What is a side extension?
A side extension builds into the space beside a semi-detached or detached house, often over a driveway or side passage. It can be single-storey, for a utility room, downstairs toilet or study, or two-storey to add a bedroom above.
Side extensions are visible from the street, so councils pay close attention to how they look. Two-storey side extensions are usually set back from the front wall and stepped down at the roof so the house does not join up with its neighbour.
What we handle
- Survey of the boundary, drains and access
- Planning drawings and application
- Structural calculations
- Foundations and drainage alterations
- Walls, roof and windows to match the house
- Opening into the existing house
- Electrics, heating and plastering
- Building control completion
Options to consider
Single-storey side
Often used for a utility, toilet or study. A single-storey side extension up to half the width of the house may be permitted development in England.
Two-storey side
Adds a bedroom above. Usually set back and with a lower ridge to stay subordinate to the house.
Garage replacement
An old side garage can be rebuilt as habitable rooms on proper foundations.
Wrap to the rear
A side extension can continue round the back to form an L-shape.
How much does a side extension cost?
A side extension typically costs £1,700–£2,650 per m² of floor area across the UK, including VAT at 20% (standard rate). A typical 28 m² project comes to roughly £47,500–£74,000. Build time is usually 10 to 16 weeks on site depending on size and storeys. Design, structural calculations and council fees are extra.
| Region | Guide range (per m² of floor area) |
|---|---|
| London | £2,200–£3,450 |
| South East | £1,900–£2,950 |
| East of England | £1,800–£2,800 |
| South West | £1,750–£2,700 |
| West Midlands | £1,650–£2,550 |
| East Midlands | £1,600–£2,500 |
| North West | £1,600–£2,500 |
| Yorkshire and the Humber | £1,600–£2,450 |
| North East | £1,550–£2,400 |
| Wales | £1,550–£2,450 |
| Scotland | £1,650–£2,550 |
| Northern Ireland | £1,450–£2,300 |
What changes the price:
- Single or two storeys
- Boundary walls and access for materials
- Drains, gas meters or soil pipes that need moving
- Roof design and tile matching
- Steel work where the side wall is opened
- Internal fit-out of the new rooms
Use the guide price tool for a figure based on your measurements, or see all guide prices.
Do I need planning permission?
England
In England a single-storey side extension can be permitted development if it is no more than 4 metres high and no wider than half the width of the original house. Side extensions of more than one storey always need planning permission, as do side extensions in conservation areas, National Parks and National Landscapes.
Wales
In Wales a single-storey side extension can be permitted development if it is no more than 4 metres high and the widest part of the house ends up no more than 50% wider than the original. Two-storey side extensions must be at least 10.5 metres from the side boundary and set back 1 metre from the front, so most need planning permission. In conservation areas single-storey side extensions are limited to 3 metres.
Scotland
In Scotland single-storey side extensions can be permitted development if they do not come forward of a side wall facing a road and stay within the height and boundary limits. Two-storey side extensions usually need planning permission, and a building warrant is needed before work starts.
Northern Ireland
In Northern Ireland a single-storey side extension can be permitted development if it is no more than 4 metres high, no wider than half the original house and not beside a wall facing a road. Side extensions of more than one storey need planning permission.
Even when planning permission is not needed, building regulations approval (a building warrant in Scotland) usually is. We make the applications and book the inspections.
How long does it take?
Typically 10 to 16 weeks on site depending on size and storeys. Surveys, design and approvals usually take 3 to 4 months beforehand.
How it works
- 01
Free visit
We look at the property, discuss what you want and measure up.
- 02
Design and fixed quote
Drawings, calculations and an itemised price for the agreed specification.
- 03
Approvals
Planning or permitted development, building regulations and neighbour matters.
- 04
Build and sign-off
Built to a written programme, with a completion certificate at the end.
Side extensions: questions and answers
Can I build a side extension without planning permission?
In England a single-storey side extension can be permitted development if it is no more than 4 metres high and no wider than half the original house, and the house is not in a conservation area or similar protected land. Two-storey side extensions always need planning permission.
How close to the boundary can I build?
You can build up to the boundary, but the eaves height near it is limited under permitted development, and planning officers often want a gap at first floor. If you build on or near a shared boundary, party wall rules may apply.
Will a two-storey side extension look odd?
Not if it is designed well. Setting it back from the front and keeping the roof lower than the main ridge usually gives a natural look and is what most councils ask for.
Can a side extension go over a drain?
Often yes, with a build-over agreement from the water company or by rerouting the drain. We check this during the survey.
How long does it take?
Roughly 10 to 16 weeks on site once approvals are in place, depending on whether it is one or two storeys.
Will I lose my driveway?
A side extension often takes the side drive. We look at whether parking can be kept at the front and whether a dropped kerb is needed.