Yes. For most homes in South Merstham, heritage rules won't get in the way. There's no Grade I listing anywhere nearby. Both conservation areas are named after the old village, not the post-war estate. Every postcode sits on clay that shrinks and swells. The planning office is the easy conversation. The structural engineer is the expensive one.
The area has 39 listed buildings. None are Grade I, one is Grade II* and the other 38 are Grade II. Listing means you need listed building consent for changes that affect the building's character, inside or out, and that includes things normal planning doesn't cover. If you're buying one of those 39, alterations will be slow and the paperwork long. If you're buying one of the other several thousand homes here, listing won't be your problem.
There are two conservation areas: Merstham Village, and Rockshaw Road, Merstham. Inside them, the council looks more closely at anything that changes a building's appearance from the street, and some changes that would normally go ahead without planning permission need an application. Check the council's conservation-area guidance for the exact address before you plan the extension, because the boundary matters more than which side of the road you're on.
There's one scheduled monument in the area, the strictest tier of protection, where nearly any works need national consent. It only matters if your land is next to it, and for most gardens it isn't.
People here clearly do extend. The council received 1,554 planning applications for the area between April 2024 and June 2026, including 657 in the last twelve months. That's a lot of loft conversions, rear extensions and new porches. Those figures cover how many applications were submitted, not how many were approved, so the queue tells you people are trying, not how many of them succeeded.
The ground is the real issue. The British Geological Survey rates shrink-swell clay at class 3 here, and every postcode reaches that class. Clay shrinks when it dries and swells when it gets wet, and a new extension sitting next to an old house on that ground can move differently from it. That's not a reason to give up. It does mean a structural engineer's input, foundations deep enough for the ground, and a careful look at nearby trees before you draw anything. A builder who says your extension doesn't need that has either built on this clay before and got lucky, or hasn't built on it at all.
For a typical post-war house outside the two conservation areas, many modest rear extensions and loft conversions may come under permitted development, so you might not need a full planning application. Confirm that with the council for the specific address before starting, because whether a project qualifies depends on the house, its history and the size of the work. Inside a conservation area, or on a listed building, assume you'll need consent and plan for the extra time.
Your freedom to extend here is about average. The heritage rules are light across most of the area, but the clay underneath means any extension needs proper engineering, not wishful thinking.