Detached House Extension London 2026: Costs, 8m PD Rules & Design Guide
Detached houses have the most generous permitted development rights of any house type. You can extend up to 8m at the rear under prior approval, build on both sides, and go to two storeys at the back. Here are the real 2026 costs, what planning allows, and how to design an extension that works with the house.
Quick Answer
Detached house extensions cost £65k–£220k+ inc VAT in London depending on type. Single-storey rear (4m×6m) costs £82k–£118k, side extension £65k–£95k, two-storey rear £140k–£220k. Detached houses get the most generous PD rights: 4m depth without any application, 8m with prior approval.
£82k–£118k
Rear 4m×6m (popular)
£65k–£95k
Side 3m×6m
10–25%
Value added
Check your specific property constraints
Free Property CheckThe Permitted Development Advantage
Detached houses benefit from the most generous permitted development (PD) allowances under the Town and Country Planning (General Permitted Development) Order. This is because they sit in their own plot with no shared walls, so the impact on neighbours is lower.
| PD allowance | Detached | Semi / Terraced |
|---|---|---|
| Rear depth (no application) | 4m | 3m |
| Rear depth (prior approval) | 8m | 6m |
| Side extension | Single storey, max half width of house | Same rule |
| Two-storey rear | 3m depth, 7m from rear boundary | 3m depth, 7m from rear boundary |
| Max site coverage | 50% of curtilage | 50% of curtilage |
The practical result: a detached house can add a single-storey rear extension up to 4m deep with zero planning paperwork. Under the Neighbour Consultation Scheme (prior approval), that increases to 8m. Most London detached houses have the plot size to take full advantage of this.
Key condition: These PD rights only apply if permitted development has not been removed by an Article 4 direction, the house is not listed, and it is not in a conservation area. Many detached houses in outer London boroughs have full PD rights, but check before assuming.
Extension Types for Detached Houses
Detached houses offer more options than any other house type because you can extend on three or even four sides. Here are the five most common configurations.
Single-storey rear extension
The most popular choice. Creates a large kitchen-diner or open-plan living space across the full width of the rear. Up to 4m depth under PD or 8m with prior approval. Flat roof with skylights or a roof lantern is standard.
Side extension
Uses the space between the house and the boundary. Common on detached houses with wide plots. Adds a utility room, home office, extra reception room, or widens the kitchen. Must not exceed half the width of the original house under PD.
Wrap-around extension
Combines side and rear into one L-shaped or U-shaped extension. Maximises ground floor space. Particularly effective on detached houses where you can extend on both sides and the rear simultaneously.
Two-storey rear extension
Adds two rooms stacked: kitchen-diner below, bedroom or bathroom above. Under PD, limited to 3m depth and must be at least 7m from the rear boundary. The most cost-efficient way to add significant floor area because the roof and foundations serve two floors.
Over-garage extension
Builds a room above an existing attached or integral garage. Often creates a bedroom, home office, or en-suite. Structural work is usually straightforward if the garage walls can support the load. Typically needs planning permission as it extends the front or side of the house upward.
2026 Costs by Extension Type
All figures include VAT at 20%. Construction cost is the build contract only. All-in adds architect, structural engineer, building regulations, party wall (where applicable), and a 10% contingency.
| Extension type | Typical size | Construction | All-in |
|---|---|---|---|
| Single-storey rear | 4m × 6m (24m²) | £68k–£98k | £82k–£118k |
| Side extension | 3m × 6m (18m²) | £54k–£79k | £65k–£95k |
| Wrap-around (side + rear) | 30–40m² | £92k–£142k | £110k–£170k |
| Two-storey rear | 3m × 6m per floor (36m²) | £118k–£184k | £140k–£220k |
| Over-garage | 3m × 6m (18m²) | £50k–£80k | £60k–£96k |
Cost per m² in London 2026
All figures inc VAT at 20%. Two-storey extensions cost less per m² because the foundations and roof serve both floors. Inner London (zones 1–2) typically runs 10–15% above these figures.
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Planning Permission: When PD Applies and When It Does Not
Three planning routes are available for extending a detached house. The route depends on the extension type, depth, and your location.
