Extension Size Cost Guide: Every Common Dimension Priced for London 2026
How much does a 3m x 5m extension cost? What about 4m x 4m, or 6m x 6m? Here are the real London prices for every common extension size in 2026, with planning implications included.
Quick Answer
In London 2026, a 3m x 5m (15m²) rear extension costs £45,000–£70,000 inc VAT. A 4m x 5m (20m²) costs £60,000–£92,000 inc VAT. A 6m x 6m (36m²) costs £108,000–£165,000 inc VAT. All prices are for construction only. Add 15–20% for architect fees, structural engineer, and contingency.
£45k–£70k
3m × 5m
£60k–£92k
4m × 5m
£108k–£165k
6m × 6m
Check your specific property constraints
Free Property CheckHow Extension Size Translates to Cost
Extension costs in London are priced per square metre (m²). In 2026, a typical single-storey rear extension costs £3,000–£4,600 per m² including VAT: budget spec at the lower end, high-spec glazing and finishes at the upper end. To calculate the cost of any size, multiply the floor area by the m² rate.
A 3m x 5m extension has a floor area of 15m². At a mid-range London rate of £3,700/m² inc VAT, that is £55,500 for construction. Add architect fees, structural engineer, and a 10% contingency and the total budget is typically £68,000–£80,000.
Important: All construction prices on this page include VAT at 20%. They do not include architect fees (typically 8–12% of build cost), structural engineer (£1,500–£3,000), party wall surveyor if needed (£1,000–£2,500), planning fees (£258 for householder application), or kitchen/fit-out costs. See our guide on hidden costs of London extensions for the full picture.
London Extension Cost by Size (2026)
The table below shows construction costs for every common extension size in London, all prices include VAT. The range spans budget spec (basic finishes, flat roof) to high-spec (bifolds, underfloor heating, premium materials).
| Size | Floor Area | Build Cost | Planning |
|---|---|---|---|
| 3m × 3m | 9m² | £32,000 – £48,000 | Usually PD |
| 3m × 4m | 12m² | £38,000 – £58,000 | Usually PD |
| 3m × 5m | 15m² | £45,000 – £70,000 | Usually PD |
| 3m × 6m | 18m² | £54,000 – £83,000 | Usually PD |
| 4m × 4m | 16m² | £48,000 – £74,000 | PD or PP* |
| 4m × 5m | 20m² | £60,000 – £92,000 | PD or PP* |
| 4m × 6m | 24m² | £72,000 – £110,000 | PD or PP* |
| 5m × 4m | 20m² | £60,000 – £92,000 | Usually PP |
| 5m × 5m | 25m² | £75,000 – £115,000 | Usually PP |
| 5m × 6m | 30m² | £90,000 – £138,000 | PP required |
| 6m × 6m | 36m² | £108,000 – £165,000 | PP required |
| 6m × 8m | 48m² | £144,000 – £220,000 | PP required |
*PD = Permitted Development (no planning application needed). PP = Planning Permission required. Highlighted rows are the most commonly searched sizes. See planning section below for depth rules.
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Planning Permission: The Depth Rules Explained
The depth of your extension (how far it extends from the back of the house) determines whether you need planning permission. This is the most important sizing decision you will make.
Terraced & Semi-Detached Houses
Detached Houses
London note: These are the national Permitted Development rules. In London, additional restrictions apply if your property is in a conservation area, Article 4 direction zone, or is a listed building. Many inner London properties have Article 4 directions that remove PD rights entirely. Check with your borough before assuming PD applies. See our guide to permitted development rights in London.
What "Depth" Means in Practice
The depth is measured from the original rear wall of the house, not the existing extension (if you have one). So a 3m x 5m extension has a depth of 3m, comfortably within PD limits for a terraced house. A 5m x 5m extension has a depth of 5m — it would need Prior Approval for a terraced house, or Planning Permission depending on other factors.
The width (how far the extension spans across the back of the house) has fewer restrictions under PD, though it must not extend beyond the side walls of the house.
Most Common Extension Sizes: Full Cost Breakdown
3m × 5m Rear Extension
Floor area: 15m² (the most popular size for terraced houses)
A 3m depth means this is Permitted Development for terraced and semi-detached houses. No planning application needed. The 5m width suits a standard terraced house kitchen-diner. This is the sweet spot for maximum space with minimum planning hassle.
Planning: Permitted Development for terraced/semi-detached. Prior Approval may apply under Neighbour Consultation Scheme in some boroughs.
3m × 6m Rear Extension
Floor area: 18m² (maximum width for most terraced houses)
At 6m wide, this extension typically spans the full width of the rear of a terraced house. Still within PD depth limits at 3m. Creates a generous kitchen-diner with room for a dining table, sofa, and kitchen island.
Planning: Permitted Development at 3m depth for terraced/semi-detached houses.
4m × 4m Rear Extension
Floor area: 16m² (popular for detached houses within PD limits)
A 4m depth is always PD for detached houses, and requires Prior Approval (not full planning) for terraced and semi-detached under the Neighbour Consultation Scheme. Adds a substantial kitchen-diner or extended living room.
Planning: PD for detached houses. Prior Approval needed for terraced/semi-detached at 4m depth.
4m × 5m Rear Extension
Floor area: 20m² (common for semi-detached and detached houses)
One of the most popular sizes for semi-detached houses. Creates a proper open-plan kitchen-diner with a generous dining area. At 4m deep, detached houses can build this under PD; semi-detached and terraced houses need Prior Approval.
Planning: PD for detached. Prior Approval for terraced/semi-detached. Full PP if in conservation area.
