Do you need planning permission for a garden room?

The limits in Class E

(e) the height of the building, enclosure or container would exceed: (i) 4 metres in the case of a building with a dual-pitched roof, (ii) 2.5 metres in the case of a building, enclosure or container within 2 metres of the boundary of the curtilage of the dwellinghouse, or (iii) 3 metres in any other case; (f) the height of the eaves of the building would exceed 2.5 metres;

Three points that catch people out

1. One corner within 2 metres caps the whole building. The government's technical guidance says that if any part is within 2 metres of the boundary, the 2.5 metre limit applies to the whole building.
2. A flat roof is held down by the eaves limit. The guidance treats the eaves as the point where a flat roof meets the outside wall, so a flat-roofed garden room is usually limited to 2.5 metres wherever it stands. The 3 metre figure suits a mono-pitched roof whose low edge stays at 2.5 metres.
3. Height is measured from the highest ground next to the building (article 2(2) of the Order). On a slope the building can stand taller than the limit on its downhill side.

The 50% test counts extensions as well as outbuildings, measured against the original house, so a house already extended may have less room than it looks.

Protected land

On article 2(3) land, which includes conservation areas, national parks, areas of outstanding natural beauty, the Broads and World Heritage Sites, no part of the building may stand between a side wall of the house and the boundary (paragraph E.3). In a national park, an area of outstanding natural beauty, the Broads or a World Heritage Site, buildings more than 20 metres from any wall of the house may cover no more than 10 square metres in total (paragraph E.2).

Building Regulations are a separate test

The "no sleeping accommodation" rule people quote comes from the Building Regulations, not from planning. Schedule 2 exempts small detached buildings:

A detached single storey building, having a floor area which does not exceed 30m2, which contains no sleeping accommodation and is a building: (a) no point of which is less than one metre from the boundary of its curtilage; or (b) which is constructed substantially of non-combustible material.

A detached building, having a floor area which does not exceed 15m2, which contains no sleeping accommodation.

An exempt building still has to meet Part P where its electricity comes from the house (regulation 9(3)(b)). A garden room over 30 square metres needs approval, and so does one between 15 and 30 square metres that is within a metre of the boundary and not built substantially of non-combustible material: see Building Regulations approval.

What this means for you

• Settle the use first. Somewhere to work or train can fit the incidental test; somewhere to sleep does not.
• Check for a listed building, an article 4 direction, or a planning condition removing these rights (article 3(4)). The permitted development checker takes you through the tests in order.

Getting certainty

A lawful development certificate under section 192 of the Town and Country Planning Act 1990 confirms that proposed work would be lawful. The council has 8 weeks to decide, and the certificate is conclusive unless something relevant changes before you build. It is worth having where the building is close to a limit or the use is borderline. See lawful development certificates.