Two-Storey Rear Extensions: Design, Approval, Build

What you get

A two-storey rear extension suits you if you need more room on both floors and have a reasonably deep garden. You get one team for the whole job: a measured survey, the design, a check on whether permitted development applies, planning drawings where they are needed, structural design, building regulations approval and the build itself, carried out by vetted teams working under S&K. Every project starts with a free site visit.

It suits you if…

You need a bigger kitchen and an extra bedroom, and the garden can give up a few metres
Your house has a plain rear wall with room to build out without blocking a neighbour's light
You would rather extend than move, and want new space on both floors
You want to know whether permitted development could avoid a planning application
Your loft is too shallow to convert, so building out is the better way to add a bedroom

Line drawing: existing house in outline, new work in brick, blue dashed line at first-floor level.

Design points that matter

Depth and neighbours' light

Councils check that a two-storey rear extension does not take too much light or outlook from neighbouring windows, and many use a 45-degree line drawn from the nearest habitable window. We test this at sketch stage, so the depth you are shown is one that can realistically be approved.

Roof form

A hipped or gabled roof that matches the pitch of the main house usually looks best, and matching the pitch is a permitted development condition in England. Keeping the ridge below the main ridge makes the extension read as secondary and avoids awkward junctions with existing gutters and windows.

Upstairs layout and windows

Bedrooms above need a window big enough for escape or a protected route to the stairs. Side-facing upstairs windows normally have to be obscure-glazed and fixed below 1.7 m, so we put the main bedroom windows in the rear wall and use roof lights where extra daylight is needed.

Opening up the ground floor

Knocking through the old rear wall means steel beams to carry the first floor and roof above. The structural engineer sets the beam sizes, padstones and any posts, and we plan them early so the open-plan layout you want is the one that gets built.

Foundations and drains

Foundations for two storeys carry more load than a single storey and may need to go deeper near trees or on clay. Rear extensions often sit near drains, so we check sewer records early, as building over or near a public sewer may need the water company's agreement.

Planning and approvals by nation

England

In England, a two-storey rear extension can be permitted development if it goes no more than 3 m beyond the rear wall of the original house, stays at least 7 m from the rear boundary and matches the house's materials and roof pitch. It is not permitted development in conservation areas, National Landscapes, National Parks, World Heritage Sites or the Broads. Otherwise a householder application costs £548 (or £575 for applications made from 8 December 2026).

Approval in England: Householder planning application: £548 (or £575 for applications made from 8 December 2026). Lawful development certificate: £274 (about £288 for applications from 8 December 2026). Councils should decide householder applications within 8 weeks of validation.

Building regulations: Building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end.

Wales

In Wales, a two-storey rear extension can be permitted development if the ground floor goes no more than 4 m and the first floor no more than 3 m beyond the original rear wall, with the first floor at least 10.5 m from the rear boundary. Nothing within 2 m of a boundary can be higher than 4 m. Conservation areas and other protected areas are excluded, and a householder application costs £585.

Approval in Wales: Householder planning application: £585. Councils normally decide householder applications within 8 weeks.

Building regulations: Building regulations approval under the Welsh Approved Documents, which differ from England's in places (notably energy efficiency).

Scotland

In Scotland, a two-storey rear extension is only permitted development under Class 1B if no part of it is within 10 m of any boundary, it is no higher than the house and the house is not in a conservation area. Most gardens cannot meet the 10 m test, so expect a householder application (£371, decided within two months) and a building warrant before work starts.

Approval in Scotland: Householder planning application: £371. Councils have two months to decide a local planning application.

Building standards: A building warrant from the council's building standards service before work starts, and a completion certificate at the end. Scotland uses its own Technical Handbooks.

Northern Ireland

In Northern Ireland, a two-storey rear extension can be permitted development if it goes no more than 3 m beyond the original rear wall and stays at least 7 m from the rear boundary, with conditions on height, materials and roof pitch. Upper side windows within 15 m of a neighbouring boundary must be obscure-glazed and non-opening. Otherwise a planning application costs £347 per dwelling.

Approval in Northern Ireland: Planning application for works to a house: £347. Councils aim to decide local applications within an average of 15 weeks; many take longer.

Building control: Building control approval from the local council under the Building Regulations (Northern Ireland), using the Technical Booklets.

