3m × 6m Extension Cost London 2026: What Every Size Really Costs All-In
How much will your extension actually cost once you add the architect, structural engineer, building regs, and a sensible contingency? Here are the real all-in figures for every common size in London, from 9m² to 36m².
Quick Answer
A 3m × 6m extension costs £72k–£110k all-in inc VAT in London 2026. That includes construction, architect, structural engineer, building regulations, and a 10% contingency. A smaller 3m × 5m costs £60k–£92k. A larger 4m × 6m costs £98k–£142k. Cost per m² drops as size increases because fixed costs are spread over more floor area.
£60k–£92k
3m × 5m all-in
£72k–£110k
3m × 6m all-in
£98k–£142k
4m × 6m all-in
Check your specific property constraints
Free Property CheckAll-In Costs by Size: London 2026
Every figure below includes VAT at 20%. The construction column is the build contract alone. The all-in column adds architect fees, structural engineer, building regulations application, and a 10% contingency. It does not include a new kitchen, party wall costs, or planning application fees (because most of these sizes qualify for permitted development).
| Size | Floor area | Construction | All-in cost |
|---|---|---|---|
| 3m × 3m | 9m² | £32k–£48k | £42k–£62k |
| 3m × 4m | 12m² | £40k–£58k | £50k–£72k |
| 3m × 5m | 15m² | £50k–£75k | £60k–£92k |
| 3m × 6m | 18m² | £58k–£90k | £72k–£110k |
| 4m × 5m | 20m² | £68k–£100k | £82k–£120k |
| 4m × 6m | 24m² | £80k–£118k | £98k–£142k |
| 5m × 5m | 25m² | £82k–£122k | £100k–£148k |
| 5m × 6m | 30m² | £100k–£148k | £120k–£178k |
| 6m × 6m | 36m² | £120k–£178k | £145k–£214k |
These ranges reflect outer London pricing. Inner London (zones 1–2) typically runs 10–15% above these figures due to higher labour rates, more expensive skip permits, and tighter site access.
Cost per m² by Spec Level
The per-square-metre rate is the most useful benchmark when comparing quotes. In London 2026, single-storey rear extensions sit in three broad spec bands. These are construction costs only, inc VAT.
London construction cost per m² (2026, inc VAT)
Smaller extensions cost more per m² because foundations, structural openings, and roof details are fixed costs regardless of floor area. A 9m² extension might hit £4,000/m² at mid-range spec, while a 30m² extension at the same spec sits closer to £3,400/m².
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What Each Size Actually Gives You
Dimensions on paper mean little until you picture the room. Here is what common sizes translate to in everyday use.
3m × 3m (9m²) — Utility room or small kitchen
Enough for a compact utility room, home office, or a small kitchen with limited worktop space. Not large enough for a kitchen-diner. This is the minimum viable extension size for most projects.
3m × 4m (12m²) — Small kitchen or dining room
A functional kitchen with a modest island, or a separate dining room. At 3m depth the room feels usable but not generous. Best for houses where the existing kitchen is very small and any additional space is a significant improvement.
3m × 5m (15m²) — Kitchen with dining for 4
The most common size for a budget-conscious kitchen extension. Room for an L-shaped or galley kitchen with a small island and dining for four. The 5m width gives breathing room. This is the sweet spot for a Victorian terraced house where a full-width rear extension replaces a narrow back addition.
3m × 6m (18m²) — Proper kitchen-diner
At 6m wide this feels like a proper room. Kitchen with island, dining for 6, and wall space for storage or a dresser. The extra width over a 3m×5m makes a meaningful difference to how the space feels day-to-day. 18m² is enough to justify bi-fold doors across the full rear wall.
4m × 5m (20m²) — Kitchen-diner with comfortable seating
The extra depth (4m vs 3m) transforms the room. You gain a proper distance between the kitchen zone and the dining table, making it feel like two connected areas rather than one cramped one. Room for an island with bar seating plus a dining table for 6.
4m × 6m (24m²) — Large open-plan living space
Kitchen, dining, and a small seating area all comfortably in one room. This is the size where an extension starts to fundamentally change how a house lives. Works particularly well when the existing reception room becomes a dedicated living room.
5m × 6m or 6m × 6m (30–36m²) — Full open-plan ground floor
A genuinely large family space. Kitchen with full island, dining for 8, and a distinct living area with sofa. At 30m²+ you are typically combining a rear extension with a side return (wrap-around). This usually requires prior approval or planning permission and a build duration of 14–20 weeks.
