Yes, most Shenley homes can be extended, as long as they're outside the old village core. The village has a couple of dozen listed buildings, almost all at the lower grade. One conservation area covers the historic centre. The newer streets have far fewer restrictions. The clay under every garden matters more to the cost than the paperwork does.
Historic England's list includes 24 buildings in Shenley: none at Grade I, one at Grade II* and 23 at Grade II. Listing covers the whole building, inside and out, so any change that affects its character needs listed building consent from the council as well as any planning permission. Replacing windows, moving an internal wall or changing the roof covering can all need consent. Unless you're buying one of those 24, though, listing won't affect your plans. Most buyers in Shenley won't be.
Shenley has one conservation area, Shenley Village, covering the historic centre. Inside it, changes that would be routine elsewhere, such as altering a frontage, adding a dormer, cladding or taking down trees, can need permission or notice. Hertsmere Borough Council can also apply extra local controls on top. The conservation area covers the old part of the village. Most of the newer housing went up beyond it, much of it on the former hospital land.
On the planning records, 758 applications were made between April 2024 and June 2026, and 313 of them came in the last twelve months. These figures say nothing about how many were approved. They do show that Shenley residents extend and alter their homes all the time. Expect a neighbour's extension to be under way somewhere nearby at most points in the year.
The British Geological Survey puts the whole of Shenley in shrink-swell class 3, its published hazard class for clay that swells when wet and shrinks when dry. In practice, an extension here probably needs deeper foundations than on sandier ground, and trees close to the new walls need thinking about because their roots draw water out of the clay. Get a structural engineer's view on foundation depth early, while the plans can still change, and allow for it in the budget. The same survey puts all of Shenley in class 3 for soluble rocks, so ask the engineer about that too.
The practical picture is clear. Outside the conservation area, and away from the 24 listed buildings, a typical Shenley house has the usual permitted development rights, and plenty of rear extensions and loft conversions can go ahead without a full application. Inside the Shenley Village boundary, or on a listed building, expect to apply for permission for most changes and to take longer over it. Ask Hertsmere Borough Council whether the house is inside the conservation area and whether any extra local controls apply to it. That one question decides most of what follows.
Your freedom to extend in Shenley is middling. The planning rules are only tight in the historic core, and the clay adds cost to building almost everywhere.