Yes, but you will usually need permission first. Nearly two hundred listed buildings stand in and around the town. Fourteen conservation areas fall within the same area. The clay underneath moves with the weather. Most homes are flats, so for most residents an extension was never on the table.
Historic England lists 194 buildings in the area around Brentford, 13 at Grade I, 17 at Grade II* and 164 at Grade II. Listing covers the whole building, inside and out, so replacing windows, removing a wall or adding to the back of a listed home needs listed building consent on top of any planning permission. The 164 Grade II buildings are the ones people are most likely to live in, and in those, changing a window frame means filling in forms.
Within the same area there are 14 conservation areas, including The Butts, St Paul's Brentford, Grand Union Canal and Boston Manor, and Kew Bridge. In a conservation area, the council takes a closer interest in anything that changes how a street looks, from front extensions and roof changes to demolition and some work on trees. Parts of The Butts date back to 1680, and the council will judge a new porch against that history.
There is one scheduled monument in the area, the strictest tier of protection. It matters mainly to anyone whose land adjoins it.
Plenty of people do get on with it. Hounslow's planning register lists 721 applications in Brentford between February 2024 and July 2026, 249 of them in the last twelve months. That is a register of what people have asked for, and it shows that changing a home here is routine, even if it is never quick.
The British Geological Survey puts the whole area in class 3 for shrink-swell, its rating for clay that swells when wet and shrinks when dry. Class 3 is a published hazard rating, not cause for alarm. In practice an extension needs foundations designed for clay that moves, often deeper than a builder might quote at first, and a structural engineer should see the plans before work starts. If you have an extension in mind, get that engineer's view at the survey stage, because the clay affects what the extension will cost.
Seven listed buildings in the area are on Historic England's Heritage at Risk register. These are buildings in poor condition whose repair may qualify for grants and will be closely watched. They make no difference to the owner of an ordinary house down the road.
For a house outside a conservation area and not listed, many smaller rear extensions and loft conversions can go ahead under permitted development, the national rights that let you build some things without applying. In a conservation area those rights are narrower, and for a listed building almost everything needs consent. With seven in ten homes in Brentford being flats, most owners need the freeholder's agreement to alter the building at all.
Freedom to extend is one of Brentford's weaker points. Heritage controls, conservation areas and shifting clay mean that almost any change involves paperwork, and the houses that might be extended are few to begin with.