Double-storey extensions across Windsor
Our design and build teams take on double-storey extensions in Windsor, from the first measured survey through planning and building control to the finished room.
- Guide price in WindsorFrom £42,500 inc VAT
- PlanningUsually a householder application
- Build time16–24 weeks on site
- Postcode districtsSL1, SL3, SL4
Will the council need to approve it?
In England a two-storey rear extension is permitted development only if it projects no more than 3 m from the original rear wall, sits at least 7 m from the rear boundary, matches the roof pitch and uses obscure, non-opening side windows upstairs. Most two-storey extensions therefore go through a householder planning application, which costs £548 from 1 April 2026 and normally takes about 8 weeks.
Before any drawing is finalised we check whether the house is listed, in a conservation area or subject to an Article 4 direction.
Building over or within 3 m of a public sewer needs the water company's consent, usually through a build-over agreement, before the foundations go in.
Building regulations approval is separate from planning. In England you can use the council's building control team or a registered building control approver; a full plans application gets the drawings checked before work starts, and the inspector signs the work off with a completion certificate.
Applications are decided by the local planning authority (your district, borough or unitary council).
How much does a double-storey extension cost in Windsor?
For Windsor we adjust our national guide rates by ×1.12, the factor we use for South East England. Figures include VAT; your written quote after the survey replaces them.
Typical rates in Windsor: £1,740 to £3,360 per m² including VAT, depending on specification.
| Size | Essential | Standard | Premium |
|---|---|---|---|
| 3 m × 4 m, two floors 24 m² | £42,500 – £51,000 | £48,000 – £63,500 | £61,500 – £81,000 |
| 4 m × 4 m, two floors 32 m² | £55,500 – £66,500 | £64,500 – £84,500 | £82,000 – £108,000 |
| 4 m × 6 m, two floors 48 m² | £83,000 – £99,500 | £96,500 – £126,500 | £123,500 – £161,500 |
Try the guide-price tool to see a range for your own size and finish.
When a double-storey extension makes sense
- Houses with enough distance to the rear or side boundary
- Homeowners who would otherwise consider moving
- Growing families who need a bedroom as well as a bigger kitchen
The roof is what makes a two-storey extension look right or wrong. We match the pitch and materials of the main house wherever possible and plan the upstairs layout around the existing landing so the new rooms feel original rather than bolted on.
Covering Windsor
Site visits are available across Windsor (SL1, SL3, SL4). We also work in nearby Slough (2 miles), Maidenhead (6 miles), Bracknell (8 miles), Wokingham (11 miles).
Egham, in Surrey, is roughly 4 miles from Windsor and also on our list. Homes across the South East range from Victorian terraces in the older town centres to large detached houses on the edges, so the right extension type varies street by street.
What happens next
Site visits are free and there is no obligation. Where drawings are needed, the design stage is priced in writing before it starts.
Free site visit
We visit your Windsor home, measure up, talk through what you want and check the planning position.
Design and approvals
We draw the design, agree it with you and handle planning or permitted development checks and building regulations.
Fixed written price
With the design and specification agreed, you get a fixed written price and a start date.
Build and sign-off
We build it with one project manager, book the building control inspections and hand over the completion certificate.
Double-storey extension questions answered
Who do I contract with for a double-storey extension in Windsor?
Your contract is with S&K Property Investments and Improvement Ltd, which designs and manages the project and the build teams who carry it out.
Do you cover all of Windsor?
Yes. We cover Windsor and the surrounding parts of Berkshire. Site visits are free and there is no obligation.
Do I need building regulations approval in Windsor?
Yes. Almost every extension needs it, whatever the planning route. We prepare the drawings and book the inspections.
Is a two-storey extension cheaper per square metre?
Usually. Our guide range is about £1,740 to £3,360 per m² of new floor area including VAT, compared with £1,960 to £3,810 per m² for single-storey work, because the foundations and roof are shared across two floors.
Does a double-storey extension need planning permission?
Often, yes. In England a two-storey rear extension can be permitted development only if it projects no more than 3 m and sits at least 7 m from the rear boundary, with matching roof pitch and obscure side windows. Side extensions over one storey always need planning permission. We confirm the route at the survey.
Book a free site visit in Windsor
We will measure up, check the planning position and give you honest guide figures.
- Free, no-obligation site visit
- Planning route checked before you spend on drawings
- Written price once the design is agreed
- Building control sign-off at the end
Prefer to talk? Call 0800 088 6341 or .
Book a free site visit
Tell us a little about the project and we will arrange a time.