Part single, part two-storey extensions in Portsmouth

Portsmouth: the key facts for this kind of project.

Postcode districts: PO1, PO2 and PO3
County / area: Hampshire, England
Price region: South East England
Guide price: £79,000–£109,000 (£2,200–£3,030 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: Southsea (1.1 miles), Gosport (1.7 miles), Cosham (3.5 miles)
Distance to Southampton: about 15.5 miles
First visit: Free, no obligation

About a part single, part two-storey extension

A part single, part two-storey extension suits you if you want a generous ground floor but only need one extra room upstairs, or if a full two-storey design would affect a neighbour too much. You get design options that test where the two-storey part works best, the approvals your nation needs, structural design, building regulations approval and the build, carried out by vetted teams working under S&K.

We cover postcode districts PO1, PO2 and PO3 in and around Portsmouth, along with neighbouring Southsea, Gosport and Cosham.

What it costs in Portsmouth

Most of the cost of a part single, part two-storey extension is set by its size and specification. For Portsmouth and the rest of South East England, our guide is £2,200–£3,030 per m² including VAT, so about £79,000–£109,000 for a 6 m × 4 m ground floor and a first floor over half of it (36 m² in total). Design fees, structural calculations and approval fees are separate and listed on our prices page.

Spec	Per m²	A 6 m × 4 m ground floor and a first floor over half of it (36 m² in total)
Essential	£2,020–£2,780 per m²	£73,000–£100,000
Standard	£2,200–£3,030 per m²	£79,000–£109,000
Premium	£2,640–£3,630 per m²	£95,000–£130,500

Planning and building regulations in Portsmouth

In England, the extension can only be permitted development if it goes no more than 3 m beyond the original rear wall and stays at least 7 m from the rear boundary, with matching materials and roof pitch. We check the whole design against the rules, including the 50% garden coverage limit. If any part fails, or extends beyond a side wall on two floors, you need 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 Portsmouth homes

Structure at the step

The first-floor wall above often sits over the new open-plan space, so it needs a steel beam or frame to carry it. Loads come down at different points than in a full two-storey design, and the engineer sizes the foundations to suit each part.

Light into the kitchen

The single-storey part is the place for roof lights or a lantern, which bring daylight deep into the plan. In England, glazing is normally limited to 25% of the new floor area plus any windows removed, unless calculations show the house still performs, so we plan the balance early.

How we'd run your project

Visit and first ideas
One of our team visits your house in Portsmouth, checks the walls, roof and drains, and sketches out options with you.
Plans, then approval
Measured survey, design, then the right approval route for your house. We explain each step before we take it.
Fixed price in writing
With approved drawings and calculations, we give you a fixed build price and a programme you can plan around.
Build
Foundations, walls, floor, roof, windows, then first and second fix. We break through into the house as late as we can.

Questions about part single, part two-storey extensions in Portsmouth

Who handles building regulations for a project in Portsmouth?

We handle it as part of the project. What's needed in England: building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end. We prepare the drawings and the engineer's calculations, book the inspections and pass you the completion paperwork at the end.

How much does a part single, part two-storey extension cost in Portsmouth?

Our guide for South East England, which covers Portsmouth, is £2,200–£3,030 per m² including VAT. A typical project with a 6 m × 4 m ground floor and a first floor over half of it (36 m² in total) comes to about £79,000–£109,000. Design and approval fees are extra, and we fix the build price in writing once the design is agreed.

Which postcodes do you cover around Portsmouth?

We cover Portsmouth (PO1, PO2 and PO3) and nearby places including Southsea, Gosport and Cosham, plus the rest of Hampshire. The first site visit is free.

Which side should the two-storey part go on?

Usually the side furthest from the nearest neighbour's windows, so the single-storey part sits nearest them and keeps their light. Other factors are where the stairs land upstairs, which room needs the extra space and where drains run. We sketch options at the design stage and explain which is most likely to be approved.

Can I add roof lights to the single-storey part?

Yes. Roof lights or a lantern over the single-storey part are one of the main reasons to choose this design, as they light the middle of a deep kitchen. In England they count towards the Part L glazing allowance, normally 25% of the new floor area plus windows removed, so we balance them with the rear doors.

Nearby

The same service in neighbouring towns:

Other extensions in Portsmouth

All two-storey extension services in Portsmouth · Part single, part two-storey extensions across Hampshire

Book a free site visit in Portsmouth and we'll tell you what's possible, what approvals you need and what the design stage would cost.