Route 1: Permitted development (no application)
You can build without any planning application if all of these conditions are met:
- Single-storey rear: maximum 4m depth from original rear wall
- Maximum eaves height 3m if within 2m of boundary, overall height 4m maximum
- Side extension: single storey only, must not exceed half the width of the original house
- No more than 50% of the curtilage (area around the original house) covered by extensions and outbuildings
- Materials similar in appearance to the existing house
- Not in a conservation area, not a listed building, no Article 4 direction in place
Route 2: Neighbour Consultation Scheme (prior approval)
For detached houses this extends the single-storey rear allowance from 4m to 8m depth. The process:
- Up to 8m depth from the original rear wall (vs 6m for terraced and semi-detached)
- Single storey only, same height limits as standard PD (4m max, 3m eaves if within 2m of boundary)
- All other PD conditions still apply (coverage, materials, no conservation area)
- Submit a prior approval application to the council (£120 fee)
- Council notifies adjoining neighbours, who have 21 days to comment
- Council has 42 days to decide. They can only consider impact on amenity of adjoining neighbours
- If no decision within 42 days, you can proceed
Practical note: An 8m rear extension on a detached house creates 40–60m² of floor space depending on width. That is enough for an entire open-plan ground floor. Very few projects actually go to 8m because garden size and proportion become an issue, but the allowance gives significant headroom for design.
Route 3: Full planning permission
Full planning is required when:
- Two-storey extensions that exceed PD limits (depth over 3m or within 7m of rear boundary)
- Over-garage extensions (extending above the original roofline at the front or side)
- Property in a conservation area, listed, or subject to Article 4 direction
- Total extensions already cover more than 50% of the curtilage
Full planning permission takes 8–10 weeks and costs £528 in application fees.
Not sure which planning route applies to your detached house? Our AI checks your address, extension type, and conservation area status instantly.
Check now — it's free →The 50% Rule: Curtilage Coverage
Under permitted development, extensions and outbuildings combined cannot cover more than 50% of the curtilage. The curtilage is the land around the original house as it was built (or as it stood on 1 July 1948, whichever is later).
This matters more for detached houses than you might expect. Garages, sheds, conservatories, and any previous extensions all count toward the 50% calculation. A detached house with an existing double garage and a large shed may already be at 30–40% coverage before any new extension.
Tip: If you are close to the 50% limit, removing an existing outbuilding (like a garage or shed) before building the extension can free up allowance. Alternatively, full planning permission is not bound by the 50% rule, though the council will still consider the impact of site coverage on the character of the area.
Side Extension Rules for Detached Houses
Side extensions are one of the most useful options for detached houses because the space either side of the house is often underused. But PD rules impose specific limits.
Width: maximum half the original house
A side extension under PD cannot be wider than half the width of the original house. For a house 8m wide, the maximum side extension width is 4m. This is measured from the widest point of the original house.
Height: 4m max, 3m eaves within 2m of boundary
If the side extension wall is within 2m of the property boundary, the eaves cannot exceed 3m. The overall height is capped at 4m. This effectively limits most side extensions to a flat roof or very low pitched roof.
Single storey only
Under PD, side extensions must be single storey. A two-storey side extension always requires full planning permission. Councils are often cautious about two-storey side extensions because of the “terracing effect”—closing the gap between detached houses so the street appears terraced.
No forward of the principal elevation
The side extension cannot project forward of the front wall of the original house facing a highway. This prevents extensions from dominating the streetscape.
Party Wall: Yes, Even for Detached Houses
A common misconception is that detached houses do not trigger the Party Wall Act 1996. They can and frequently do. The Act applies in three situations:
- Section 6: Excavating foundations within 3m of a neighbouring structure (or 6m if the excavation goes below the bottom of the neighbour's foundations). On a typical London plot, side extension foundations are almost always within 3m of a neighbour's wall or outbuilding.
- Section 1: Building a new wall on or at the boundary line. If your extension wall sits on the property boundary, you must serve notice.
