The short answer
With planning permission there is no fixed maximum; the council asks whether the annexe is still clearly part of the main house. On the caravan route the limit is fixed by law at 20 m by 6.8 m, with 3.05 m inside height. Most family annexes are between 20 and 50 m².
The answer depends on which route the annexe takes through planning. There are two, and they set limits in very different ways.
With planning permission: no fixed number
When you apply for permission for an annexe, nothing in the regulations sets a maximum floor area. What the council weighs is whether the building will work as part of the existing home or is, in practice, a second house in the garden.
Size is one of the clearest signs. An annexe that is plainly smaller than the house, shares its garden and drive, and sits close to it reads as ancillary. One that approaches the size of the house, or comes with its own parking and a fence round it, starts to look like a separate dwelling, which councils judge far more strictly.
There is no national rule of thumb, and some councils publish their own guidance on annexes. Read it before drawing anything up. If your council offers advice before an application, a sketch plan and a short note about who will live there is a good use of it.
The garden-building limits do not apply
You may have read that outbuildings can be up to 2.5 m high near a boundary and can cover up to half the garden. Those are permitted development limits for buildings used for things like hobbies and storage. A building with a bedroom, kitchen and bathroom is not covered by them at any size, so they are not a guide to how big an annexe can be.
On the caravan route: fixed by law
A mobile-home style annexe can avoid a planning application only if it meets the legal definition of a caravan. For a twin-unit caravan, made in no more than two sections and joined on site, the limits are:
- length: 20 m, not counting the towbar;
- width: 6.8 m;
- height inside, floor to ceiling at the highest point: 3.05 m.
That allows a floor of up to about 136 m² in theory, far more than most families need or most gardens hold. The real constraint is getting two sections that size into the garden.
Space standards for new homes
England has a nationally described space standard for new homes. It does not apply to an annexe that is part of an existing house, but some councils mention it. For reference, it asks for 37 m² for a one-person, one-bedroom home with a shower room and 50 m² for a two-person one-bedroom home. Most annexes are smaller than that, and work well.
What families actually build
| Layout | Usual floor area inside |
|---|---|
| Studio | 16 to 26 m² |
| One bedroom | 26 to 42 m² |
| Two bedrooms | 40 to 65 m² |
Start from who will live there and what help they may need in ten years’ time, then find the smallest plan that does it well. Floor area sets the price more than anything else. Our granny annexe cost calculator shows what each extra square metre adds.