Do I need planning permission for a single storey rear extension?

Usually not. Most single-storey rear extensions up to 4m deep (8m for detached) fall under Permitted Development. However, PD rights can be removed in conservation areas, National Parks, AONBs, and for listed buildings. A Certificate of Lawfulness (£1,500) provides legal proof.

How much does a single storey extension cost in 2026?

In Dorset, expect £1,800–£2,800 per m². A typical 4m x 6m (24m²) extension costs £43,000–£67,000 for the build alone. Add £2,000–£5,000 for professional fees, £8,000–£25,000 for a kitchen, and 15% contingency.

How long does a single storey extension take to build?

The build itself takes 10–16 weeks. Add 4–12 weeks beforehand for design, planning (if required), structural calculations, and builder procurement. Total timeline from first contact to completion: 4–7 months.

Can I build a single storey extension myself?

The design, structural calculations, and Building Regulations submission need professional input. The build itself requires experienced tradespeople — foundations, steelwork, roofing, and electrics are not DIY tasks. Our design service (£250) gives you professional plans to take to any builder.

Does a rear extension need a party wall agreement?

If the extension is within 3m of your neighbour's property (or 6m for foundations deeper than the neighbour's foundations), you need to serve a Party Wall Notice under the Party Wall Act 1996. This is not planning permission — it is a separate legal requirement.

What roof type is best for a single storey extension?

Flat roofs are the most common and cost-effective. A pitched roof looks more traditional but adds cost and may affect PD compliance (height limits). Lantern roofs and skylights add natural light at modest extra cost (£2,000–£5,000).

Single Storey Rear Extension: Complete Guide in New Forest

Q: Do I need planning permission for a single storey rear extension: complete guide in New Forest?

It depends on your specific property. Much of the New Forest falls within the New Forest National Park, which has its own planning authority (NFNPA). Properties inside the National Park have significantly reduced Permitted Development rights. Properties outside the park boundary but within the district follow standard PD rules. New Forest District Council approves approximately ~85% of householder applications. We assess your project and property designations before recommending the planning route — many New Forest homeowners find they only need a Certificate of Lawfulness (£1,500).

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. Here is what New Forest homeowners — covered by New Forest District Council — need to know about costs, planning, and timelines.

Local cost range — New Forest

£2,000–£3,000/m²

New Forest properties — especially those inside the National Park — require premium natural materials to comply with NFNPA design guidelines. Expect 10–20% higher build costs than the Dorset average.

Single Storey Rear Extension: Complete Guide rules in New Forest

New Forest falls under New Forest District Council (New Forest District Council). Much of the New Forest falls within the New Forest National Park, which has its own planning authority (NFNPA). Properties inside the National Park have significantly reduced Permitted Development rights. Properties outside the park boundary but within the district follow standard PD rules.

Planning applications (annual)

~2,500

Average decision time

8 weeks

National Park coverage

Significant

Approval rate

~85%

Fixed prices, no surprises

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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 in New Forest?

Local New Forest rates: £2,000–£3,000/m². The table below shows national Dorset figures for reference.

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. Local New Forest range: £2,000–£3,000/m².

Planning permission in New Forest

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.

New Forest District Council — local planning context

Much of the New Forest falls within the New Forest National Park, which has its own planning authority (NFNPA). Properties inside the National Park have significantly reduced Permitted Development rights. Properties outside the park boundary but within the district follow standard PD rules.

New Forest District Council processes approximately ~2,500 planning applications per year, with an approval rate of ~85%. Average decision time is 8 weeks.

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