A rear extension is the most common home improvement project in the UK. It extends the back of the house into the garden, adding floor space that is typically used to enlarge a kitchen, create an open-plan kitchen-diner, or add a new reception room. Unlike loft conversions, rear extensions are visible from the garden but virtually invisible from the street — which is why planning authorities treat them more generously than front or side additions.
This guide explains the difference between single and double storey options, gives realistic 2026 costs, maps out the Permitted Development rules, and covers the glazing and design choices that make the most of a rear extension.
Single Storey vs Double Storey Rear Extension
The fundamental choice is whether to go up as well as out.
A single-storey rear extension sits on the ground floor only, typically with a flat, lean-to, or pitched roof. It is cheaper to build, faster to complete, and available under more generous permitted development rules. It suits households that primarily need more kitchen or living space.
A double-storey rear extension adds a first-floor room (usually a bedroom, bathroom, or home office) above the ground-floor extension. The extra floor costs significantly less than the ground floor because it shares the foundations, walls, and roof — making it much better value per square metre when you also need upstairs space.
| Feature | Single Storey | Double Storey |
|---|---|---|
| Typical size | 12–30 m² floor area | 24–60 m² combined |
| Build cost (London) | £2,200–£4,500/m² | £1,800–£3,500/m² (combined) |
| Build cost (Midlands/North) | £1,900–£3,200/m² | £1,500–£2,800/m² (combined) |
| PD rear projection (terrace/semi) | Up to 6 m | Up to 3 m only |
| PD rear projection (detached) | Up to 8 m | Up to 3 m only |
| Planning application required? | Usually not | Often yes, or Prior Approval |
| Construction programme (site) | 10–16 weeks | 16–24 weeks |
Rear Extension Cost (2026)
Full costs including professional fees for a typical project:
| Project type | London & SE | Midlands / North |
|---|---|---|
| Single storey, 12 m² (small kitchen extension) | £26,000–£42,000 | £19,000–£31,000 |
| Single storey, 25 m² (large open-plan) | £52,000–£90,000 | £38,000–£65,000 |
| Double storey, 25 m² per floor | £70,000–£130,000 | £50,000–£95,000 |
These figures include structural engineer (£800–£1,800), architect/designer (£3,000–£8,000), building control (£900–£1,600), and VAT at 20%. Kitchen fit-out and internal decoration are excluded. A quality bi-fold or sliding door set adds £4,500–£12,000 depending on width and glazing specification.
Permitted Development Size Limits
In England, rear extensions can be built under Permitted Development rights without a full planning application, subject to strict conditions under Schedule 2, Part 1, Class A of the GPDO 2015.
Projection limits from the original rear wall:
- Terraced and semi-detached houses: 6 m single storey, 3 m double storey
- Detached houses: 8 m single storey, 3 m double storey
The 6 m and 8 m limits for single-storey extensions are subject to the Neighbour Consultation Scheme (sometimes called Prior Approval) if they exceed 4 m (detached) or 3 m (other). You notify the council; neighbours have 21 days to object; the council has 42 days to respond. If no objection is upheld, works can proceed.
Height limits:
- Maximum eaves height: 3 m where within 2 m of a boundary
- Maximum overall height for single-storey: 4 m (dual-pitch) or 3 m (flat/lean-to)
- Double-storey maximum height: must not exceed the existing ridge height of the original house
Other conditions:
- Extension must not project beyond the side elevation of the original house
- Cladding materials must be of similar appearance to the existing dwelling
- Combined extensions must not cover more than 50% of the original curtilage
- PD rights may be removed by Article 4 directions in conservation areas — check with your LPA
If your proposed extension falls outside these limits, a full householder planning application is required (fee: £258 in England). Double-storey extensions at the rear almost always require planning if the house is in a conservation area, an AONB, or if the design departs significantly from the host building.
