03 / 04 House extension types
Double-Storey Extensions
A double-storey extension, also called a two-storey extension, adds space on two floors in one build. It usually means a larger kitchen, dining or living room on the ground floor and one or two extra bedrooms or a bathroom above. It is the biggest single step up in space most houses can make without moving.
We design and build two-storey extensions as part of our house extensions service, covering the foundations, structure and roof tie-in through to the finished rooms on both floors.
- Part ofHouse Extensions
- Our process8 clear steps
- Answered here6 common questions
- FreeSurvey & itemised quote
01Types
Types of Double-Storey Extension
Double-Storey Rear Extension
The extension is built across the back of the house on both floors. It suits semi-detached and detached houses with a reasonable garden depth. It can also work on some terraces, depending on the effect on neighbours.
Two-Storey Side Extension
The extension is built on the side of a semi-detached or detached house, over a driveway, garage or side garden. It often adds a garage or utility room and a larger kitchen downstairs, with a bedroom and en-suite above. Councils usually expect it to be set back from the front of the house and to have a lower roof, so it looks secondary to the original house.
Combined Side and Rear
On a corner or wide plot, the extension may wrap around the side and rear on both floors. These designs need careful planning, because of their size and the effect on neighbours.
02Cheaper per square metre
Is a Double-Storey Extension Cheaper per Square Metre?
Usually, yes. A double-storey extension shares one set of foundations and one roof between two floors, and these are among the most expensive parts of any extension. Scaffolding, design fees and approvals are also shared. So although the total cost is higher than a single-storey extension, the cost of each square metre of new floor space is usually lower. Building both floors at once also avoids the disruption and extra cost of adding a second storey later.
03Planning
Do I Need Planning Permission for a Two-Storey Extension?
Usually, yes. A two-storey rear extension can sometimes be permitted development on a house in England, but only within tight limits:
- Depth: No more than 3 metres beyond the original rear wall.
- Distance from the rear boundary: At least 7 metres.
- Near side boundaries: Within 2 metres of a boundary, the eaves must be no higher than 3 metres.
- Roof and materials: The roof pitch should match the original house, and the materials should look similar.
- Side windows: Upper-floor side windows must be obscure-glazed and non-opening below 1.7 metres.
Two-storey side extensions are not permitted development and always need planning permission. The same applies to flats and maisonettes, listed buildings, conservation areas and Article 4 areas.
When deciding an application, councils look closely at the effect on neighbours. Many use the 45-degree rule: a line drawn at 45 degrees from the nearest neighbouring window should not be crossed by the extension. They also consider overlooking, the design and the space left to the boundary. We prepare the drawings and planning application with these tests in mind.
04Structure
Structure, Foundations and Roof Tie-In
A two-storey extension carries more load than a single-storey one, so the foundations, steelwork and walls are designed by a structural engineer to suit the ground conditions. On clay or near trees, deeper or piled foundations may be needed. The new roof must tie into the existing roof neatly and weathertight. That means matching tiles, pitch and ridge lines, often with a lower ridge on side extensions. Openings are formed in the existing walls on both floors, supported by steel beams.
05Party wall and regulations
Party Wall and Building Regulations
Where the extension is built on or near the boundary, or its foundations are dug close to a neighbour's house, notices are needed under the Party Wall etc. Act 1996. The extension also needs building regulations approval, covering:
- foundations and structure
- fire safety, including escape from the new first-floor rooms
- insulation and glazing to energy standards
- ventilation
- drainage
- new electrical circuits
Building control inspects the work at key stages and issues a completion certificate.
06Cost
What Affects the Cost of a Double-Storey Extension in London?
Every extension is priced after a free survey, and you receive a written, itemised quote. The main factors are:
Every factor is priced line by line after your free survey — no guesswork, staged payments.
Book a free survey- 01
Size and depth on both floors.
- 02
Rear, side or combined design.
- 03
Foundations and ground conditions
Including piling.
- 04
Steelwork to open up both floors of the existing house.
- 05
Roof tie-in and matching materials.
- 06
Kitchen, bathroom and en-suite fit-out.
- 07
Glazing
Rear doors, windows and rooflights.
- 08
Fees
Drawings, planning, structural calculations, building control and party wall surveyors.
Timescale · 07
How Long Does a Two-Storey Extension Take?
A two-storey extension takes longer than a single-storey one because there are two floors to build and finish, and the roof has to be tied into the existing house. Before building starts, allow time for design, planning permission, building regulations approval and party wall notices. The new structure is built and closed in before the existing walls are opened up, which keeps the house weathertight. Your quote includes a programme for each stage.
One team · 08
One Team for Both Floors
A double-storey extension usually includes a kitchen downstairs and a bedroom or bathroom upstairs, so we coordinate every trade on both floors:
- tiling, plastering and decorating
If you only need more ground-floor space, see our kitchen extension service.
09Process
Our Double-Storey Extension Process
Planned before we start, managed by one project manager, signed off at the end.
- 01
Free survey
We look at the house, the plot, the boundaries, neighbouring windows and access.
- 02
Design and quote
Drawings, structural calculations and an itemised quote with a programme.
- 03
Approvals
Planning application, building regulations approval and party wall notices.
- 04
Groundworks
Foundations dug and inspected, and drains laid.
- 05
Structure and roof
Ground- and first-floor walls, floor, roof and tie-in, closed in and weathertight.
- 06
Opening up and first fix
Openings formed on both floors, then wiring, plumbing and heating.
- 07
Finishing
Plastering, kitchen and bathroom fitting, tiling, flooring and decorating.
- 08
Sign-off and handover
Building control completion, certificates and a final walk-round.
10Areas
Double-Storey Extensions Across London
We build double-storey extensions across London, with a focus on east and south-east London, including the semis and detached homes of Orpington and Ilford. Find out more on our pages for double-storey extension in London, double-storey extension in Orpington, double-storey extension in Ilford, double-storey extension in Barking, double-storey extension in Dagenham and double-storey extension in East Ham.
11FAQs
Double-Storey Extension FAQs
How much does a double-storey extension cost?
It depends mainly on the size on both floors, the foundations, the steelwork, the roof tie-in and the kitchen and bathroom fit-out. It costs more in total than a single-storey extension but usually less per square metre. We give you a fixed, itemised quote after a free survey.
Do I need planning permission for a two-storey extension?
Usually, yes. Two-storey side extensions always need permission. A two-storey rear extension can only be permitted development if it is no more than 3 metres deep and at least 7 metres from the rear boundary, among other limits.
Is a double-storey extension cheaper per square metre?
Usually, yes. The foundations and roof, two of the most expensive elements, are shared between two floors. Scaffolding, fees and approvals are also shared.
How long does a two-storey extension take?
Longer than a single-storey extension, because there are two floors to build and finish and a roof to tie in. Design, planning and party wall notices come first. Your quote includes a programme.
What is the 45-degree rule?
It is a test many councils use to protect neighbours' light. A line drawn at 45 degrees from the nearest ground- or first-floor window next door should not be crossed by the extension.
Can I stay in the house during a two-storey extension?
Usually, yes. The extension is built and made weathertight before the existing walls are opened up. Some rooms, such as the kitchen, may be out of use for part of the build.
04House Extensions
Explore other house extension types
Next step
Book Your Free Survey
Tell us how much space you need on each floor. We will survey the house, explain what your council is likely to allow and send you a written, itemised quote.
Send us photos of the back and side of your house on WhatsApp to get started, or use our contact form.
- Free survey
- Itemised written quote
- One project manager