Single Storey Rear Extension: Complete Guide 2026
A single-storey rear extension is the most common home improvement project in the UK — and for good reason. It adds usable living space, connects your home to the garden, and typically falls under Permitted Development. This guide covers everything from costs to completion.
What you need to know
What is a single-storey rear extension?
A single-storey rear extension projects from the back of your house at ground-floor level. It typically creates a larger kitchen-diner, an open-plan living space, or a garden room. The most popular sizes range from 3m to 6m deep, though Permitted Development allows up to 4m (or 8m for detached homes under the Larger Home Extension scheme).
Design options
The three most popular layouts are: (1) a full-width extension across the entire rear wall, creating one large open-plan space; (2) a partial-width extension leaving a courtyard or side access; and (3) a side-return extension on Victorian terraces that fills the narrow alley alongside the kitchen. Roof options include flat roofs (most common), pitched roofs, and lantern roofs for extra light.
Structural considerations
Most rear extensions require removing or partially removing the existing rear wall, which is typically load-bearing. This means a steel beam (RSJ) is needed to support the weight above. Foundations typically need to be 1m deep, though tree proximity and clay soil can require deeper foundations. A structural engineer calculates the exact requirements.
The build process
A typical rear extension build follows this sequence: (1) excavate and pour foundations; (2) build walls to damp-proof course; (3) complete walls and install the roof structure; (4) install windows and make weathertight; (5) first fix (plumbing, electrics, insulation); (6) plastering; (7) second fix (sockets, lights, kitchen); (8) decoration and snagging. Allow 10–16 weeks for the build itself.
Adding value to your home
A well-designed rear extension typically adds 5–10% to your property value, often exceeding the cost of the work. The key to maximising value is creating genuine usable space (not just extra square footage) and ensuring quality finishes. A kitchen-diner that flows to the garden is the single most desirable feature for buyers.
How much does it cost?
| Item | Typical Cost Range |
|---|---|
| Build cost (per m²) | £1,800–£2,800 |
| 4m x 5m (20m²) extension | £36,000–£56,000 |
| 4m x 6m (24m²) extension | £43,000–£67,000 |
| 6m x 6m (36m²) extension | £65,000–£100,000 |
| Professional fees (design, planning, structural) | £2,000–£5,000 |
| Kitchen (if included) | £8,000–£25,000 |
| Party wall agreements (per neighbour) | £700–£1,500 |
| Building control fees | £400–£800 |
| Contingency (15%) | £5,000–£15,000 |
Prices based on Dorset rates, 2026
Planning permission
Most single-storey rear extensions in Dorset fall under Permitted Development, meaning no planning application is needed. The key conditions are: maximum depth of 4m from the original rear wall (8m for detached under Prior Approval), maximum eaves height of 3m, and no higher than the existing roof. If any condition is not met, or if your property is in a conservation area, you will need full planning permission.
Under Class A of the GPDO (General Permitted Development Order), you can extend up to 4m from the original rear wall (semi-detached/terraced) or 8m (detached, subject to Prior Approval). The extension must not exceed 4m in height, must not be within 2m of the boundary at a height above 2.5m, and must not cover more than 50% of the curtilage.
Building regulations
Building Regulations approval is always required, regardless of whether planning permission is needed. Key requirements include: foundations to a minimum of 1m depth, cavity wall insulation achieving current U-values (0.18 W/m²K for walls), adequate drainage, structural adequacy of any new openings, fire safety provisions, and electrical certification.
Typical timeline
| Phase | Duration |
|---|---|
| Design and planning | 2–8 weeks |
| Building Regulations drawings | 2 weeks |
| Builder procurement | 2–4 weeks |
| Foundation and substructure | 2–3 weeks |
| Superstructure and roof | 3–4 weeks |
| First fix and insulation | 2–3 weeks |
| Second fix and finishing | 3–4 weeks |
| Snagging and sign-off | 1–2 weeks |
Frequently asked questions
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