Yes, you can extend most houses in Woolmer Green, because most of the village was built too recently for anyone to protect it. Listed buildings are scattered around the area. Two conservation areas reach in from neighbouring villages. Most houses are post-war. Neighbours apply for planning permission constantly, and that tells you it can be done.
Historic England lists 58 buildings in and around Woolmer Green. None is Grade I, three are Grade II* and 55 are Grade II. Listing protects the whole building, inside and out, so almost any change to one needs listed building consent on top of any planning permission. On the council-tax valuation list, about 19% of the village's homes were built before 1930. The listed stock is a small share of what is here, though anyone who owns one of those houses will feel the rules on every job.
There are two conservation areas in the surrounding area, Stockens Green Knebworth and Welwyn. Inside a conservation area, councils control demolition and front-facing changes more tightly, and some rights that apply elsewhere are reduced. Both carry the names of neighbouring villages. A house in Woolmer Green is affected only if the boundary reaches its plot.
There is one scheduled monument in the wider area. That is the strictest tier of protection, and it matters mainly to anyone digging or building on land right next to it.
The planning registers recorded 1,075 applications in and around the village between April 2024 and June 2026, and 456 of them in the last twelve months. These figures cover how many applications were made. They say nothing about how many were approved. They do show that people here change their houses all the time, and that the planning office sees the same extension ideas repeatedly. The bungalows, about 16% of the village's homes, are the obvious candidates for an extra storey. Whoever owns the house next door to one should expect that application eventually.
For an ordinary post-war house outside a conservation area, some smaller works, such as many single-storey rear extensions, can go ahead under permitted development rights without a full planning application. A listed building gets none of that freedom, and a conservation area trims it back. Woolmer Green is at the point where three council districts meet, so check which council covers the exact address, and whether past permissions on the house removed its development rights. Welwyn Hatfield is the parent authority, but the boundary is close by.
Your freedom to extend here is middling. Most houses are straightforward to alter, while the listed few and anything inside a conservation boundary will take extra consents and patience.