Yes, and with less red tape than most buyers expect. The whole village has nine listed buildings, and every one of them is Grade II. There is no conservation area and no scheduled monument. For most houses the main obstacle to an extension is underground, in the clay.
Welham Green has 9 listed buildings: none at Grade I, none at Grade II*, and all 9 at Grade II. Listing protects the whole building, inside and out, so changing one of those nine needs listed building consent from the council on top of any planning permission, and the work has to respect the original fabric. Of the 2,390 homes on the village's council-tax list, only a few are listed, and most owners will never deal with a heritage officer. Look the house up on the national heritage list anyway. It takes a minute, and finding out after you've bought it costs a great deal more.
Planning records for the area show 1,010 applications between April 2024 and June 2026, and 455 in the last twelve months. That is a lot of drawings for a village of post-war semis and bungalows, and it shows the owners treat the houses as a first draft. The local planning arguments over development are about the fields between Welham Green and Hatfield. Back gardens rarely come into it.
The ground is where Welham Green asks more of you. The British Geological Survey puts the whole area in class 3 for shrink-swell, which means clay soils that swell when wet and shrink when dry. That shapes how new foundations are built. An extension here will usually need foundations dug to a depth that suits the clay, especially near trees, and it makes sense to get a structural engineer involved at the design stage. The rules allow the extension. The clay decides how deep the foundations go.
With no conservation area, an unlisted house in Welham Green keeps the standard permitted development rights, so many modest rear extensions, loft conversions and outbuildings can go ahead without a full planning application, as long as they stay within the national size limits. Larger projects need planning permission from Welwyn Hatfield Borough Council, and building regulations apply to almost everything either way. Some houses lost their permitted development rights through conditions attached when they were first built or later extended, so check the planning history of the specific house. Many owners find out only when the builder is booked.
Your freedom to extend or alter a home is one of Welham Green's real strengths: the listing touches only a few buildings, and the clay is an engineering job to plan for, with no planning officer involved.