Why Smaller Extensions Cost More per m²
Every extension has a set of fixed costs that barely change whether the floor area is 9m² or 36m². These fixed costs dominate the budget on small extensions and explain why a 3m×3m does not cost half as much as a 3m×6m.
The fixed costs that barely change with size
These items total £15,500–£30,000 regardless of floor area. On a 9m² extension, that is £1,700–£3,300 per m² before any walls, floor finish, or glazing. On a 36m² extension, it is £430–£830 per m². This is why doubling the size does not double the cost.
The practical takeaway: if you are choosing between a 3m×5m and a 3m×6m, the extra 3m² of floor area adds roughly £10k–£18k to the all-in budget. That is significantly less per square metre than the first 15m² cost you. Building slightly larger is almost always better value than building small and wishing you had gone bigger.
Want to know exactly what your extension will cost? Our AI gives you a personalised estimate based on your property, size, and spec level.
Get your free estimate →Which Sizes Fit Under Permitted Development?
The planning route for your extension depends on the depth from the original rear wall and your house type. Width does not matter for permitted development (as long as you stay within other rules like garden coverage and boundary distances).
| Depth | Terraced / semi | Detached |
|---|---|---|
| Up to 3m | Permitted development | Permitted development |
| 3m – 4m | Prior approval needed | Permitted development |
| 4m – 6m | Prior approval needed | Prior approval needed |
| 6m – 8m | Full planning permission | Prior approval needed |
| Over 8m | Full planning permission | Full planning permission |
What this means for common sizes
3m × 6m (3m depth, 6m wide)
Permitted development for all house types. No application needed. The 3m depth is within PD limits for terraced, semi-detached, and detached houses. The 6m width is unrestricted (subject to boundary and garden coverage rules).
4m × 6m (4m depth, 6m wide)
Permitted development for detached houses. Prior approval required for terraced and semi-detached (the 4m depth exceeds their 3m PD limit). Prior approval costs £120 and takes 6–8 weeks.
5m or 6m depth
Prior approval for all house types. For terraced/semi-detached the maximum under prior approval is 6m. For detached it is 8m. Beyond these limits, full planning permission is needed (£528, 8–10 weeks).
All of the above assumes your property is not in a conservation area, not listed, and not subject to an Article 4 direction that removes permitted development rights. If any of those apply, full planning permission is almost always required regardless of size.
Additional Costs Beyond Construction
The build contract is the largest single cost, but it is not the whole picture. These are the professional fees and extras that make up the difference between the construction figure and the all-in total.
Professional fees and extras (all inc VAT)
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Inner London vs Outer London: The Price Gap
The cost table above reflects outer London pricing (zones 3–6). If you are in inner London (zones 1–2), expect to add 10–15% to the all-in figure. The reasons are straightforward:
- Higher labour rates. Skilled tradespeople charge more to work in central London due to commuting time and congestion charges.
- Restricted site access. Narrow streets, residents' parking, and limited space for skips and materials storage increase time on site.
- Skip permits and parking suspensions cost more in inner boroughs and take longer to arrange.
- Conservation areas and Article 4 directions are more common in inner London, which means more planning work and potentially more design constraints.
3m × 6m all-in cost comparison
How to Decide Which Size Is Right
The right size is a balance between what the space needs to do, what the site allows, and what the budget can absorb. Work through these questions:
1. What room are you creating?
A dedicated kitchen (no dining) works at 9–12m². A kitchen-diner needs 15–20m² minimum. An open-plan kitchen-living-dining space needs 24m²+. Start from the room you want, not the depth you think you can afford.
2. What does the garden allow?
Permitted development limits total garden coverage to 50% of the original plot. A 3m depth on a terraced house with a 10m garden is comfortably within this. A 6m depth on a short garden may not be. Measure your garden before committing to a size.
3. Does depth or width matter more?
On a typical London terraced house (5m wide at the rear), a 3m×5m extension gives 15m². Going wider by infilling a side return (making it 3m×7m = 21m²) adds more usable space than going deeper to 4m×5m = 20m². Width creates a more generous room; depth can make a narrow space feel like a corridor.
4. What is the planning route?
If staying at 3m depth means no planning application and no delay, that may outweigh the benefits of going to 4m and needing prior approval. Conversely, if you need prior approval anyway (perhaps for other reasons), you might as well use the full 6m depth allowance.
5. What is the cost-per-m² sweet spot?
Extensions under 12m² are disproportionately expensive per m² because fixed costs dominate. Between 15–24m² is the value sweet spot where you get the most additional space for each pound spent. Above 30m² the savings per m² are smaller and the total budget climbs steeply.
