Yes, in most of Potters Bar you can, and the neighbours plainly are. Listed buildings are few. The two conservation areas are small corners of a large post-war town. Hundreds of planning applications go in every year. The thing most likely to argue with your extension is the ground underneath it.
Potters Bar has 41 listed buildings. None is Grade I, 3 are Grade II* and the remaining 38 are Grade II. Listing protects the whole building, inside and out, so even modest changes to one of them need listed building consent on top of any planning permission. In a town where the semi and the bungalow make up most of the housing, 41 is a short list, and most buyers will never own anything on it.
The area has two conservation areas, Potters Bar Darkes Lane (West) and Potters Bar The Royds. Inside them, the council takes a closer interest in anything that changes how a street looks, from front extensions to demolition and some kinds of tree work, and work that is straightforward elsewhere can need an application. A side extension that goes through without fuss on a post-war close can turn into a longer conversation about materials and rooflines in The Royds. Whether a house falls inside either boundary is on Hertsmere Borough Council's conservation area maps, and it is worth looking up on day one.
Hertsmere Borough Council's planning records show 1,275 applications across the area between April 2024 and June 2026, with 581 of them in the most recent twelve months. That is a town busy adding loft rooms, rear extensions and new porches to houses built for a smaller family. The paperwork is well-trodden here, and a planning officer in Potters Bar has seen your idea before.
The ground deserves more attention than the planners. The British Geological Survey rates the area's shrink-swell hazard at class 3, and all of the area reaches that class. Shrink-swell means clay that swells when wet and shrinks when dry, moving foundations as it goes. For an extension, that usually means deeper foundations, a careful look at nearby trees, and a structural engineer's view before the builder prices the job. The clay in Potters Bar has firmer views about your extension than the council does, and it will keep expressing them long after the planning certificate is framed.
For most homes outside the two conservation areas and off the listed register, a modest rear extension or a loft conversion can often go ahead under permitted development, the national rights that allow certain work without a full planning application, as long as the size and design limits are met. Anything larger, anything on a listed building and much of what changes the street scene in a conservation area will need consent from the council. Building regulations apply either way, and on this clay the foundations are where they will bite.
Freedom to extend in Potters Bar is middling, easy enough on paper for most post-war semis and bungalows, with the conservation areas, the listed handful and the clay setting the limits.