It is one of the first questions almost every Horsham homeowner asks us: “Will I need planning permission for this?” It is a sensible place to start, because the answer shapes your timeline, your budget and, in some cases, the design itself. The good news is that a great many extensions across Horsham and the surrounding West Sussex villages can be built without a full planning application at all, under what are known as permitted development rights. The important part is knowing where the line sits — and when your project crosses it.
In this guide we explain the difference between permitted development and full planning permission, when your extension is likely to fall on each side of that line, and the local factors around Horsham that can change the rules entirely. As always, treat this as a helpful overview rather than a substitute for advice on your specific property, because the finer points genuinely do turn on the details.
Permitted development versus planning permission
Permitted development is a national grant of planning permission that lets you carry out certain building work without applying to the council each time. It exists to save homeowners and local authorities from processing applications for modest, uncontroversial projects. If your extension fits inside the permitted development limits, you can usually proceed without a planning application — though you may still want a Lawful Development Certificate to prove the work was lawful, which is invaluable when you come to sell.
Full planning permission is what you apply to Horsham District Council for when a project falls outside those limits. It involves submitting drawings, paying a fee and waiting for a decision, typically around eight weeks for a householder application. Planning permission is not something to fear — thousands of extensions are approved every year — but it does add time to the front of a project, and it is far better to know you need it at the design stage than to discover it halfway through.
When your extension is likely to be permitted development
For a typical house in Horsham that has not already been extended and sits outside a designated area, the following broad allowances usually apply. A single storey rear extension can generally project up to three metres from the original rear wall for an attached or terraced house, or four metres for a detached house, under standard permitted development. There is also a “larger home extension” route that can allow up to six or eight metres respectively, but this requires a prior approval application to the council and a neighbour consultation, so it is not quite as automatic as it sounds.
Height matters too. A single storey rear extension should not exceed four metres in height, and where it comes within two metres of a boundary the eaves height is limited to three metres. Side extensions must be single storey, no more than half the width of the original house, and no higher than four metres. Materials should be similar in appearance to the existing house, and the extension must not cover more than half the land around the original property. Get inside all of these and you are usually in permitted development territory.
When you will almost certainly need planning permission
Some projects fall outside permitted development by their very nature. Double storey and two storey extensions have their own, tighter set of rules and frequently need a full application, particularly on the side of a property or where they come close to a boundary. Anything that extends forward of the principal elevation — in other words, towards the road at the front of the house — will normally require permission. So will an extension that is taller than the existing roof, that sits on designated land, or that pushes beyond the depth and height figures above.
It is also worth remembering that permitted development rights apply to houses, not flats or maisonettes. If you own a flat in Horsham town centre, you will need planning permission for almost any external alteration. And if your home has already been extended by a previous owner, some or all of your permitted development allowance may already have been used up, which catches a lot of people out.
The Horsham factors that can remove your rights
This is where local knowledge earns its keep. Permitted development rights are restricted or removed altogether in a number of situations that are common around Horsham. If your home sits within one of the district’s conservation areas — and there are several, including parts of the historic town centre and Causeway — your rights are curtailed, particularly for side extensions and anything visible from the street. Listed buildings need listed building consent for most alterations, entirely separately from planning permission, and the penalties for getting that wrong are serious.
Some estates and newer developments, including parts of Broadbridge Heath and other recent schemes, are covered by an Article 4 direction or by planning conditions that strip out permitted development rights so that the character of the area is preserved. On these properties even a modest rear extension may need a full application. Because these designations are not always obvious from the street, it is well worth checking your property’s status with Horsham District Council, or asking us to check as part of your initial consultation, before you settle on a design.
Planning is not the same as Building Regulations
One point that causes endless confusion is the difference between planning permission and Building Regulations. They are two completely separate approvals. Planning is about whether you can build something and what it looks like. Building Regulations are about whether it is built safely and to standard — covering structure, insulation, fire safety, drainage, ventilation and energy efficiency. Almost every extension needs Building Regulations approval even when it does not need planning permission. So an extension being “permitted development” does not mean it is free of paperwork; it simply means you skip the planning application, not the building control sign-off.
Do not forget the Party Wall Act
If your extension involves work on or near a shared boundary with a neighbour — building up to the line, cutting into a party wall, or excavating foundations close to their property — the Party Wall etc. Act 1996 may require you to serve formal notice on your neighbours. This is separate again from both planning and Building Regulations. It is rarely a problem in practice, especially when neighbours are kept informed early, but it is another box that needs ticking on many Horsham extension projects, particularly on the town’s Victorian and Edwardian terraces where houses sit close together.
How we help you get it right
The honest reality is that the rules are layered, and a single detail — a conservation area boundary, a previous extension, the distance to a fence — can move your project from permitted development into a full application, or the other way around. That is exactly why we start every extension with a free site visit. We look at your property, check its planning status, and tell you plainly which route yours falls under before you commit to anything. Where a full application is needed, we can guide you through it and work alongside architects and planning officers to give your project the best chance of a smooth approval.
If you are thinking about extending your home anywhere in Horsham, Broadbridge Heath, Southwater, Roffey or the surrounding villages, get in touch for a free, no-obligation consultation. We will help you understand exactly what your project needs — and then build it properly, from first drawing to final handover.