House Extensions in Portsmouth: Costs & Planning Guide
House extensions in Portsmouth are a different job from extensions almost anywhere else we work. Portsea Island is one of the most densely built places in the country, so most of the homes we're asked to extend are Victorian and Edwardian terraces in Southsea, Fratton, North End and Copnor, with narrow gardens, shared walls on both sides and a sewer running somewhere behind the back wall.
That doesn't make extending harder in a bad way. It just means the planning, the drainage and the access need thinking about before anyone talks about bifold doors. This guide covers what a Portsmouth extension costs in 2026, where planning catches people out, and how long the build really takes.
What a house extension costs in Portsmouth
We price every job from a survey visit, because two terraces on the same Southsea street can hide very different foundations and drains. As a working guide, though, here's what our customers in Portsmouth are typically looking at for a standard-spec build, including our labour, materials and project management:
Single-storey rear extension, around 4m x 3m: roughly £22,000 to £34,000
Larger single-storey kitchen extension, around 6m x 4m with a quality finish: roughly £42,000 to £60,000
Two-storey rear extension, around 6m x 4m: roughly £66,000 to £100,000
Build rate for standard spec: around £2,600 to £3,100 per square metre
Those figures cover the structure and a finished shell ready for decoration. The kitchen itself, sanitaryware, and anything like rooflights or a steel-framed glazed screen sit on top. Structural engineer's calculations for the steels usually come in at around £500 to £1,100, and you'll have the council's planning or building control charges as well.
A few things push a Portsmouth price up more than people expect. Rear access is the big one: plenty of terraces have no side passage, so every bag of cement, every block and all the spoil goes through the house or over a back alley. Ground conditions near the seafront and around Milton and Eastney can mean deeper or wider foundations. And the drainage, which we'll get to in a minute.
Do you need planning permission in Portsmouth?
Often you don't. A modest single-storey rear extension on a terraced house can usually go ahead under permitted development, which on a terrace allows up to 3m deep (or up to 6m under the prior approval route, where neighbours get a say). Two-storey extensions on terraces almost always need a full householder application.
Where Portsmouth gets tricky is its conservation areas. Owen's Southsea, for example, is covered by several Article 4 directions that strip back permitted development rights on particular streets, so things like replacing front windows, altering chimney stacks or changing front boundary walls need permission there. Rear extensions are less affected, but we check the exact street every time rather than assume.
If you're unsure, Portsmouth City Council runs a householder pre-application advice service for £40, which is about the cheapest peace of mind you'll find in construction. We're happy to help pull the information together for it.
Building regulations apply whether or not you need planning. That covers the foundations, the steel over any new opening, insulation to current Part L standards, fire escape routes on two-storey work and ventilation. We handle the building control side as part of the job, and you get the completion certificate at the end.
Party walls and the sewer behind your back wall
On a terrace you'll be digging foundations within 3m of at least one neighbour's wall, so the Party Wall Act will almost certainly apply. That means serving notice on both neighbours at least a month before work starts. Most people sign it off without fuss when they've been told early and properly; the surprises are what cause disputes.
The other Portsmouth regular is a shared sewer running across the back gardens of a whole row. If your extension goes over or near it, you'll need a build-over agreement with Southern Water before work starts. We've written a fuller guide on building over a sewer and build-over agreements which applies just as much here. We usually run a CCTV survey of the drain first through our drainage and civil works team, so we know its condition before anything gets built over it.
How long does a Portsmouth extension take?
For a typical single-storey rear extension, plan for 10 to 14 weeks on site once we start. Before that, allow time for drawings, any planning decision (eight weeks is the target for householder applications), party wall notices and the build-over agreement. Realistically that's three to five months from first conversation to first dig.
Two-storey work runs longer, usually 16 to 22 weeks on site. Restricted access adds a week or two on some streets, simply because everything moves by wheelbarrow. If you're on a tight terrace with no rear lane, tell us early and we'll plan deliveries and skips around it.
Getting the most from a terraced house extension
On a Southsea or Fratton terrace, the side return is often the best space you've got. Infilling it and opening up into the back reception room can transform a dark galley kitchen. We covered this in detail in our side return extension ideas for Southampton homes post, and the same housing stock is all over Portsmouth.
A few things we'd always push for on these houses: a rooflight or glazed roof strip along the party wall to pull light into the middle of the house, underfloor heating rather than radiators eating into a narrow room, and a proper steel calculation rather than a guess. Our general construction and building service covers the whole lot, from groundworks to the final skim, so you deal with one team rather than five.
Related reading
Planning an extension in Portsmouth?
If you're in Southsea, Fratton, North End, Drayton or anywhere across Portsmouth and the surrounding Hampshire towns, we'll come out, look at the access, the drains and the ground, and give you a clear written price. Book a survey visit with Complete Builds and we'll take it from there.
Comments