Yes, in most cases you can extend a house in Brookmans Park without heritage rules getting in the way. The village has only a handful of listed buildings. There are no conservation areas. The neighbours are extending constantly. The main thing to plan around is the clay under the garden.
Brookmans Park has 9 listed buildings. None is Grade I, 1 is Grade II* and 8 are Grade II. Listing means listed building consent for almost any change, inside or out, on top of ordinary planning permission. With only nine of them in a village of big twentieth-century houses, the odds are your house is not one. If it is, the council will want a say in everything from the windows to the internal walls.
The neighbours are making full use of that freedom. Between April 2024 and June 2026 the area saw 1,814 planning applications, 835 of them in the last twelve months. In a village built mostly of detached houses on generous plots, that is the sound of people adding a fifth bedroom because the four they have look small next door.
The ground matters more than the heritage. The British Geological Survey puts the area in shrink-swell class 3, and all of it reaches at least that class. Clay at that level swells when it's wet and shrinks when it dries, so extensions usually need deeper foundations and sometimes a different design near trees. Get a structural engineer to look at the ground and any nearby trees early in the design, because the foundation depth can change the cost of an extension as much as its size does.
With no conservation area and few listed buildings, a lot of ordinary work, such as many rear extensions and loft conversions within the national size limits, can go ahead as permitted development without a full planning application. Bigger or more unusual schemes still go to Welwyn Hatfield council. Find out which side of that line your plan falls on before an architect draws it.
Brookmans Park gives homeowners plenty of freedom to extend, and the paperwork is easier than the clay underneath.