You can extend in Hertford, but in a large part of the town you will need someone's permission first. Nearly 400 listed buildings. Three conservation areas. A Norman castle under official protection. Plenty of homes here are ordinary post-war and modern houses with the usual freedoms, and plenty are not.
Historic England lists 378 buildings in and around Hertford: five at Grade I, 28 at Grade II* and 345 at Grade II. Listing covers the whole building, inside and out, so for any of those homes a new window, a removed wall or a replaced staircase needs listed building consent on top of any planning permission. Grade II is the most common tier and the least restrictive, but the consent rules are the same. Changing a listed building without consent is a criminal offence, so it's no use hoping the council won't notice.
Three conservation areas cover parts of the map here: Hertford itself, Hertingfordbury and Little Amwell. Inside them, East Hertfordshire District Council looks harder at anything visible from the street, and some of the extensions and alterations normally allowed without an application need permission. The Hertford conservation area covers the old centre, so a terrace on the medieval street plan is judged by different rules from a semi on a 1980s estate across the river. The town's biggest change of the last century was the 1960s dual carriageway that cut through the historic centre, and the conservation officers now judging a dormer window are working in a town that remembers what that cost it.
Four scheduled monuments are close enough to count, including the motte and bailey of Hertford Castle south of the River Lea. Scheduling is the strictest protection there is, and even digging near one can need consent. It matters mostly to anyone whose garden backs onto the castle grounds, and those owners already know it.
People in Hertford alter their homes all the time anyway. The council's planning register holds 1,239 applications between April 2024 and June 2026, 531 of them in the most recent twelve months. That is a town that keeps applying for extensions, loft conversions and porches, and whoever you hire to draw your plans will almost certainly have dealt with these officers before.
In practice, a house outside the conservation areas that isn't listed usually keeps the normal permitted development rights for rear extensions, loft conversions and outbuildings, within the national size limits. A listed house loses most of that freedom, and a house in a conservation area loses some of it, especially for anything facing the street. The council's online maps show both the listing and the conservation area boundary for any address, and that takes ten minutes to check when you are looking at a house you would want to extend.
Freedom to extend is limited in Hertford. The modern estates are fine, but the old town needs consent for most changes and the council will want a say in the details.