension in London?
No. A 3m single-storey rear extension on a terraced or semi-detached house falls within permitted development and needs no planning application or prior approval. You must still meet conditions: max eaves height 3m (within 2m of boundary), max overall height 4m, not more than half the garden covered, and not in a conservation area or listed building.
What is the maximum extension depth without planning permission?
Under standard permitted development: 3m for terraced and semi-detached houses, 4m for detached houses. Under the Neighbour Consultation Scheme (prior approval): up to 6m for terraced and semi-detached, 8m for detached. Prior approval costs £120 and takes 6–8 weeks. Beyond these limits, full planning permission is required.
What kitchen layout works in a 3m extension?
At 3m depth (roughly 2.7m internal), an L-shaped kitchen with a peninsula works best. A freestanding island is tight — you need 900mm clearance on each side plus 600mm for the island, which leaves very little for worktop depth along the wall. A peninsula extending from one wall is more practical. For wider houses (5m+), a galley layout with dining at the glazed end also works well.
How long does a 3m kitchen extension take to build?
Construction on site: 10–14 weeks including kitchen fit-out. Total project from design to completion: 3–5 months. The main stages are: design and drawings (3–4 weeks), building regulations (3–5 weeks, runs in parallel), party wall notices (2+ months), then construction. No planning delay since 3m is permitted development.
Do I need a party wall agreement for a 3m extension?
Almost certainly yes on a terraced or semi-detached house. Excavating foundations within 3m of a neighbour's structure triggers the Party Wall Act 1996. Serve notice at least 2 months before work starts. Budget £800–£1,400 per neighbour. In a terrace with neighbours on both sides: £1,600–£2,800.
Why does a 3m extension cost more per square metre than a larger one?
Fixed costs. Every rear extension needs the same foundations across the full width, the same structural steel beam, the same flat roof and drainage, and the same bi-fold doors. Whether you build 3m or 6m deep, these cost roughly the same. A smaller extension has less floor area to spread those fixed costs across, pushing the per-m² rate higher.
Should I go to 4m instead of 3m?
If your layout would benefit from the extra depth (a proper island, a dedicated dining zone), the extra metre adds roughly £8k–£15k to construction cost but gives you 4–6m² more space at a much better per-m² rate. The trade-off is a prior approval application (£120, 6–8 weeks). If you're not in a rush, 4m is often better value.
Summary
A 3m kitchen extension is the fastest, simplest way to transform a London terraced or semi-detached house. No planning application, no council delay, just building regulations and party wall. The 3m × 5m size is the most popular for good reason: it fits most London houses, creates a proper kitchen-diner, and stays within a realistic budget.
Design the kitchen before building starts, not after. Work from internal dimensions (2.7m, not 3m). Start party wall notices early. And if your layout genuinely needs more depth, the prior approval route to 4–6m is straightforward and rarely refused.
Budget 15–20% above the construction cost for professional fees and contingency, and expect the whole project to take 3–5 months from first appointment to moving back into the kitchen.
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