What permitted development actually covers

Permitted development rights are the reason many homeowners can build an extension without ever submitting a planning application. They are granted automatically by national legislation, but they come with strict conditions. Step outside those conditions and your project becomes unauthorised development, which can lead to enforcement action and a very difficult conversation if you later try to sell.

For a house, the most useful permitted development rules for a single-storey rear extension are broadly these:

• Up to 4 metres beyond the rear wall of a detached house, or 3 metres for a semi-detached or terraced house.
• A maximum height of 4 metres to the ridge, and 3 metres to the eaves if the extension is within 2 metres of a boundary.
• Materials that match the existing house in appearance.
• No extension forward of the principal elevation (usually the front), and no balconies, verandas or raised platforms.
• The extension, together with any previous additions, must not cover more than half the land around the original house.

Side extensions are also possible under permitted development, but only single storey, no more than 4 metres high, and no wider than half the width of the original house.

When your extension will need full planning permission

Certain situations remove your permitted development rights entirely, or require more than the national rules allow. You will need a full householder application if any of the following apply:

• Your home is a flat, maisonette or converted building — these have no permitted development rights at all.
• You live in a conservation area, where side extensions, cladding and extensions to the front generally need permission. Rear extensions may still qualify, but the permitted depth is often reduced.
• Your property is listed, in which case you need listed building consent as well as planning permission for almost any alteration.
• Your house sits in a National Park, Area of Outstanding Natural Beauty or the Broads, where the limits tighten considerably.
• A local Article 4 Direction has withdrawn permitted development rights in your area — a common move in streets where councils want to control the cumulative impact of extensions.
• You want a two-storey extension that goes beyond what the rules allow, or a side extension more than one storey.

If you are unsure, the quickest route is a pre-application enquiry to your local planning authority. It costs a modest fee, takes a few weeks, and gives you written feedback before you commit to drawings and builders.

Two-storey extensions and the boundaries that catch people out

Two-storey extensions are much harder to build under permitted development, and in most cases they need permission. Where they are allowed, the extension must not extend beyond the rear wall of the original house, must not be closer than 7 metres to the rear boundary, and must not include a window in a side elevation facing a neighbour's property unless it is obscure-glazed and fixed shut below 1.7 metres.

The 7-metre rule surprises a lot of people, particularly on shallow suburban plots. Measure it before you brief an architect — it often eliminates a two-storey scheme altogether and pushes you towards a single-storey design instead.

The larger rear extension route: prior approval

There is a middle path for larger single-storey rear extensions on houses that still hold permitted development rights. Through the neighbour consultation scheme, you can apply for prior approval for an extension of up to 8 metres on a detached house and 6 metres on a semi-detached or terraced one. Rather than a full planning application, this is a lighter process in which the council notifies adjoining neighbours and considers the impact on amenity.

The council can refuse if the impact on neighbours is unacceptable, so this is not a formality. It is, however, often faster and cheaper than a full application, and it has helped many homeowners gain the extra kitchen-dining space they wanted without a drawn-out planning battle.

Proving your position: lawful development certificates

Even when you are certain your extension is permitted development, it is worth applying for a Lawful Development Certificate. This is a formal confirmation from the council that the work is lawful, and it costs far less than a planning application.

Without one, you may run into problems when you sell. Solicitors routinely raise enquiries about extensions, and a certificate is the cleanest evidence you can provide. If you cannot produce one, buyers may ask for indemnity insurance or, worse, walk away.

Planning is only half the picture

Planning permission and building regulations are separate regimes, and approval under one does not imply the other. Building regulations cover structure, fire safety, drainage, energy performance and ventilation, and you will need to submit a building notice or full plans application regardless of your planning route.

Two other practical points are worth diarising early. First, the Party Wall etc. Act 1996 applies if you are excavating near a neighbour's foundations or building on a party wall, and notices must be served in advance. Second, if you are extending over or near a public sewer, you may need permission from your water authority.

Get these three strands — planning, building regulations and party wall — moving in parallel rather than in sequence, and your extension will stay on programme. Start with the planning question, settle it in writing, and the rest of the project becomes far more straightforward.