Yes, for most houses in Knebworth, and with paperwork for a few. Only a handful of buildings are listed. Two small pockets are conservation areas. Everywhere else, the usual national rules apply. The neighbours are applying for planning permission at a steady rate.
Knebworth has 14 listed buildings: none at Grade I, 1 at Grade II* and 13 at Grade II. Listing covers the whole building, inside and out, so almost any change that affects its character needs listed building consent from North Hertfordshire Council on top of any planning permission. Owners of those 14 buildings will need to plan every change carefully. For the other homes in the village, listing doesn't come into it.
There are 2 conservation areas in Knebworth: Deard's End Lane and Stockens Green. Inside them, some changes that would go ahead without an application elsewhere, such as certain alterations seen from the street, can need planning permission, and the council looks harder at how a design fits its surroundings. Deard's End was one of the few farmhouses standing before the railway village was built around it, and the conservation area keeps the planners paying close attention to what grows there now.
The council received 249 planning applications for Knebworth between April 2024 and June 2026, with 103 of them in the last twelve months. Those are applications made, and they say nothing about how many were approved. Even so, that many applications show that people in Knebworth extend, convert and rebuild their homes as a matter of course, and most of them are not dealing with a listed building at all.
For most houses here, smaller projects such as a modest rear extension or a loft conversion can often go ahead under permitted development, the national rules that let some work proceed without a planning application, subject to their size limits and conditions. Larger schemes need a full planning application whatever the street. The two conservation areas and the 14 listed buildings are where the room to manoeuvre shrinks. To check a specific house, look it up on North Hertfordshire Council's conservation-area maps and the national heritage list, so you know which rules apply before you plan a single wall.
Knebworth gives home extenders a middling amount of freedom. Most of the village is ordinary planning territory, but in the two conservation areas and in the listed houses, a simple extension can become a long consultation with the council.