First-floor extensions in Southampton

The basics for Southampton at a glance.

Postcode districts: SO14 and SO15
County / area: Hampshire, England
Price region: South East England
Guide price: £27,500–£40,000 (£1,980–£2,860 per m²)
Approval route: It may be permitted development if it goes no more than 3 m beyond the original rear wall (counting any single-storey part) and stays 7 m from the rear boundary; otherwise it needs planning permission.
Nearest towns: Woolston (1.4 miles), Portswood (1.7 miles), Hythe (Hampshire) (3 miles)
First visit: Free, no obligation

About a first-floor extension

A first-floor extension suits you if your house already has a single-storey extension and you need more space upstairs. You get a structural check of the existing walls and foundations, a design that ties the new floor into the house, the approvals your nation needs, building regulations approval and the build, managed by us and carried out by vetted teams working under S&K.

We cover postcode districts SO14 and SO15 in and around Southampton, along with neighbouring Woolston, Portswood and Hythe (Hampshire).

What it costs in Southampton

For budgeting in Southampton, allow around £27,500–£40,000 for a first-floor extension with 4 m × 3.5 m of new first-floor space (14 m²). That's based on £1,980–£2,860 per m² including VAT for South East England. We'll give you a written design-stage price first, then a fixed build price once the drawings are agreed.

Spec	Per m²	4 m × 3.5 m of new first-floor space (14 m²)
Essential	£1,820–£2,630 per m²	£25,500–£37,000
Standard	£1,980–£2,860 per m²	£27,500–£40,000
Premium	£2,380–£3,430 per m²	£33,500–£48,000

Planning and building regulations in Southampton

In England, adding a first floor turns a single-storey extension into one of more than one storey. To be permitted development, the extension as a whole must go no more than 3 m beyond the rear wall of the original house and stay at least 7 m from the rear boundary, with matching materials and roof pitch. Over a side extension it always needs planning permission, costing £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.

Design points for Southampton homes

Escape and fire safety

A new first-floor bedroom needs an escape window with a clear opening of at least 0.33 m², at least 450 mm wide and 450 mm high, with the bottom no more than 1,100 mm above the floor, unless there is a protected route to the stairs.

Matching the roofline

A first-floor extension often changes how the back or side of the house looks more than the original single storey did. Matching the main roof pitch and materials, and keeping the ridge below the main ridge, usually gives the best result and is expected for permitted development in England.

How we'd run your project

Free site visit
We visit your home in Southampton, look at the structure, access and boundaries, and talk through what you want the new rooms to do.
Drawings and permission
Our designers produce plans and elevations, then we take care of planning or permitted development checks with the council covering Southampton.
Technical design
We prepare the building regulations drawings, structural calculations and specification so the build price can be fixed.
Finish and certify
Second fix, plastering and making good, then building control's final inspection and the completion paperwork.

Questions about first-floor extensions in Southampton

Do I need planning permission for a first-floor extension in Southampton?

Southampton is in England, so England planning rules apply. It may be permitted development if it goes no more than 3 m beyond the original rear wall (counting any single-storey part) and stays 7 m from the rear boundary; otherwise it needs planning permission. If in doubt, a lawful development certificate or a planning application gives you certainty before you build.

How long does a first-floor extension take?

Allow 4 to 8 weeks for survey and design, then the approval period (councils should decide householder applications within 8 weeks of validation), then 4 to 8 weeks of technical design, which can overlap. The build itself usually takes 14 to 20 weeks, longer for large or complex designs.

Which postcodes do you cover around Southampton?

We cover Southampton (SO14 and SO15) and nearby places including Woolston, Portswood and Hythe (Hampshire), plus the rest of Hampshire. The first site visit is free.

What happens to the room below while the first floor is built?

The old roof is removed, so the room below is exposed for a short period. We fit temporary weather protection, plan the work so the roof comes off only when the new structure is ready, and agree with you which rooms will be out of use and for roughly how long.

Can I build on top of my existing single-storey extension?

Often yes, but only after the structure has been checked. Foundations, walls and lintels built for one storey may not carry two. We dig trial holes, look for the original building control approval and ask the structural engineer to confirm what can stay, what needs strengthening and what must be rebuilt.

Nearby

We also cover places near Southampton, including:

Other extensions in Southampton

All two-storey extension services in Southampton · First-floor extensions across Hampshire

The quickest next step is a visit. We'll look at your house in Southampton and give you an honest view of the options.