You can extend a house in Portsmouth, but you will spend more of the project asking permission than most places on this coast require. There are 697 listed buildings inside the area. There are 41 conservation areas layered over the top. The ground under all of it carries a shrink-swell rating of three. People build here constantly, and almost none of them do it quietly.
Start with the listings, because they are the hardest constraint and the most commonly discovered late. Of those 697 listed buildings, 19 are grade one, 51 are grade two star, and 627 are grade two. Listing attaches to the whole building rather than the pretty bit at the front, so it covers interiors, chimneys, staircases and window frames as readily as the elevation. Any alteration affecting the building's character needs listed building consent before it happens, not after, and doing the work first and asking later is an offence rather than a paperwork problem. If you are buying a flat carved out of something Georgian near the harbour, this applies to you and not just to the museum next door.
The 41 conservation areas catch far more houses than the listings do, and that is where most owners meet the planning system. Designation covers stretches of Old Portsmouth's High Street and King Street, Owen's Southsea, Gunwharf, Highland Terrace and Stoke Road among others. Inside one, the controls extend to demolition and to trees, and the street's appearance becomes a material consideration in a way it simply is not two roads over. The practical effect is that things you assumed were your own business, like the roof covering or the front windows, become the council's business too.
There are also 28 scheduled monuments in the area, which sit under the strictest protection of the lot. For most buyers this is background, since scheduled monument consent applies to the monument and its ground rather than to the house down the road. If a plot backs onto one, it stops being background.
None of which means nothing gets built. Portsmouth City Council received 4,321 planning applications between April 2024 and June 2026, of which 1,941 landed in the most recent twelve months. That is a city where altering your home is a routine activity conducted in public, with a form, in front of neighbours who can read it.
The ground adds its own cost. The British Geological Survey rates shrink-swell across the area at class 3, and every part of the area reaches that class or above. In practical terms this is a clay-movement rating, and it is the reason foundation depth stops being a detail on a drawing and becomes a line in the quote, particularly near mature trees. It is not a reason to abandon an extension. It is a reason to have a structural engineer look at the design before your builder tells you what he usually does.
Thirteen of the area's listed buildings appear on Historic England's Heritage at Risk register. That register records buildings in poor condition whose repair may attract grant support and will certainly attract close attention from the conservation officer. It is a live consideration if you are buying one and largely irrelevant if you are buying an ordinary terrace nearby.
The practical rule is simple enough. If the property is listed or sits in a conservation area, assume you are making an application rather than exercising permitted development rights, and get that confirmed by the council before anyone orders materials. Outside those areas the ordinary rules apply, though the housing stock imposes its own limits: dense terraced streets leave little room for a side return, and shared walls mean your neighbours are involved whether the planning system requires it or not.
Freedom to alter your own home is the weakest thing about living here. The city is beautiful in places precisely because so much of it is protected, and you will pay for that beauty in consultants, consents and waiting, whether or not you were the one who wanted it protected.