Structural Considerations
Rear extensions require a structural design for:
- Strip or pad foundations: typical depth 900 mm–1,200 mm in clay soils; up to 2,000 mm near trees (heave risk). A structural engineer will specify based on a soils report.
- Steel beams (RSJs): opening up the rear wall of the existing house almost always requires a steel beam. Typical sizes for a 4 m opening: 203 × 133 × 25 kg/m UB; for 5–6 m: 254 × 146 × 31–37 kg/m. Padstone design and temporary support propping the floors above is part of the structural package.
- Roof structure: flat roofs use a warm-deck build-up (insulation above deck) with a single-ply membrane or GRP finish; pitched roofs use engineered timber rafters. Building Regulations Part A governs.
- Lintels and openings in party walls for semi-detached houses may require Party Wall Act notices — see the planning section above.
Always engage a structural engineer before tendering; their design drawings allow contractors to price accurately.
Glazing and Rooflights
The rear wall is where you make the architectural statement. Common options:
Bi-fold doors: fold flat against jambs to create a full-width opening. Typical width 3–6 m; 3-panel set costs £4,500–£7,500; 5-panel set costs £8,000–£13,000 (aluminium, thermally broken frames, double-glazed). Require a stiff concrete floor slab as a running track base.
Sliding doors: slimmer frame profiles than bi-folds; better for narrow openings (2–4 m); large 3-panel systems cost £6,000–£10,000.
Full-width glazed screen with pivot/hinged door: architectural look, fixed glass panels with a single or double door leaf. Allows very slim sightlines (30–50 mm) with structural glass or Schüco-type aluminium systems; budget £10,000–£20,000 for a 4 m run.
Rooflights: flat or lean-to roofs benefit greatly from rooflights. Fixed frameless flush units give a cleaner look than opening skylights; a 1,000 × 2,000 mm unit costs £900–£1,800 installed. Building Regulations Part L requires total glazed area to not exceed 25% of the new floor area (though rooflights are treated more generously than vertical glazing in some compliance pathways).
Design Ideas
Kitchen-diner: the most popular outcome. Remove the rear wall, span with steel, and create a 5–8 m wide open kitchen and dining space running from front to back. Rooflights over the old rear wall line avoid a dark junction between old and new.
Garden room / snug: a single-storey extension that is fully glazed on three sides acts as a garden room in summer and a warm snug in winter. Works particularly well on south- or west-facing rear elevations.
Bedroom and en-suite above: a double-storey extension on a detached house can add a large master bedroom suite above the extended kitchen — without touching the existing internal configuration.
Study or home office: a 10–15 m² single-storey extension dedicated to a home office, acoustically separated from the main house, is increasingly popular since 2020.
Building Regulations (Key Parts)
All rear extensions, whether PD or not, require Building Regulations approval:
- Part A: structural design of foundations, beams, and roof
- Part B: fire escape maintained from all habitable rooms above ground
- Part L: new walls U-value ≤ 0.28 W/m²K, roof ≤ 0.18 W/m²K, glazing ≤ 1.6 W/m²K (whole unit)
- Part F: adequate ventilation in new and altered spaces
- Part P: new electrical circuits certified by a Part P registered electrician
Submit a Full Plans application to your local authority building control or an Approved Inspector (now registered building control approver under the Building Safety Act 2022 regime). Fee: £900–£1,600 depending on project value and LPA.
Programme Overview
| Stage | Typical Duration |
|---|---|
| Architect design and planning / Prior Approval | 8–14 weeks |
| Building Regulations approval | 5–8 weeks |
| Groundworks and foundations | 2–3 weeks |
| Structural frame and roof | 3–5 weeks |
| Windows, doors, external finish | 2–3 weeks |
| First fix M&E, insulation, plasterboard | 2–3 weeks |
| Second fix, tiling, decoration | 3–4 weeks |
| Total site time (single storey) | 12–18 weeks |
Double-storey projects add 4–8 weeks of site time. Allow 7–12 months elapsed from first meeting to practical completion on a straightforward project.
