Most homeowners in High Barnet can extend, but whether a given house can depends on which street it's on. The area has well over a hundred listed buildings. Three conservation areas are on its doorstep. Hundreds of planning applications go in every year. The clay underneath matters for the foundations whatever the paperwork says.
There are 144 listed buildings in the area. None is grade I, 10 are grade II* and 134 are grade II. Listing covers the inside of a building as well as the outside, so changes that affect its character need listed building consent on top of any planning permission, and a like-for-like repair can still need a conversation with the council. The ten grade II* buildings get the closest scrutiny of all, and owning one involves more paperwork than most extensions.
Three conservation areas fall within the area's bounds, Monken Hadley, Totteridge and Wood Street. Inside them, changes to the outside of a house that would be routine on an ordinary street can need planning permission, and demolition, roof alterations and work on trees are watched more closely. Barnet Council's maps show whether a particular house is listed or inside one of the three boundaries, and that's worth checking first, because it decides which set of rules the house comes under.
People here do extend and alter their homes. The council logged 1,377 planning applications in the area between April 2024 and June 2026, and 615 of those came in the last twelve months. Those figures cover applications only, and they say nothing about how many were approved. Barnet's council took over Arkley in 1905 and Totteridge in 1914 without asking the neighbours. A homeowner adding a side extension doesn't get the same freedom.
The British Geological Survey rates the ground for shrink-swell at class 3, and the whole area reaches that class. Shrink-swell clay swells when wet and shrinks as it dries, and nearby trees make the movement worse. For an extension, that usually means deeper foundations and a structural engineer's view, which a builder should price in from the start and not discover halfway through. It doesn't stop anyone building. It means the foundations cost more than the bare footprint would suggest.
For an unlisted house outside the conservation areas, many smaller rear extensions and loft conversions can go ahead under permitted development, the national rules that allow some work without a full planning application. A listed building or a house inside one of the three conservation areas loses much of that freedom, and the same project may need a full application. A rear extension on a post-war semi in New Barnet and the same extension on a period house near Monken Hadley can face very different planning processes.
High Barnet gives homeowners a middling amount of freedom to extend. Most streets are workable. The listed buildings and the conservation areas are where it gets difficult, and the clay affects every project regardless of the planning rules.