You can, but the town has opinions about it. More than 160 buildings are listed. Seven conservation areas carve up the map. The neighbours still lodge over a thousand planning applications a year. Extending here is common, and it is rarely quick.
Historic England's register lists 163 buildings in the area: none at Grade I, 18 at Grade II* and 145 at Grade II. Listing covers the inside as well as the outside, so changing a listed building usually needs listed building consent on top of any planning permission, including for work that would be routine anywhere else. Swapping the windows, knocking through a wall or replacing a staircase can all need consent. Owners of a Grade II* building should expect even closer attention. With no Grade I buildings in the area, nobody is guarding a cathedral, but 18 Grade II* buildings is plenty for a commuter town that mostly went up after the railway arrived.
There are seven conservation areas: Franklands Village, Lewes Road, Lindfield, Lucastes, Mill Hill Close, Muster Green and The Heath. Inside one, the automatic rights to extend without applying are narrower, work that changes the look of the street tends to need permission, and trees get protection that surprises people with a chainsaw and a free Saturday. The details vary by area, so the council's appraisal for the specific area is the document to read, ideally before the offer rather than the architect.
There is 1 scheduled monument in the area. That is the strictest level of protection there is, and it matters mainly to anyone whose land sits on or next to it rather than to the average semi.
The planning register shows what people here are doing anyway. There were 2,288 planning applications between April 2024 and June 2026, and 1,053 in the twelve months to June 2026. That is a town full of kitchen extensions, loft rooms and garden offices in progress. Plenty of households are altering their homes; the protections shape how they do it, not whether anyone tries.
In practice, the answer depends on which house you buy. A post-war semi outside the conservation areas and not listed will usually have the normal permitted development rights, meaning some rear extensions and loft conversions can go ahead without a full application, subject to the national limits and a check that no earlier permission removed those rights. A listed building, or anything inside one of the seven conservation areas, will usually need a formal application for changes that would sail through elsewhere. Ask the seller's solicitor for the planning history, and check the listing and conservation-area status before you pay for a survey.
Freedom to extend is one of Haywards Heath's weaker points: below average, with the heritage rules biting hardest exactly where the prettiest houses are. Pick an unlisted house outside the conservation areas and the path is ordinary; pick a period one inside them and budget for patience as well as builders.