A rear extension in 2026 costs £1,500–£2,500 per m² outside London, or £2,000–£3,500/m² in the capital. A 3m-deep extension starts from £20,000–£35,000. A 4m-deep extension costs £30,000–£50,000. Under prior approval, a 6m-deep extension costs £45,000–£75,000, and an 8m-deep extension on a detached house runs £60,000–£100,000. Rear extensions are the cheapest extension type per m² because the structure is simple and site access is usually good.
Cost by Depth
Rear extension costs depend heavily on how far you go into the garden. These prices assume a typical house width of 5–6m and include the build to plastered shell with basic finishes.
3m deep (15–18m²)
Maximum PD depth for semi-detached and terraced. A solid kitchen extension or dining room. 8–10 weeks build. No planning application needed.
4m deep (20–24m²)
Maximum PD depth for detached houses. The classic kitchen-diner size. Room for a table, bi-folds, and a proper layout. 10–12 weeks.
6m deep (30–36m²)
Maximum prior approval depth for semi/terraced. Serious open-plan living — kitchen, dining, and seating area. £120 fee, 42-day process. 12–14 weeks.
8m deep (40–48m²)
Maximum prior approval for detached houses. A major ground-floor transformation. Full-width open-plan with kitchen island, dining, and living zone. 14–16 weeks.
3m × 3m (9m²)
The minimum practical extension — a utility room, boot room, or study. Cheapest option. Flat roof, standard window, PD-compliant. 8 weeks.
Full-width 8m (48m²+)
The maximum you can get without full planning. Needs careful design — structural steels, multiple roof lanterns, full heating system extension. A genuine house transformation.
Permitted Development & Prior Approval
Rear extensions are the extension type most likely to qualify for permitted development (PD), which means no planning application, no fees (or minimal fees), and no 8-week wait. Here's exactly what you can build:
Standard Permitted Development (no application needed)
- Semi-detached or terraced: Up to 3m from the original rear wall
- Detached: Up to 4m from the original rear wall
- Maximum height: 4m to the ridge (pitched roof), 3m to the eaves
- Flat roof maximum: 3m height
- Must not cover more than 50% of the original garden (including all previous extensions, outbuildings, and sheds)
- No application or fee required — you can apply for a Lawful Development Certificate (£132) if you want written confirmation
Prior Approval (larger home extension scheme)
- Semi-detached or terraced: Up to 6m from the original rear wall
- Detached: Up to 8m from the original rear wall
- Fee: £120 (England, 2026)
- Process: Submit a notification to your council. They consult adjoining neighbours, who have 21 days to comment. If no objections, approval is granted after 42 days. If objections, the council decides.
- Must be single storey only
- Maximum height: 4m (same as standard PD)
- Cannot be used in conservation areas, listed buildings, or AONB
💡 Prior approval vs full planning — the savings
Prior approval costs £120 and takes 42 days. Full planning permission costs £462 and takes 8–12 weeks. If you can design your extension within the prior approval limits, you save £342 and several weeks. More importantly, prior approval is rarely refused unless there's a genuine impact on amenity — it's a much more predictable process than full planning.
Important restrictions: PD rights can be removed by Article 4 directions (common in conservation areas). If your property has been converted from a flat, maisonette, or commercial building, PD rights may not apply. If you've already extended using PD, the combined extensions must still meet the depth and area limits. Always check with your council's planning department before assuming PD applies.
Flat Roof vs Pitched Roof
This is one of the biggest design and cost decisions for a rear extension. Both work — the right choice depends on your budget, the look you're going for, and what planning allows.
Flat roof (EPDM or GRP)
- Cost: £80–£120 per m² of roof area
- Lifespan: 25–30 years for EPDM rubber; 20–25 years for GRP fibreglass
- Pros: Cheaper, faster to install, modern look, works brilliantly with rooflights and roof lanterns, lower profile keeps it under PD height limits
- Cons: Shorter lifespan than tiles, slight risk of ponding if fall is insufficient, some people don't like the aesthetic
- Best for: Modern extensions, extensions with large roof lanterns, budget-conscious builds
Pitched tile roof
- Cost: £120–£180 per m² of roof area
- Lifespan: 50–80 years for concrete tiles, 80–100+ years for clay or slate
- Pros: Matches the existing house, longer lifespan, better resale appeal in traditional areas, more loft space above
- Cons: More expensive, takes longer to build, higher profile (may need planning), harder to incorporate large rooflights
- Best for: Extensions visible from the street, conservation areas, houses where a flat roof would look incongruous
💰 The cost difference in practice
On a typical 20m² rear extension, the flat roof option saves £500–£1,500 compared to pitched. On a 36m² extension, the saving grows to £1,000–£2,500. Many homeowners choose a flat roof with a large roof lantern (£2,500–£5,000) — the total is still cheaper than a pitched roof, and you get a flood of natural light.
