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.
How much does a side extension cost in West Sussex?
In West Sussex, a side extension typically costs £1,900–£2,950 per m² of floor area, including VAT at 20% (standard rate). For a typical 28 m² project that is roughly £53,000–£82,500. We start from a UK-wide guide and adjust it for the South East, where labour and materials are typically about 12% above average. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (20 m²) | £38,000–£59,000 |
| Typical project (28 m²) | £53,000–£82,500 |
| Larger project (39 m²) | £74,000–£115,000 |
| Rate per m² | £1,900–£2,950 |
The main things that change the cost:
- Roof design and tile matching
- Drains, gas meters or soil pipes that need moving
- Steel work where the side wall is opened
- Single or two storeys
For a figure based on your own measurements, try the guide price tool, then book a free home visit for a fixed quote.
Do you need planning permission for a side extension in West Sussex?
Before anything is built in West Sussex, we confirm which approvals apply in 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.
Where work involves a shared wall, building on the boundary, or digging foundations within 3 metres of a neighbour’s building and deeper than its foundations (or within 6 metres for deeper work), the Party Wall etc. Act 1996 applies and notices must be served before work starts.
Our first visit includes a check of your planning history, so you know the route before paying for drawings.
What we handle
From survey to sign-off, the job normally involves:
- Opening into the existing house
- Walls, roof and windows to match the house
- Building control completion
- Foundations and drainage alterations
- Survey of the boundary, drains and access
- Electrics, heating and plastering
How long does it take?
Allow 10 to 16 weeks on site depending on size and storeys. Design and approvals come first and typically take 3 to 4 months, depending on the approval route.
Building in West Sussex
Commuter towns across the South East have many 1930s and post-war semis that suit rear extensions and hip-to-gable loft conversions.
Towns we cover in West Sussex include Crawley, Worthing, Bognor Regis, Littlehampton, Horsham and Haywards Heath.
How it works
- 01
Free visit
We look at the property, talk through what you want and measure up. There is no charge and no obligation.
- 02
Design and fixed quote
Drawings, structural calculations and an itemised, fixed price for the agreed specification.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about side extensions in West Sussex
Do you build side extensions in West Sussex?
Yes. We build side extensions across West Sussex, including postcodes BN11, BN12, BN13, BN14 and BN15. Every project starts with a free home visit.
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 much does a side extension cost in West Sussex?
In West Sussex, a side extension typically costs £1,900–£2,950 per m² of floor area, including VAT at 20% (standard rate). A typical 28 m² project comes to roughly £53,000–£82,500. Your fixed price depends on the survey and drawings.
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 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.