Yes, you can extend most houses in Bishop's Stortford, though the clay underneath limits what you can do more often than the planning rules. Only about one home in eight here was built before 1930. Two conservation areas cover the old town and Great Hallingbury. The ground under every part of the town is shrinkable clay. Planners will rarely be the problem, and the foundations need more thought.
The town and its surroundings have 166 listed buildings. Three are Grade I, 9 are Grade II* and 154 are Grade II. If a building is listed, almost any change that affects its character, inside or out, needs listed building consent on top of any planning permission. Doing the work without that consent is a criminal offence, and it is the owner who is liable. With 166 listed buildings spread across the town and its villages, most buyers will never own one, but the ones who do will need consent for most changes.
There are two conservation areas here, one covering the historic core of Bishop's Stortford and one covering Great Hallingbury. Inside them, the council looks more closely at anything visible from the street, and demolition and some works that would otherwise be allowed without an application can need consent. Check the council's conservation area map and the listing register for the exact house early on, because it decides how much of what follows applies.
The area also has two scheduled monuments, the strictest tier of heritage protection. They mostly matter if your land is next to one.
People here are not put off by any of this. East Hertfordshire's planning register shows 2,228 applications in the area between April 2024 and June 2026, and 1,026 in the most recent twelve months. A third of the town's homes already have four or more bedrooms, and judging by those numbers, plenty of the rest are planning to join them.
The ground is the main constraint. On the national geological survey's shrink-swell hazard rating, the whole of the town reaches class 3, the band at which clay swells when wet and shrinks in dry summers enough to matter for buildings. For an extension, that usually means deeper foundations than a builder would dig on firmer ground, more care around trees, and a structural engineer's input at the design stage. It is a routine part of building here, and the cost belongs in the budget from the start.
Outside the conservation areas and away from listed buildings, plenty of standard extensions can go ahead under permitted development without a full application. Inside them, assume you will need permission, and allow time for it.
Your freedom to extend is middling in Bishop's Stortford. Most of the town is open to a sensible extension, the protected old core makes you ask first, and the clay sets the depth of the foundations wherever you build.