Foundations Near Drains & Sewers
Rear gardens are where the drains live. Most UK houses have their main drain run going straight down the rear garden to the public sewer. Before designing your extension, you need to know exactly where these drains are — and plan around them.
Build Over Agreement
If your extension footprint crosses or comes within 3m of a public sewer (a sewer maintained by the water company), you need a Build Over Agreement. This is not the same as a private drain — a CCTV drain survey will help clarify what's private and what's public.
- Application: Free to apply (Thames Water, Severn Trent, etc.)
- Timeline: 6–8 weeks for approval — start this early
- What they require: Your foundations must bridge the sewer (not load onto it). You'll need an engineer to design the foundations accordingly, which may mean deeper or wider footings, or piled foundations near deep sewers.
- CCTV survey: The water company will usually require a pre-build CCTV survey of the sewer (£250–£400). They may require a post-build survey too.
Diverting drains
If the drain position makes it impractical to build over, you may need to divert it around the extension footprint.
- Diverting a private drain: £1,500–£3,500 — your builder can do this with building regs approval
- Diverting a public sewer: £3,000–£8,000+ — must be done by or approved by the water company
- Moving the soil stack: £2,000–£4,000 — if your extension swallows the existing soil pipe
🔧 Builder's tip: drain survey first
Before you spend money on architect's drawings, get a drain survey (£250–£400). This maps every drain, manhole, and connection in your garden. Design the extension footprint around the drains wherever possible — it's always cheaper to adjust the plan by 300mm than to divert a sewer. A survey also reveals whether you're dealing with a private drain (simpler) or a public sewer (more bureaucracy).
Regional Pricing (Per m²)
These per-m² rates are for a standard-specification rear extension — blockwork cavity walls, flat or pitched roof, standard windows, plastered and ready for decoration. Kitchen, premium glazing, and luxury finishes are extra.
What Affects the Price
Depth of the extension
Going deeper isn't a simple multiplier. A 6m extension is not double a 3m extension because you still have the same rear wall opening, the same connection to the house, and similar drainage. But deeper means more foundations, more floor slab, more roof, and a wider steel span. Expect costs to increase at roughly 70–80% per additional metre of depth.
Foundation conditions
Standard strip foundations cost £150–£200 per linear metre. Clay soil near trees needs deeper foundations — sometimes 2m+ deep on shrinkable clay near mature oaks. Piled foundations: £1,500–£3,000 per pile. Near drains: specialised bridging foundations. A trial hole (£300–£500) before quoting saves nasty surprises.
Roof type
Flat EPDM roof: £80–£120/m² — cheapest and fastest. Pitched tile roof: £120–£180/m² — longer lasting, better kerb appeal. Zinc standing seam: £200–£350/m² — premium aesthetic. A flat roof with a roof lantern is the most popular choice for rear extensions. Saves £500–£2,500 versus pitched.
Glazing package
The glazing on a rear extension is often the single biggest visible cost. Bi-fold doors (3m): £2,500–£5,000. Bi-fold doors (4m): £4,000–£7,000. Sliding doors (3m): £3,000–£5,500. Roof lantern: £2,500–£5,000. Individual rooflights: £800–£1,500 each. Glazing typically accounts for 15–25% of the total build cost.
Structural steelwork
Opening up the rear wall needs a steel beam (RSJ). A simple 3m opening: £800–£2,000. A 4m opening: £1,500–£3,500. A goalpost frame for full-width bi-folds: £2,500–£5,500. Removing the entire rear wall to create seamless open-plan: £3,000–£8,000 in steels alone.
Drainage and sewer proximity
Building over a public sewer: build-over agreement (free, 6–8 weeks) but foundation design is more complex. Diverting drains: £1,500–£8,000 depending on private or public. Moving the soil stack: £2,000–£4,000. The closer your extension is to existing drains, the more it costs.
Width of the extension
A narrow 3m-wide extension is structurally simpler than a full-width 6m extension. Full width means a larger steel span, more foundation, and more external wall. Going full-width adds 20–30% over a partial-width extension of the same depth.
Internal specification
Basic plaster and paint: £15–£25/m². Engineered oak flooring: £50–£80/m². Polished concrete floor: £80–£130/m². Underfloor heating: £50–£70/m². A handleless kitchen with quartz worktops adds £10,000–£20,000+ on top of the build. Fit-out can be 20–30% of total cost.
Site access
Good rear access (wide gate, driveway to the back): materials come straight in, mini-digger for foundations, concrete pump from the road. No side access (terrace house, no gate): everything goes through the house or over the top. Adds £2,000–£6,000 in labour, protection, and skip logistics.
