Yes, most homes in Stanmore can be extended, and the bigger constraint is the clay under them, not the rules above them. Heritage protection is concentrated on a small number of old buildings. Most of the town is ordinary interwar housing with no special status. Plenty of neighbours are already building. The ground is the thing that asks for a proper engineer.
There are 115 listed buildings in and around Stanmore. One is Grade I, seven are Grade II* and 107 are Grade II. Listing covers the whole building, inside and out, so even small changes such as new windows, a removed wall or a fresh render need listed building consent from Harrow Council on top of any planning permission. For a town that hosted a queen consort, a prime minister and the founder of a hotel empire, the protected list is short, and almost none of it is semis.
Three conservation areas fall within the wider area mapped around Stanmore. Inside one, demolition, front extensions, roof changes and some alterations that would normally go ahead without permission can need consent, and the council judges each one against the character of the street. Whether a particular road is in one is the first thing to ask Harrow Council, because the answer changes what you can do without permission.
Three scheduled monuments lie in the same wider area. That is the strictest level of protection there is, though it matters mainly to anyone whose land adjoins one.
Harrow Council's register shows 983 planning applications across Stanmore between March 2024 and June 2026, and 408 in the last twelve months alone. That is a town full of people adding rooms, and the 1930s semi has been the standard starting point for loft conversions and side extensions for a long time. The builders from the Metropolitan Railway era left a lot of plots wide enough to build out to the side.
The ground needs more thought than the paperwork. The British Geological Survey puts Stanmore in class 3 for shrink-swell, and the whole area reaches that class. Clay in that class swells when wet and shrinks when dry, and it moves most under trees and in long dry summers. For an extension, that means deeper foundations than on sand or chalk, a structural engineer's view on the design, and some thought about the trees nearby, both yours and the neighbours'. Plenty of houses here have been extended without trouble, and they were the ones built on foundations designed for clay.
Six listed buildings in the area are on Historic England's Heritage at Risk register. They are buildings in poor condition whose repairs may attract grant funding and will certainly attract scrutiny. For the owner of an ordinary house nearby, they are someone else's problem.
In practice, an unlisted semi outside a conservation area can usually be extended at the back or into the loft under permitted development rights, within the national size limits, and anything bigger goes through an ordinary planning application. Listed buildings and homes in conservation areas need consent for more of the work, so check a property's status with Harrow Council and get its view in writing for anything you plan to do without permission.
Extending in Stanmore is about average in difficulty: the planning side is easy for most houses, and the clay underneath sets the rules on the foundations.