Yes, but more of Swindon gets a say in it than you would expect. Hundreds of buildings are listed. More than a dozen conservation areas cover the older streets. The clay under your garden gets a say too. The neighbours are extending anyway, but most of them had to get consent first.
Swindon has 282 listed buildings. Five are Grade I, 11 are Grade II* and 266 are Grade II. Grade II is the everyday tier, but it still means listed building consent for works that affect the building's character, inside as well as out. That can include windows, doors, internal walls and the extension you sketched on the back of an envelope. Carrying out that kind of work without consent is a criminal offence. With 266 of them, a listed building is far more likely to turn up on a Swindon viewing than most buyers assume.
There are 13 conservation areas across the area. They include Swindon Old Town, Swindon's Railway Conservation Area, Devizes Road, Prospect Place and the Town Gardens. In a conservation area, changes that are routine elsewhere can need planning permission, and the council will look at how an extension fits the street, not just whether it fits your plot. The railway village looks the way it does because the council keeps it that way.
There are also 11 scheduled monuments, the strictest tier of heritage protection. They matter mainly if your land borders one. Most extensions won't come near one, but check before digging anything deep near one.
People here extend anyway, and in large numbers. The council received 3,387 planning applications between April 2024 and June 2026, and 1,567 of them in the most recent twelve months. That is a borough full of people building kitchens, loft rooms and garages. The rules add paperwork, but they don't stop people building.
The ground is the other thing to plan around. The British Geological Survey rates shrink-swell clay at class 3 across the area, and every postcode reaches that class. Clay soils swell in wet weather and shrink in dry, which moves foundations, and trees near the house make that worse. It shouldn't put you off extending. It means deeper foundations, a structural engineer's view on the design, and building control asking about nearby trees. Include those costs from the start rather than finding out when the builder quotes.
One listed building in the area is on Historic England's Heritage at Risk register. That describes a historic building in poor condition, where repairs may attract grant support and will certainly be scrutinised. It doesn't pose a risk to anyone buying an ordinary house nearby.
In practice, a post-war semi on an ordinary estate, outside a conservation area, may be able to take a modest extension under permitted development rights without a full application. A listed house, or any house in a conservation area, should be treated as needing consent until the council says otherwise. Find out which kind of house you're buying before you commission the architect.
Freedom to extend is one of Swindon's weaker points: the old streets are well protected, the clay needs proper engineering, and the easiest plans are for the ordinary post-war houses.