Yes, most Mill Hill homes can be extended, and plenty of owners are doing it. Listed buildings are few. Conservation areas cover the older parts. The ground needs proper foundations. The house that will give you trouble is the old one up on The Ridgeway, and the interwar semi down the hill is the easy one.
Historic England lists 90 buildings in Mill Hill. None is Grade I, 1 is Grade II* and the other 89 are Grade II. Listing covers the inside as well as the outside, so on a listed house almost any alteration, from an extension to a replacement window or a moved wall, needs listed building consent on top of any planning permission. Out of a district of thousands of homes, 90 is a short list, and the chance that the house you are viewing is on it is small. If it is, the council will have a view on the staircase.
Five conservation areas fall within and around the area: Mill Hill itself, Totteridge, the Watling Estate, Finchley Garden Village and Finchley Church End. Inside them, work that would be routine elsewhere, such as demolition, changes visible from the street and work to trees, needs more consent and more patience, and the council judges extensions on how well they fit the character of the area. The Mill Hill conservation area takes in the old village along The Ridgeway, which has green belt running along both sides of it. A house there can be extended in exactly two directions, and the council has an opinion on both.
Owners clearly are not put off. Barnet's planning register lists 1,139 applications in Mill Hill between April 2024 and June 2026, 508 of them in the last twelve months. That is a district full of people adding rooms, converting lofts and arguing about rear walls, and it makes the street's planning history the best guide to what a particular house might get. The council's register shows every application on a road, and five minutes reading what the neighbours tried says more about your chances than any general rule.
The ground matters as much as the paperwork. The British Geological Survey places every postcode in the area in class 3 or above for shrink-swell, which means clay soil that swells when wet and shrinks when dry. For an extension, that means deeper foundations than a builder might quote by habit, extra care near trees, and a structural engineer's view on the design before work starts. It is a known, manageable condition across the whole district and a common one in this part of London. The clay was here before the semis were, and it has no intention of moving out quietly.
For most of Mill Hill's interwar and later housing, outside the conservation areas and off the listed register, the usual permitted development rules for rear extensions and loft conversions should apply, subject to their limits on size and height. Inside a conservation area those rights are narrower, and on a listed building they mostly fall away. A flat is a different case altogether, since flats do not carry the same permitted development rights as houses.
Freedom to extend in Mill Hill is middling: most semis and newer homes can grow without much trouble, while the old village, the conservation areas and the clay underneath all add cost, consent or both.