Guide price

As a guide, a two-storey rear extension costs £56,000–£75,500 including VAT at 20% for a 4 m × 3.5 m footprint on two floors (28 m² of new floor space), based on West Midlands prices and our Standard spec; London and the South East are higher.

Guide prices by region
Region	Per m² of new floor	Typical project
London	£2,400–£3,240 per m²	£67,000–£90,500
South East England	£2,200–£2,970 per m²	£61,500–£83,000
East of England	£2,060–£2,780 per m²	£57,500–£78,000
South West England	£2,060–£2,780 per m²	£57,500–£78,000
West Midlands	£2,000–£2,700 per m²	£56,000–£75,500
East Midlands	£1,960–£2,650 per m²	£55,000–£74,000
North West England	£1,900–£2,560 per m²	£53,000–£72,000
Yorkshire and the Humber	£1,840–£2,480 per m²	£51,500–£69,500
North East England	£1,800–£2,430 per m²	£50,500–£68,000
Wales	£1,840–£2,480 per m²	£51,500–£69,500
Scotland	£1,880–£2,540 per m²	£52,500–£71,000
Northern Ireland	£1,760–£2,380 per m²	£49,500–£66,500

Typical project: a 4 m × 3.5 m footprint on two floors (28 m² of new floor space). Including VAT at 20%, Standard spec. Essential spec is about 8% less, Premium about 20% more. See all prices and fees or work out your own guide price.

What's included

Free site visit
Measured survey, priced in writing with the design stage
Design options with guide costs
Permitted development check and lawful development certificate where it applies
Planning drawings and submission where permission is needed
Structural engineering and beam design
Building regulations drawings and approval
Party wall notices in England and Wales where needed
The build, carried out by vetted teams working under S&K

How it works

Free site visit
We look at your house, the boundaries, access and drains, and talk through what you need the new space to do.
Design and approval
Measured survey and drawings, then planning permission or a lawful development certificate, depending on the rules where you live.
Engineering and fixed price
Structural calculations and building regulations approval, then a fixed build price in writing.
Build and sign-off
Vetted teams working under S&K build it, and we arrange the completion certificate.

Questions about two-storey rear extensions

How far can a two-storey rear extension go out?

It depends on your nation and on whether you use permitted development. In England and Northern Ireland, permitted development allows up to 3 m beyond the original rear wall, at least 7 m from the rear boundary. In Wales the first floor can go 3 m and the ground floor 4 m. With planning permission, the council judges depth on light, outlook and design.

Do I need a lawful development certificate for a rear extension?

A lawful development certificate is not compulsory, but it is the safest way to prove a two-storey rear extension is permitted development. In England it costs £274 (about £288 for applications from 8 December 2026). We recommend one before you commit to a build, and a buyer's solicitor may ask for it when you sell.

Can I have a balcony on a two-storey rear extension?

In England, balconies, verandas and raised platforms are not permitted development, so a balcony on a two-storey rear extension needs planning permission. A Juliet balcony, a guard rail across full-height doors with no platform to stand on, is often a practical alternative, but it still needs checking against the rules and the council's approach to overlooking.

Will a two-storey rear extension block my neighbour's light?

It can if it is too deep or too close to the boundary, which is why councils look closely at its effect on neighbouring windows and gardens. We test the design against light and outlook early, keep the first floor shorter or set in where needed, and use a hipped roof to reduce bulk on the boundary side.

How long does a two-storey rear extension take to build?

The build itself usually takes 14 to 20 weeks once work starts, or 16 to 24 weeks for a larger or more complex design. Before that, allow 4 to 8 weeks for survey and design, 8 weeks for a planning decision in England or Wales, and time for building regulations approval. Most projects take roughly 9 to 12 months from first visit to completion.

Related services

Part single, part two-storey extensions

A part single, part two-storey extension is an extension where the ground floor is wider or deeper than the floor above, so only part of it rises to two storeys.

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Kitchen extensions with bedrooms above

A kitchen extension with bedrooms above is a two-storey extension designed so the ground floor becomes a larger, usually open-plan kitchen and dining space, with one or two bedrooms or a bathroom on the first floor.

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Planning permission for two-storey extensions

Getting planning permission for a two-storey extension involves checking whether permitted development applies, preparing drawings and supporting information, submitting a householder application to the council and dealing with questions until a decision is made.

Two-storey rear extensions