Time of year
Winter builds (November–February) can save 5–10% — builders have more availability and may sharpen prices. Groundwork is slower in wet weather, and concrete pours need frost protection below 5°C. But the overall savings usually outweigh the minor inconvenience.
How to Save Money on a Rear Extension
💡 Practical cost-cutting strategies
- Stay within permitted development. A 3m extension on a semi (or 4m on a detached) costs nothing in planning fees and avoids weeks of delay. If 3m is enough space, don't push to 4m just because you can.
- Use prior approval for bigger extensions. Going to 6m or 8m? Prior approval costs £120 and takes 42 days — much cheaper and faster than full planning (£462, 8–12 weeks). Design to the prior approval limits wherever possible.
- Choose a flat roof. Saves £500–£2,500 over a pitched roof, builds faster, and works perfectly with a roof lantern. Most rear extensions suit a flat roof — the extension sits under the existing first-floor windows.
- Design around the drains. Moving your extension footprint 300mm to avoid a drain saves £1,500–£4,000 in diversion costs. Get a drain survey first (£250–£400) and design around what you find.
- Standard-size bi-folds. A 3-panel 2.4m set is a stock size at £2,500–£4,000. Going bespoke adds 30–50%. Even better: large sliding doors are often cheaper than bi-folds and thermally superior.
- Keep the soil stack where it is. Don't relocate the main soil pipe — it's expensive and creates complications. Design the kitchen and utility layout to work with the existing plumbing run.
- Get three itemised quotes. Not "£40,000 for the job." Line-by-line breakdowns showing steelwork, foundations, glazing, roof, electrics, plumbing. This exposes where one builder is overpricing and another has forgotten something.
- Do your own demolition and clearance. Knocking down an old conservatory, removing a patio, and filling the first skip saves £1,000–£3,000 in labour.
- Shell and fit out separately. Get the builder to complete the watertight shell with basic electrics and plumbing. Then fit the kitchen yourself, lay your own flooring, and paint yourself. The shell costs the same regardless — you're saving on the premium trades.
What Should Be in a Builder's Quote
Every rear extension quote should clearly include or exclude the following. If any of these are missing, ask before signing:
- Groundwork and foundations — excavation, concrete, DPM, drainage connections, ground-floor slab
- Structural work — blockwork, steels, lintels, cavity insulation, wall ties, DPC
- Roof — structure, insulation, covering (flat EPDM/GRP or pitched tiles), fascias, soffits, guttering
- Windows and doors — including bi-folds or sliders, frame colour, handle specification
- Rooflights or lantern — if applicable, make/model should be specified
- First and second fix electrics — sockets, switches, downlights, consumer unit upgrade
- First and second fix plumbing — radiators or UFH, hot and cold supply extensions
- Plastering — walls and ceiling throughout
- Decoration — mist coat and two coats emulsion (often excluded — always ask)
- Floor screed — ready for tiling or flooring (the flooring itself is usually excluded)
- Drainage — connections to existing system, any new manholes or inspection chambers
- Scaffolding — even single storey may need a scaffold tower for the roof
- Skip hire and waste removal — typically 2–4 skips for a rear extension
- Building regs compliance — compliance itself, though the council fee is usually separate
Common exclusions: Kitchen fitting and appliances, flooring, tiling, landscaping, building regs fees (£300–£900), party wall costs, architectural and structural engineer fees, decoration, external landscaping. Clarify everything upfront so there are no surprises at final account.
Hidden Costs to Budget For
Building Regulations
Full plans application required for every rear extension. The fee depends on size and your council. Some builders include this — most don't.
Structural Engineer
Steel beam calculations, foundation design, structural report for building regs. Essential for any opening wider than a doorway.
Architectural Drawings
Design, planning drawings (if needed), and building regs package. A straightforward rear extension is at the lower end.
Drain Survey
CCTV drain survey to map all drainage under the extension footprint. Essential before finalising the design. Saves thousands in drain diversion costs.
Party Wall Surveyor
Needed if digging within 3m of a neighbour's structure (6m if deeper). Your neighbour can appoint their own surveyor at your cost.
Planning / Prior Approval
Prior approval: £120 (42 days). Full planning: £462 (8–12 weeks). If your extension is within PD limits: £0. Lawful Development Certificate: £132 (optional written confirmation).
Build Over Agreement
Free to apply but takes 6–8 weeks. You'll need a CCTV survey pre and post build. If the sewer must be diverted: add £3,000–£8,000.
Kitchen Fitting
Almost always excluded from the builder's quote. A separate budget item. Supply-only kitchens from £3,000; installation from £2,000. Premium kitchens with appliances: £15,000+.
Garden Reinstatement
New patio or decking, re-turfing, fencing, planting. The garden always suffers during a build — budget for putting it back together.
Frequently Asked Questions
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