Rear Extension: Planning Permission, Costs and Ideas
A complete guide to rear extensions in the UK, covering planning rules, permitted development limits, costs, and design ideas for single and double storey.
Rear extensions are the most common type of home extension in the UK - and for good reason. They extend your living space into the garden, create opportunities for open-plan layouts, and many can be built without planning permission. This guide covers everything you need to know.
Single Storey vs Two Storey
The choice between single and two storey depends on your needs, budget, and site constraints:
| Feature | Single Storey | Two Storey |
|---|---|---|
| Typical use | Kitchen-diner, living room, garden room | Kitchen + bedroom/bathroom above |
| Cost per m² | £1,800 – £3,000 | £1,500 – £2,500 (lower per m² due to shared foundations/roof) |
| PD limits (depth) | 3m (semi/terrace) or 4m (detached), up to 6m/8m with prior approval | 3m for all house types |
| Boundary distance (PD) | No specific distance (but eaves must be 3m max within 2m of boundary) | Must be 7m from rear boundary |
| Planning permission | Often not needed under PD | More likely to need PP (7m rule, conservation areas) |
| Build time | 8 – 14 weeks | 12 – 20 weeks |
For detailed PD rules, see our guides to single storey extension PD and two storey extension PD.
Permitted Development Rules for Rear Extensions
Single storey rear extensions under permitted development:
- Standard limits: 3m depth (semi/terrace) or 4m (detached), max 4m height
- Larger home extension scheme: up to 6m (semi/terrace) or 8m (detached) with prior approval. Neighbours are notified and can object, and the council has 42 days to respond
- Eaves within 2m of boundary: max 3m high
- Not in conservation area for larger scheme (standard limits still apply)
Two storey rear extensions under PD:
- Max depth: 3m from the original rear wall
- Min 7m from rear boundary
- Materials must match existing house
- Side-facing upper windows must be obscure glazed
- Not PD in conservation areas
How Much Does a Rear Extension Cost?
Costs vary significantly by size, specification, and location:
Single Storey Rear Extension Costs
- Basic (3m depth, standard finish): £25,000 – £45,000
- Mid-range (4m depth, good finishes): £40,000 – £70,000
- High spec (6m+ depth, premium finishes, large glazing): £65,000 – £120,000
Two Storey Rear Extension Costs
- Basic (3m depth): £45,000 – £75,000
- Mid-range (3m depth, good finishes): £70,000 – £110,000
- High spec: £100,000 – £160,000+
A two storey extension typically costs only 40–60% more than single storey for double the space, because the foundations, ground floor structure, and roof are shared. This makes two storey extensions excellent value if you need both ground and first floor space.
Design Ideas
Open-Plan Living
The most popular rear extension layout creates an open-plan kitchen, dining, and living area. Key design elements:
- Bi-fold or sliding doors along the garden-facing wall - creating a seamless indoor-outdoor connection
- Kitchen island - practical for cooking and socialising, and works as a visual room divider
- Vaulted ceiling - raise the ceiling height for a more spacious feel
- Roof lantern or skylights - bring light deep into the plan, especially important for deeper extensions
Garden Room / Orangery Style
For a lighter, more traditional feel, an orangery-style extension with a lantern roof and floor-to-ceiling glazing on two or three sides creates a bright, airy space that feels connected to the garden year-round.
Split Level
On sloping sites, a split-level extension can work with the natural topography, creating distinct zones at different levels - for example, a kitchen on the upper level stepping down to a garden room at ground level.
The Build Process
- Design - appoint an architect to develop designs and prepare drawings
- Planning / PD confirmation - submit an application or apply for a Lawful Development Certificate
- Building regulations - submit detailed drawings for approval
- Party wall - if building within 3m (6m for deeper foundations) of a neighbour, serve party wall notice
- Tender to builders - get at least 3 quotes. Find a contractor near you
- Construction - groundworks, foundations, walls, roof, fit-out
- Sign-off - obtain building control completion certificate
Key Considerations
Neighbours
Even if your extension is permitted development, maintaining a good relationship with neighbours is important. Discuss your plans before construction starts, and consider the impact on their light and privacy.
Drainage
Many rear extensions cross or come close to existing drainage runs. If you are building over or within 3 metres of a public sewer, you will need a build-over agreement from your water company.
Trees
If mature trees are close to the extension, their roots may affect foundations and their canopy may affect light. Check for tree preservation orders and consider the Root Protection Area when positioning foundations.
Access for Construction
Rear extensions can be challenging for access - all materials and equipment must reach the back of the house. Narrow side passages, shared access paths, and mid-terrace properties can all increase costs. Discuss access with your builder at the quoting stage.
Value Added
A well-designed rear extension typically adds 5–10% to your home's value for a single storey, and 10–15% for two storeys. In many areas, the value added exceeds the build cost, making this one of the best investments you can make in your property.
Planning Applications for Rear Extensions
If your extension exceeds PD limits or you are in a conservation area, you will need to submit a householder planning application. The good news is that householder applications for rear extensions have a high approval rate - typically 85-90% nationally - because the impact on the streetscape is minimal and the design parameters are well understood.
Key factors the planning officer will consider:
- Impact on neighbours - will the extension reduce their light or privacy?
- Design and materials - does it complement the existing house?
- Scale - is it proportionate to the house and plot?
- Compliance with local plan policies - separation distances, parking, garden space retention
You can check how your local council typically decides applications for rear extensions by searching on Planning Signal - look at similar applications in your area to see what has been approved or refused.
Common Questions
Can I build right up to the boundary?
Under PD, a single storey extension can be built up to the boundary line. However, if you are within 2 metres of the boundary, the eaves height is limited to 3 metres. If you need planning permission, the council may require you to set the extension back from the boundary - check your local plan's policies on separation distances.
Do I need a party wall agreement?
If you are building within 3 metres of a neighbour's building (or 6 metres for deeper foundations), you must serve a party wall notice under the Party Wall Act 1996. This is a legal requirement regardless of whether planning permission is needed. The notice must be served at least 2 months before work begins.
How long will construction take?
A single storey rear extension typically takes 8 to 14 weeks from breaking ground to completion. A two storey extension takes 12 to 20 weeks. The main stages are: groundworks and foundations (1-2 weeks), walls (2-4 weeks), roof (1-2 weeks), first fix (plumbing, electrics, plastering - 2-4 weeks), and second fix (kitchen, bathroom, decoration - 2-4 weeks).
A rear extension is the most straightforward way to create more living space. With clear PD rules, predictable costs, and proven designs, it is the go-to project for families who need more room.