Common Mistakes When Choosing Size
Over-specifying a small extension
Putting £12,000 bi-fold doors, underfloor heating, and a roof lantern on a 9m² extension pushes the cost per m² above £6,000. At that point you are spending high-spec money for a room that feels modest. Either increase the size or keep the spec simple on a small footprint.
Under-sizing for how you actually want to use the space
A 3m×4m extension sounds affordable, but if you want a kitchen island, dining table for 6, and somewhere to sit with a coffee, 12m² is not enough. You will use the extension every day for decades. Spending an extra £12k–£20k to go from 12m² to 18m² is almost always worth it.
Ignoring the connection to the existing house
A large extension is only as good as the opening into the existing house. If the structural opening is narrow (2m or less), the extension will feel disconnected regardless of its size. Budget for the widest opening your structure allows. This usually means a longer steel beam and more padstones, adding £1,000–£3,000.
Not accounting for wall thickness
A 3m × 6m extension is 3m from the external face of the new rear wall to the original rear wall. But walls are 300–400mm thick once you include block, insulation, and plasterboard. The internal room is closer to 2.6m deep. Factor this into your furniture layout before committing to a size.
Forgetting that width is free (from a planning perspective)
Permitted development restricts depth, not width. Going from 3m×5m to 3m×6m adds 3m² of floor area without changing the planning route. If your house is 6m wide at the back, build the full width. The construction cost increase is modest and the room feels significantly more generous.
Frequently Asked Questions
How much does a 3m x 6m extension cost in London?
A 3m × 6m (18m²) single-storey extension in London costs £58k–£90k for construction and £72k–£110k all-in including architect, structural engineer, building regulations, and 10% contingency. All figures include VAT at 20%. Inner London adds 10–15% to these figures.
How much does a 3m x 5m extension cost in London?
A 3m × 5m (15m²) extension costs £50k–£75k for construction and £60k–£92k all-in in London 2026. This is the most common size for a budget-conscious kitchen extension on a Victorian terraced house. It fits within permitted development at 3m depth.
Do I need planning permission for a 3m x 6m extension?
A 3m × 6m extension has a depth of 3m, which is within permitted development limits for all house types (terraced, semi-detached, and detached). No planning application is needed, provided your property is not in a conservation area, is not listed, and is not subject to an Article 4 direction. The 6m width is not restricted by permitted development rules.
Why does a small extension cost so much per square metre?
Every extension has fixed costs (foundations, structural opening, roof structure, drainage connections, scaffolding) that total £15,500–£30,000 regardless of size. On a 9m² extension these fixed costs add £1,700–£3,300 per m² before any finishes. On a 36m² extension the same costs add only £430–£830 per m². This is why doubling the floor area does not double the price.
Is a 3m x 6m extension big enough for a kitchen-diner?
Yes. At 18m², a 3m × 6m extension is large enough for a kitchen with island, dining for 6, and some storage furniture. The 6m width is particularly helpful as it allows you to spread the kitchen and dining areas side by side rather than in a narrow line. It is a popular size for London terraced houses.
What is the cheapest extension size worth building?
A 3m × 3m (9m²) extension costs £42k–£62k all-in and is the minimum size most architects recommend. It works for a utility room, home office, or small kitchen. Below 9m² the fixed costs make the project poor value. For a kitchen-diner, 15m² (3m × 5m) is the practical minimum.
How much more does an inner London extension cost?
Inner London (zones 1–2) typically costs 10–15% more than outer London for the same extension. A 3m × 6m extension that costs £72k–£110k in outer London would cost £80k–£125k in inner London. The premium comes from higher labour rates, restricted site access, more expensive skip permits, and a higher likelihood of conservation area constraints.
Should I build 3m deep or 4m deep?
The extra metre of depth transforms the room. A 3m-deep extension is a functional kitchen; a 4m-deep extension is a comfortable kitchen-diner with separation between cooking and eating areas. The cost difference is roughly £12k–£20k all-in. However, going from 3m to 4m depth on a terraced or semi-detached house means you need prior approval instead of permitted development, adding 6–8 weeks and £120 in fees.
Summary
A 3m × 6m extension at 18m² is one of the most popular sizes in London: large enough for a proper kitchen-diner, within permitted development at 3m depth, and at a cost point (£72k–£110k all-in) that delivers strong value per square metre.
If budget is tight, a 3m × 5m at £60k–£92k is the minimum for a meaningful kitchen extension. If budget allows, a 4m × 6m at £98k–£142k gives a genuinely transformative open-plan space.
The key insight is that smaller extensions are disproportionately expensive per square metre. Building slightly larger almost always delivers better value than building small and adding expensive finishes. Start from the room you want to create, check the planning route, and work backwards to a budget.
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