Yes, you can extend in Horsham, but in the old town you will be asking permission for things other towns let you just get on with. Nearly two hundred buildings are listed. Four conservation areas cover the historic streets and one neighbouring village. Well over a thousand planning applications went in last year. Plenty of people are extending, and plenty of them are filling in forms first.
Historic England lists 195 buildings in and around the town: two at Grade I, 10 at Grade II* and 183 at Grade II. Listing covers the whole building, inside and out, so a new window, an internal wall or a replacement door on a listed house needs listed building consent on top of any planning permission. Grade II is the everyday tier and the one most buyers will meet, but it is not a light-touch tier. The owner of a listed home is not really its owner so much as its current custodian, and the paperwork will remind you.
There are four conservation areas: Horsham, Horsham London Road, Horsham Richmond Road and Warnham. Inside them, the council takes an interest in how a building looks from the street, and work that would be routine elsewhere, such as demolition, some roof changes and work to trees, can need consent. The town's oldest streets, including the Causeway down to the parish church, are where these rules bite hardest. Buy on a period street and the council has a say in what it looks like.
There are also four scheduled monuments in the area, including the motte and bailey castle north of Chennells Brook Farm and the moated site and fishponds south of Chesworth House. Scheduling is the strictest protection there is, but it matters mainly to anyone with land near one, and most Horsham gardens are nowhere close.
The neighbours are not put off. Horsham District Council recorded 3,257 planning applications across the area between April 2024 and June 2026, including 1,488 in the most recent twelve months. That is live evidence that people alter their homes here all the time. It says nothing about how many were approved, only that the queue at the planning office is never empty.
In practice, where you buy decides how much freedom you get. Most of Horsham's housing is post-war estates such as the Needles, Holbrook and North Heath, largely outside the conservation areas and away from listed buildings, where many modest extensions can be done under permitted development without a full application. In the conservation areas, or on anything listed, assume you will need consent and budget time for it. Before buying with a specific project in mind, check whether the property is listed or inside a conservation area, then ask the council what the project needs. Finding out after completion that your kitchen extension needs listed building consent is an expensive way to learn about Grade II.
Freedom to extend is limited in Horsham. The post-war estates give reasonable room, but the historic centre is tightly controlled, and any house with character has probably already caught the council's eye.