- Section 2: Carrying out work to an existing party wall or party fence wall. Shared boundary walls (even garden walls) can be party fence walls.
Typical cost for detached house: £800–£1,400 per neighbour. A side extension near one boundary might involve only one neighbour. A large wrap-around near both boundaries could involve two.
Timing: Serve notice at least 1 month before work starts (2 months for excavation notices). Start the process as soon as you have drawings.
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Design Considerations for Detached Houses
Detached houses present design opportunities and challenges that terraced and semi-detached houses do not. The extension is visible from multiple sides, which raises the bar for design quality.
Maintaining symmetry
Many detached houses have a symmetrical front elevation. A side extension on one side disrupts this. Architects often use set-back or set-down techniques (stepping the extension back from the front wall and setting the roofline lower) to signal that the extension is secondary to the original house.
Roof form matching
For two-storey and side extensions that are visible from the street, matching the roof form (hip, gable, or half-hip) to the existing house makes the extension look intentional. Flat roofs are fine at the rear but can look incongruous on a visible side elevation.
Garage integration
Where a detached house has an attached garage, extending above or alongside the garage is a natural design move. An over-garage extension can look well-integrated if the roof pitch and materials match. Where the garage is integral, converting it to living space and extending the footprint sideways creates a seamless addition.
Material matching
Because a detached house extension is visible from multiple angles, material matching is more critical than on a terraced house where only the rear is visible. Brickwork, render, tile, and window proportions all need to be considered. Some architects use a deliberate material contrast (such as zinc cladding or render against brick) as a design feature, but this works best with a strong architectural concept.
Building Regulations
All extensions require building regulations approval regardless of whether planning permission is needed. Building control inspects at key stages and issues a completion certificate on sign-off.
Foundations
Strip foundations are standard for single-storey extensions. Typical depth in London clay is 900mm–1.2m. Trees near the extension (common with detached houses that have mature gardens) may require deeper foundations to account for clay shrinkage. Building control will inspect the trenches before the concrete pour.
Structural openings
Creating the opening between the existing house and the extension requires a structural steel beam (RSJ). A structural engineer sizes the beam and submits calculations to building control. Two-storey extensions require more substantial steelwork and may need internal structural modifications on the first floor.
Insulation (2026 standards)
- Walls: U-value 0.18 W/m²K or better
- 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
Fire safety (two-storey extensions)
A two-storey extension that creates a new first-floor room requires mains-wired smoke detection and a protected escape route. If the new room is an inner room (accessed only through another room), additional fire safety measures may be needed including fire doors and escape windows.
Drainage
Detached houses often have private drainage runs around the property. If building over or near a public sewer (within 3m), you need a build-over agreement with Thames Water. Your architect or drainage consultant checks this during design.
Common Mistakes
Ignoring the 50% curtilage rule
Detached houses with existing garages, sheds, and conservatories can be closer to the 50% limit than expected. Measure all existing structures before designing the extension. If you exceed 50% coverage, the extension falls outside PD and needs full planning permission.
Assuming party wall does not apply
Detached does not mean exempt from the Party Wall Act. If your foundations are within 3m of any neighbouring structure—including their garage, garden wall, or extension—you must serve notice. Failing to do so gives your neighbour the right to seek an injunction stopping work.
Not accounting for tree roots
Detached houses often have more trees than terraced properties. Mature trees near the extension footprint can require deeper foundations (up to 2.5m in shrinkable London clay) or root protection barriers. If the tree is subject to a Tree Preservation Order (TPO), you cannot remove it and must design around it.
Building a side extension that looks like a separate house
Under PD, side extensions must use materials similar to the existing house. But even within PD, a large side extension with no design relationship to the original house looks awkward and reduces value rather than adding it. Invest in architectural design to ensure the extension reads as part of the original house.
Measuring permitted development depth from the wrong point
The 4m (or 8m under prior approval) is measured from the original rear wall, not from an existing single-storey addition, conservatory, or outrigger. If the house has already been extended, the original rear wall determines the baseline. Check your council's planning portal for the original footprint.
