Yes, in most of North Harrow you can extend, because the interwar streets carry very few heritage restrictions. Few of the ordinary houses are listed. There is one conservation area to watch for. The clay underneath causes more trouble than the planning rules do. Your neighbours have been extending their houses for years.
Historic England's list counts 164 listed buildings in the box drawn around North Harrow. Three are Grade I, 10 are Grade II* and 151 are Grade II. Listing covers the whole building, inside and out, so any change that affects its character needs listed building consent on top of planning permission, including changes that would normally need nothing. That box takes in a much wider area than the interwar streets, and most of the 164 are churches, farm buildings and manor houses, not semis. Most buyers here will never deal with listed building consent.
There is one conservation area within the same box. Inside a conservation area the council takes a closer interest in changes to front elevations, roofs and anything visible from the street, and demolishing even part of a building needs consent. Outside it, the rules are the ordinary ones.
There are also 2 scheduled monuments nearby, which is the strictest level of protection there is. They matter only if your garden backs onto one.
Harrow council received 1,354 planning applications for the area between March 2024 and June 2026, and 578 of them came in the twelve months to June. That is a steady flow of people wanting more space in houses built in the 1930s. Whatever extension you have in mind, someone in the next road has probably applied for something similar.
The British Geological Survey rates the ground here as shrink-swell class 3, and about 97% of the area reaches that class. Shrink-swell clay swells when wet and shrinks in dry summers. For an extension, that usually means deeper foundations than a builder would pour on firmer ground, and a structural engineer looking at nearby trees before anyone starts digging. None of this stops an extension, but it adds to the cost.
Two listed buildings in the area are on Historic England's Heritage at Risk register. That means they are in poor condition and their repair is likely to attract grants and close scrutiny. They will not affect the owner of an ordinary semi a few streets away.
For an unlisted house outside the conservation area, a modest rear extension or loft conversion can often go ahead under permitted development, without a full planning application. Flats have far fewer of these rights, and listed buildings effectively none. Ask Harrow council for a lawful development certificate before the builder orders anything, because a builder's view of what counts as permitted development is no protection if the council disagrees.
You have a fair amount of freedom to extend in North Harrow. The planning rules are ordinary for most houses, and the clay underneath will cost you more than the paperwork.