Usually yes, provided the clay underneath agrees. Very few homes are listed. There is a single conservation area. The neighbours apply for planning permission constantly. Most of the time the planning rules will let you build, and the ground decides how much the extension costs.
Historic England lists 66 buildings in and around Ashtead, none of them Grade I, 4 at Grade II* and 62 at Grade II. Listing covers the whole building, inside and out, so on any of those 66 almost every change needs listed building consent, from an extension down to replacement windows or taking out an internal wall. For a village with 7,790 dwellings on the council-tax list, that is a short register. Sir Robert Howard enclosed a park and built a mansion in the 1680s without filling in a single form. The owner of a Grade II cottage today needs consent to change a window frame.
There is one conservation area around Ashtead's historic core. Inside it, permission is generally needed for more of the work that elsewhere counts as permitted development, demolition comes under tighter control, and the council considers how changes look from the street. No additional local restrictions are on record here, so the general conservation-area rules are the ones that apply. Outside the boundary, a post-war semi is subject to nothing beyond the ordinary rules.
One scheduled monument falls within the area, the Roman villa in Ashtead Forest, which has the strictest protection of all. It matters only if your plans include digging on or near the site, which most domestic extensions will not.
The planning register shows how much building goes on. There were 2,655 planning applications across the area between April 2024 and July 2026, and 1,003 in the most recent twelve months. Not all of them will be domestic extensions and the register gives no outcomes, but at that volume the council's planners see a steady stream of rear extensions, loft conversions and side additions. You will not be the first on your street to apply.
The bigger issue is the ground. The British Geological Survey's ground-stability mapping puts every postcode in the area at class 3 or above for shrink-swell, and class 3 is the highest found here. That means clay which swells when wet and shrinks when dry, and trees and dry summers make it move more. For an extension, that usually means deeper foundations than a builder working on chalk would dig, and a structural engineer's view of the trees and drains nearby. The Romans dug claypits on Ashtead Common to feed a brickworks, and the same clay now decides how deep your footings go.
In practice, most of Ashtead's housing stock outside the conservation area and off the listed register falls under the national permitted development rules, which cover modest rear extensions, many loft conversions and outbuildings. Anything bigger, at the front, or inside the conservation area needs a planning application. The practical check is to ask Mole Valley District Council for a lawful development certificate or a planning decision in writing before work starts, and to find out at the same stage whether the house is listed or inside the conservation boundary. A builder's assurance that it counts as permitted development will carry no weight if the council disagrees.
Ashtead gives you moderate freedom to extend. The planning rules are fairly light outside one small historic core, and the clay underneath does more to shape an extension than anything the council asks for.