Do I need planning permission for a two storey extension?

Almost certainly. While PD allows some two-storey rear extensions up to 3m deep with strict conditions, the majority of two-storey projects require a full planning application. We assess your property and advise on the best route.

How much does a two storey extension cost?

In Dorset, a two-storey extension costs roughly 50–60% more than a single-storey on the same footprint (not double, because you share foundations and roof). Budget £50,000–£120,000 depending on size and specification.

How long does a two storey extension take?

The build takes 14–20 weeks. Add 12–16 weeks for design, planning, structural calculations, and builder procurement. Total from start to finish: 7–10 months.

Is a two storey extension worth it vs single storey?

If you need space on both levels (e.g. bigger kitchen + extra bedroom), a two-storey extension offers significantly better value per square metre. The shared foundations and roof mean the upper floor costs roughly 40% of a standalone single-storey.

Can I build a two storey extension on a bungalow?

Yes, but this is effectively adding a full first floor and always requires planning permission. It is a significant structural undertaking that needs careful design, particularly regarding roof structure and foundations.

Two Storey Extension: Planning, Cost & Design Guide in New Forest

Q: Do I need planning permission for a two storey extension: planning, cost & design 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 two-storey extension doubles the space you gain compared to a single-storey extension on the same footprint — and it costs significantly less than twice as much, because you share foundations and a roof. The trade-off is that you almost always need planning permission. Here is what New Forest homeowners — covered by New Forest District Council — need to know about costs, planning, and timelines.

Two Storey Extension: Planning, Cost & Design 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 two-storey extension?

A two-storey extension adds space at both ground and first floor level. At ground floor, you typically gain a larger kitchen-diner or family room. Upstairs, you add a bedroom, bathroom, or home office. The most common size is 3m–4m deep across part or all of the rear wall.

Planning permission for two-storey extensions

Two-storey extensions almost always require planning permission. Permitted Development does allow some two-storey rear extensions up to 3m deep, but only if the extension is at least 7m from the rear boundary and does not extend beyond the rear wall of the original house by more than 3m. In practice, most homeowners need a full planning application.

Design considerations

The upper floor must be set in from the boundary by at least 2m (often more for planning). Windows must avoid overlooking neighbours. Roof pitch should match the existing house for visual harmony. The extension should look like it was always part of the house, not an afterthought. We design for planning compliance from the outset.

Structural requirements

Foundations for a two-storey extension are deeper and wider than single-storey (typically 1m+ deep, 600mm+ wide). The ground floor structure must support the upper floor loads. Party wall considerations are more significant because the extension is closer to the boundary at height.

How much does it cost in New Forest?

Item	Typical Cost Range
Build cost (per m² of ground floor)	£1,500–£2,200
3m x 5m (30m² total)	£50,000–£80,000
4m x 6m (48m² total)	£75,000–£120,000
Professional fees	£4,000–£8,000
Planning application fee	£258
Party wall (per neighbour)	£700–£1,500
Contingency (15%)	£8,000–£18,000

Prices based on Dorset rates, 2026.

Planning permission in New Forest

A full householder planning application is required for most two-storey extensions. The application fee is £258 (paid to the council). Our fee of £2,200 covers all drawings, the design and access statement, submission, and liaison with the planning officer through to decision.

Permitted Development allows two-storey rear extensions up to 3m deep, but only if: the extension does not extend beyond the rear wall by more than 3m, the eaves do not exceed the height of the existing eaves, the ridge does not exceed the existing ridge, the extension is at least 7m from the rear boundary, and the property is not in a designated area.

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 for two-storey extensions are more demanding than single-storey. Additional requirements include: fire escape windows or protected staircase, sound insulation between floors, and more stringent structural calculations for the additional loadings.

Typical timeline

Phase	Duration
Design	2–4 weeks
Planning application	8–12 weeks
Building Regs and structural	2–3 weeks
Builder procurement	2–4 weeks
Build	14–20 weeks
Snagging and sign-off	1–2 weeks