What is the maximum garden room size without planning permission?

People often ask for a number of square metres, but the rules for garden buildings in England do not set one. Instead, permitted development limits the height, the position and the share of the garden a room can take up. Within those limits, a garden room can be surprisingly large. This is a guide, not legal advice; if anything is unclear, ask your council before you build.

The national limits

These come from Class E of the General Permitted Development Order 2015. They apply to buildings “incidental to the enjoyment of the dwellinghouse”, which covers offices, studios, gyms and hobby rooms used by the household.

Heights are measured from the highest ground next to the building, which matters on a sloping plot.

So how big can it actually be?

Take a typical semi-detached garden in Chesterfield, about 8 metres wide and 15 metres long. Half of that is 60 m², minus anything already built. In theory you could build a very large room. In practice, three things bring the size down:

• The 2-metre rule. Most people want the room near the back fence, which means a flat roof and a total height of 2.5 metres. That limits the roof build-up and the internal ceiling.
• The building regulations. Under 15 m² with no sleeping accommodation, normally exempt. Between 15 and 30 m², exempt only if at least 1 metre from every boundary or built substantially of non-combustible materials. Over 30 m², building regulations approval is needed.
• Budget. At around £1,600 to £2,000 per square metre built, a 30 m² room costs £48,000 to £60,000. The cost calculator shows what each size costs.

That is why so many garden rooms land between 10 and 15 m²: big enough for a desk and a sofa, close enough to the fence to save garden, and clear of the building regulations.

Where the limits are tighter

Conservation areas and other protected land

In conservation areas, national parks, Areas of Outstanding Natural Beauty and World Heritage Sites, a garden room cannot sit between a side wall of the house and the boundary. Behind the house, the usual limits apply.

Chesterfield has 12 conservation areas, including Chatsworth Road, Queens Park, Somersall, Brimington, Old Whittington, Spencer Street, Staveley and the town centre. Check the council’s map before you pick a spot.

Article 4 directions

A council can remove some permitted development rights in a particular area with an Article 4 direction. Chesterfield Borough Council lists directions in the Abercrombie Street, Barrow Hill and Town Centre conservation areas, and one covering Upper Newbold Close, Moorhay Close and Water Meadow Lane. If you live there, ask the council what applies before you plan anything.

The Peak District National Park

A large part of Derbyshire lies inside the national park, where the Peak District National Park Authority decides planning. On top of the side-wall rule, there is one more limit: buildings more than 20 metres from any wall of the house must not cover more than 10 m² in total. The same cap applies in World Heritage Sites, which in Derbyshire means the Derwent Valley Mills corridor from Matlock Bath to Derby. More on the county on our Derbyshire page.

Listed buildings

There are no permitted development rights for outbuildings within the grounds of a listed building. Any garden room needs planning permission, and you may need listed building consent too.

New-build estates

Some planning permissions for new estates remove permitted development rights by condition. Check the original permission for your house, or ask the council.

Use matters as much as size

A garden room used as an office for the person who lives there is normally fine. If you plan to sleep in it, let it out, or run a business with staff and customers visiting, it is no longer “incidental” to the house, and planning permission is likely to be needed whatever the size.

How to be sure

1. Measure your garden and anything already built, and check the half-garden rule.
2. Decide where the room will go and how close it is to each boundary.
3. Check whether you are in a conservation area, an Article 4 area, the Peak District or a World Heritage Site, and whether the house is listed.
4. If you are still unsure, apply to the council for a Lawful Development Certificate. It confirms in writing that the room is permitted development, which also helps when you sell.

A good builder will ask these questions on the first visit. If you would like one to look at your garden in Chesterfield, Derbyshire or Doncaster, use the form below.