Yes, for most homes in Harrow an extension is an ordinary planning question, though the clay underneath may cost more thought than the paperwork. Listed buildings are a small minority. There is one conservation area. Neighbours are applying all the time. The ground under every postcode shrinks and swells, and that is the part a builder will price.
Harrow has 112 listed buildings in the area. Three are Grade I, seven are Grade II* and 102 are Grade II. Listing covers the inside as well as the outside, so almost any change to one of those buildings needs listed building consent on top of planning permission, and Historic England takes an interest in the higher grades. For everyone else, 112 is a small number spread across a town of interwar semis and blocks of flats, and the odds that your house is one of them are low. The odds that it is near one, on Harrow on the Hill in particular, are rather better.
There is one conservation area in the area. Inside a conservation area, changes that would normally go ahead without asking, such as some roof alterations, cladding or demolishing a front wall, can need consent, and the council looks harder at anything visible from the street. The simplest check is to ask Harrow Council whether a particular address falls inside the boundary, since a single street can make the difference between a builder's quote and a design statement.
There is also one scheduled monument in the area, the strictest tier of protection there is. It matters mainly to anyone whose land touches it, and for the rest of Harrow it is a historical footnote.
People here alter their homes constantly. Harrow Council received 1,108 planning applications in the area between March 2024 and June 2026, and 474 of them came in the last twelve months. That is a town that treats the loft and the back garden as unfinished business.
The ground is the catch nobody puts on a sales board. The British Geological Survey's shrink-swell hazard class reaches class 3 across the area, and all of it sits at that level or above. Clay that swells when wet and shrinks when dry moves foundations, so an extension here usually needs deeper footings than a builder from a chalk county would expect, and a structural engineer's view on nearby trees and drains. The ground under Harrow moves more than the planning department does.
One listed building in the area is on Historic England's Heritage at Risk register. That means a building in poor condition whose repair may qualify for grant money and will attract close scrutiny. It is a concern for that building's owner and for whoever ends up restoring it, not for the buyer of an ordinary house down the road.
For a typical house outside the conservation area and not listed, many rear extensions and loft conversions can go ahead under permitted development, the national rights that allow certain works without a full planning application, though they come with size limits and conditions. Flats have far fewer of those automatic rights, which matters in a town where so many homes are flats. Anything bigger, closer to the boundary or visible from the street goes through a normal application.
Harrow gives a middling amount of freedom, with light heritage rules for most streets and a clay subsoil that makes every extension a little more expensive to dig than it looks.