6m × 6m Rear Extension
Floor area: 36m² (large single-storey, planning permission required)
A 6m depth requires full planning permission for all property types. This is a large single-storey that can accommodate a complete open-plan kitchen, dining, and living area. Often combined with a side return for a wrap-around effect.
Planning: Full planning permission required. Budget 8–10 weeks for application plus 8-week determination period.
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Check now — it's freeSide Return Extension Costs by Size
Side return extensions fill the narrow passage beside terraced houses. They are measured differently: the key dimension is the depth of the infill (typically 3–5m) and the width of the side return passage (typically 1–1.8m).
| Side Return Size | Floor Area | Build Cost |
|---|---|---|
| Small (3m × 1.2m) | ~4m² | £38,000 – £48,000 |
| Medium (4m × 1.5m) | ~6m² | £48,000 – £62,000 |
| Full run (5m × 1.8m) | ~9m² | £62,000 – £80,000 |
| Side return + rear combo | 15–25m² | £90,000 – £145,000 |
Why side returns cost more per m²: Despite their smaller footprint, side returns involve complex structural work: removing the original side wall, installing RSJ steelwork, and managing drainage in the narrow passage. Expect £6,000–£9,000 per m² rather than the standard £3,400–£4,600/m² for open rear extensions.
What Else Affects the Cost Beyond Size?
Glazing specification
Bifold or sliding doors cost £800–£1,500 per m² of glass. A full-width glazed rear can add £15,000–£25,000 to the build. Standard French doors are £2,500–£4,500 and much more cost-effective.
Roof type
Flat roofs are standard and least expensive. A pitched roof matching the house adds 10–15%. A roof lantern or skylight adds £3,000–£8,000 but dramatically improves natural light. See our guide on roof lanterns vs skylights.
Ground conditions
Clay soil (common in inner London) requires deeper foundations, adding £3,000–£8,000. Proximity to trees adds further cost. Drainage diversion can add £2,000–£5,000 if pipes cross the footprint.
Borough and access
Inner London boroughs (Islington, Hackney, Camden) add 8–12% due to parking restrictions, skip permits, and access challenges. Conservation areas require matching materials, adding 5–10%. See our full London area cost breakdown.
Structural complexity
Opening up the existing rear wall to create open-plan flow requires RSJ steelwork, typically £3,000–£6,000 depending on span. Larger openings cost more. This is not optional if you want an open-plan result.
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Frequently Asked Questions
How much does a 3m x 5m extension cost in London?
A 3m x 5m (15m²) rear extension in London costs £45,000–£70,000 for construction in 2026; all prices include VAT. Budget spec (flat roof, basic finishes) starts around £45,000; high-spec (bifolds, underfloor heating) reaches £70,000. The total project budget including architect fees, structural engineer, party wall surveyor (if needed), and 10% contingency is typically £70,000–£80,000 at mid-range. This size is Permitted Development for terraced and semi-detached houses at 3m depth.
How much does a 3m x 6m extension cost in London?
A 3m x 6m (18m²) rear extension in London costs £54,000–£83,000 for construction in 2026 including VAT. Mid-range spec costs £66,000–£72,000. The all-in budget including professional fees and contingency is typically £84,000–£95,000. At 3m depth this is Permitted Development for terraced and semi-detached houses. No planning application required.
How much does a 4m x 5m extension cost in London?
A 4m x 5m (20m²) rear extension in London costs £60,000–£92,000 for construction in 2026 including VAT. Mid-range spec costs £73,000–£80,000. Total all-in budget is typically £90,000–£103,000 before kitchen costs. At 4m depth, detached houses can build this under Permitted Development. Terraced and semi-detached houses need Prior Approval under the Neighbour Consultation Scheme.
How much does a 4m x 4m extension cost in London?
A 4m x 4m (16m²) rear extension in London costs £48,000–£74,000 for construction in 2026 including VAT. Mid-range spec costs £58,000–£64,000. All-in budget is typically £74,000–£85,000. For detached houses, 4m depth is the maximum under Permitted Development. No planning permission needed. Terraced/semi-detached houses need Prior Approval at this depth.
What is the average cost of a 3m rear extension in London?
A 3m deep rear extension in London (typically 3m x 4m–6m wide) costs £38,000–£83,000 for construction including VAT, depending on width and specification. The most common 3m x 5m size costs £45,000–£70,000 inc VAT. All 3m-depth extensions are within Permitted Development limits for terraced and semi-detached houses.
What is the average cost of a side return extension in London?
A side return extension in London costs £38,000–£80,000 for construction depending on length and specification. Side returns cost more per m² (£6,000–£9,000/m²) than open rear extensions because of the complex structural work involved. A combined side return and rear extension costs £90,000–£145,000.
Do I need planning permission for a 3m extension?
No. A 3m depth rear extension is within Permitted Development limits for terraced and semi-detached houses, meaning no planning application is required. You may still need to notify your local council via the Neighbour Consultation Scheme (Prior Approval). In conservation areas or Article 4 zones, PD rights may be restricted. Always check with your borough.
Do I need planning permission for a 4m extension?
It depends on your house type. For detached houses, 4m depth is within Permitted Development limits. No planning permission needed. For terraced and semi-detached houses, 4m depth requires Prior Approval under the Neighbour Consultation Scheme (not a full planning application). In conservation areas, full planning permission is required regardless of depth.
Summary: Extension Size and Cost at a Glance
- 3m depth = always PD for terraced and semi-detached. Maximum 6m wide for most properties.
- 4m depth = PD for detached, Prior Approval for terraced/semi-detached.
- Over 6m depth = planning permission required for all house types.
- Budget £3,000–£4,600/m² inc VAT for construction, then add 20–25% for all professional fees and contingency.
- Conservation area? Always check with your borough. PD rights may not apply regardless of size.
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