Usually, yes, unless your house is one of the old ones or sits in the old centre. Most of the town was built after the heritage-worthy part. Listed buildings are here, but they're a small minority. Two conservation areas cover the historic core. The neighbours are applying for planning permission all the time, which tells you most extensions get considered rather than blocked outright.
East Grinstead has 87 listed buildings. Two are listed at Grade I, six at Grade II star and 79 at Grade II. Listing covers the whole building, inside as well as out, so replacing windows, removing an internal wall or adding a rear extension all need listed building consent on top of any planning permission. Doing the work without consent is a criminal offence, not a paperwork slip. If you're buying one of the 87, you're taking on the building's history as well as the house, and the council's conservation officer comes with it.
There are two conservation areas, East Grinstead itself and Estcots and East Court. Inside them, changes that would be routine elsewhere get looked at more closely. Materials, roof lines and anything visible from the street are judged against the character of the area. The rules can extend to trees, demolition and some changes you'd assume need no permission at all. Ask the council what applies to the specific address before you commission an architect. Leave it until afterwards and you'll be paying for the same drawings twice.
The town isn't frozen, though. From April 2024 to June 2026, 1,155 planning applications were made across the area, 530 of them in the twelve months to June 2026. Those are the people you'd be living among, and they're adding loft conversions, rear extensions and conservatories while you're still deciding. The count tells you how often people apply, not how often they succeed, so treat it as evidence of demand and not as a promise of approval.
For most buyers, this comes down to what they buy. Only about 16% of homes on the council-tax valuation list were built before 1930. Most of the rest are later houses outside the conservation areas, where many modest extensions fall under permitted development rights and need no planning application at all. A listed building or a house in a conservation area reverses that: assume you need consent until the council confirms you don't. A lawful development certificate is cheap compared with an enforcement notice.
Your freedom to extend in East Grinstead is middling. Buy a post-war house outside the historic core and you'll have plenty of room to extend. Buy something old in the centre and every change will need permission first.