Realistic Timeline
Design and drawings
3–6 weeksArchitect site visit, measured survey, planning and building regulations drawings. Detached houses often need more drawing detail because the extension is visible from multiple sides.
Prior approval (if 4–8m depth)
6–8 weeksSubmit to council, neighbour notification period, decision
Full planning (if required)
8–12 weeksTypical for two-storey, over-garage, and conservation area extensions
Building regulations
3–5 weeksFull plans submission, runs in parallel with planning or prior approval
Party wall process (if needed)
1–3 monthsFaster than terraced houses because there are fewer neighbours. Runs in parallel but must start early.
Construction on site
10–24 weeksSingle-storey rear or side: 10–14 weeks. Wrap-around: 14–18 weeks. Two-storey: 16–24 weeks. Larger projects on detached houses benefit from better site access, which can speed up build times.
Total project
4–10 monthsSingle-storey under PD: 4–5 months. Prior approval with party wall: 5–7 months. Two-storey with full planning: 7–10 months.
Frequently Asked Questions
How much does a detached house extension cost in London in 2026?
All costs inc VAT: single-storey rear (4m×6m) £82k–£118k all-in, side extension (3m×6m) £65k–£95k, wrap-around £110k–£170k, two-storey rear £140k–£220k, over-garage £60k–£96k. Cost per m² is £3,000–£5,500 for single-storey and £2,600–£3,800 for two-storey.
Can I extend a detached house 8m under permitted development?
Yes, under the Neighbour Consultation Scheme (prior approval), detached houses can extend up to 8m from the original rear wall. This is single-storey only, the same height limits apply (4m max, 3m eaves if within 2m of boundary), and all other PD conditions must be met. You submit a prior approval application (£120 fee) and the council has 42 days to decide.
Do I need planning permission for a side extension on a detached house?
A single-storey side extension usually falls under permitted development if it does not exceed half the width of the original house, is no higher than 4m (3m eaves within 2m of boundary), and does not project forward of the front wall. A two-storey side extension always requires full planning permission.
Does the Party Wall Act apply to detached houses?
Yes. The Party Wall Act 1996 is triggered when you excavate foundations within 3m of a neighbouring structure, build on the boundary line, or work on a shared boundary wall. Detached houses frequently trigger the Act for side extensions near a neighbour’s garage or garden wall. Budget £800–£1,400 per affected neighbour.
What is the 50% curtilage rule for extensions?
Under permitted development, extensions and outbuildings cannot cover more than 50% of the curtilage (the area around the original house). Existing garages, sheds, and conservatories all count. If you exceed 50%, you need full planning permission. Removing an existing outbuilding can free up allowance.
Can I build a two-storey extension on a detached house without planning permission?
A two-storey rear extension up to 3m depth can qualify for permitted development if it is at least 7m from the rear boundary, uses similar materials, and the eaves match the existing house height. In practice, the 7m rear boundary rule is difficult to meet on most London plots, so most two-storey extensions require full planning.
How long does a detached house extension take?
Total project time: 4–5 months for a single-storey under permitted development, 5–7 months with prior approval and party wall, 7–10 months for a two-storey with full planning. Construction on site ranges from 10–14 weeks (single-storey) to 16–24 weeks (two-storey). Detached houses usually have better site access, which helps build speed.
Is it worth extending a detached house in London?
Yes. Detached house extensions in London typically add 10–25% to property value. A well-designed extension that creates open-plan living, adds a bedroom, or integrates the garage often delivers a return exceeding the construction cost. Detached houses in high-demand outer London boroughs see particularly strong returns.
Summary
Detached houses are the easiest house type to extend in London. You get the most generous PD allowances (4m without any application, 8m with prior approval), you can build on both sides, and site access is better for construction. The trade-off is that the extension is visible from multiple angles, so design quality and material matching matter more than on a terraced house.
Start by understanding the 50% curtilage rule and the planning route for your specific project. Then invest in an architect who understands how to integrate the extension with the character of the original house.
This completes our property type series alongside the terraced house and semi-detached house guides. Each house type has different PD limits, party wall considerations, and design constraints. Know your house type and you are halfway to a successful project.
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