It is the first question almost everyone asks, and the candid answer is that a great many extensions never require a planning application at all. They are built under permitted development, which is a standing national right rather than something for which one applies. The difficulty is that permitted development consists of a set of quite precise limits, and a proposal either sits within them or it does not.
What follows is the shape of those limits in England. It is general guidance rather than a view upon your own house: whether a particular property retains its rights is a question about that property alone.
Single-storey rear extensions
A single-storey rear extension may project up to four metres from the original rear wall of a detached house, or three metres in the case of a semi-detached or terraced house. It may not exceed four metres in height. There is a prior approval route, sometimes known as the larger home extension scheme, which doubles those depths to eight and six metres respectively: it is not a planning application, but it is a notification to the council, and neighbours are consulted before it may proceed.
Two limits tend to catch owners unawares. The first is that the depth is measured from the original rear wall, meaning the house as it was built or as it stood in 1948, rather than as you found it. A property that has already been extended will often have spent some part of its allowance. The second is that extensions may not cover more than half the land around the original house, which is the constraint that most often proves decisive on a modest garden.
Side extensions and two storeys
A side extension must be single storey, no more than four metres in height, and no wider than half the width of the original house. A two-storey rear extension may not project more than three metres, must sit at least seven metres from the boundary opposite the rear wall, and must match the existing roof as closely as is practicable.
What removes those rights
Permitted development is not universal, and it is here that a project most often proves to require an application after all.
- Designated land. Within a conservation area, an Area of Outstanding Natural Beauty or a National Park, the prior approval route is unavailable, and side extensions and two-storey rear extensions require permission outright.
- Listed buildings. Listed building consent is separate from planning permission, and it is required for work affecting the character of the building, within as well as without.
- Flats and maisonettes. These carry no permitted development rights for extensions.
- Article 4 directions. A council may withdraw permitted development rights across an area. They are common within conservation areas and are easily overlooked.
- Previous conditions. Newer estates frequently have permitted development removed by a condition attached to the original planning consent for the estate.
Permitted development is not invariably the better answer
There is a habit of regarding a permitted development scheme as the safe course and a planning application as the risky one. That is worth questioning. Designing to remain a few centimetres within a national limit can produce a poorer house than designing what the property genuinely wants and then making the case for it, particularly upon a larger plot where those limits bear little relation to what the site would comfortably accommodate.
What we would always do first is establish which rights the property still holds, since that is a question of fact and may be answered before any design work begins. The choice between the two routes then becomes a real choice rather than a guess.
A lawful development certificate
Where a scheme does fall within permitted development, it is usually worth applying to the council for a lawful development certificate. It is not a permission, because none is required; it is the council confirming in writing that none is required. Buyers and their solicitors ask for it, and it is considerably easier to obtain before the work is built than afterwards.
We prepare and manage both routes, and you will have our candid view of the prospects before you commit to drawings. What we will never do is promise an outcome that is not ours to give.
Written by Thurston King Architects, a RIBA Chartered Practice in Sutton Coldfield. This is general guidance rather than advice on your property: what a particular site will support depends on the property, its setting and the view your local planning authority takes.
