The short answer
To stay within permitted development, a garden room must be single storey, no more than 2.5m to the eaves and 3m overall (4m for a dual-pitch roof), drop to a maximum of 2.5m overall if any part is within 2m of a boundary, sit behind the principal elevation of the house, and — with other outbuildings — cover no more than half the garden. There is no fixed maximum floor area, but the 50% garden coverage rule and the height limits effectively cap the size. This is general information — always confirm with your Local Planning Authority.
People often ask “how big can a garden room be without planning permission?” expecting a single number. There isn’t one — permitted development controls a garden room through a combination of height, position and garden-coverage rules rather than a maximum floor area. This guide sets out each limit precisely, explains the all-important 2m boundary rule, and shows how the rules combine to determine the largest room you can build without a planning application.
The permitted development limits
- Storeys One
- Eaves height Max 2.5m
- Overall height (flat / mono-pitch) Max 3m
- Overall height (dual-pitch) Max 4m
- Within 2m of a boundary Max 2.5m overall
- Garden coverage (all outbuildings) ≤ 50%
- Maximum floor area None set — limited by 50% rule
Height limits in detail
Height is measured from the highest ground level next to the building. A garden room must be single storey. If it has a flat or mono-pitch (single-slope) roof, the maximum overall height is 3m, with a maximum eaves height of 2.5m. If it has a dual-pitched (apex) roof, the maximum overall height rises to 4m, again with eaves no higher than 2.5m. These extra centimetres matter: most modern garden rooms use a flat or shallow mono-pitch roof, which keeps them well under the 3m limit while still allowing a comfortable internal ceiling height of around 2.2–2.4m once the floor build-up and roof structure are accounted for. The base height counts toward the total, so a raised platform can eat into your height allowance.
| Roof type | Max eaves height | Max overall height |
|---|---|---|
| Flat or mono-pitch | 2.5m | 3m |
| Dual-pitch (apex) | 2.5m | 4m |
| Any roof within 2m of a boundary | 2.5m | 2.5m |
The 2m boundary rule
This is the rule that catches people out most often. If any part of the garden room is within 2m of a boundary — a fence, wall or the edge of your property — the maximum overall height drops to 2.5m regardless of roof type. So if you want a taller room with a pitched roof, it must sit at least 2m clear of every boundary. If your garden is narrow and you want the room close to a fence, you are restricted to 2.5m overall. There are also fire-spread considerations near boundaries that affect Building Regulations, which is a separate matter from planning but worth checking at the same time.
The 50% garden coverage rule
Permitted development requires that outbuildings — taken together with any other sheds, garages or structures — do not cover more than 50% of the total area of land around the “original house” (the house as it stood in 1948, or as built if later). This is what effectively caps the size of a garden room: there is no stated maximum floor area, but you cannot build out so far that outbuildings exceed half your garden. The 50% calculation excludes the footprint of the house itself but includes existing outbuildings, so a garden with a large shed already in it has less room to spare. If you are planning a large room, measure your garden carefully and account for everything that already stands on it.
How big a garden room can you actually build?
For most suburban gardens, the practical answer is a room of up to around 15–30m² built with a flat or mono-pitch roof under 3m, set behind the house and within the 50% coverage rule. Below 15m² with no sleeping use, you usually avoid Building Regulations too; between 15m² and 30m² there are conditions; above 30m² Building Regulations approval is normally required — see our cost guide and the planning overview for how these thresholds interact. Larger rooms are entirely possible, but they may take you out of permitted development, into a planning application, or into full Building Regulations. This is general information; rules vary locally and can be removed by an Article 4 direction, so always confirm with your Local Planning Authority before building.
Frequently asked questions
How big can a garden room be without planning permission?
There is no fixed maximum floor area. The size is limited by the rule that outbuildings must not cover more than 50% of the garden, plus the height limits (2.5m eaves, 3m overall, or 4m for a dual-pitch roof). In practice many permitted-development garden rooms are 15–30m². Always confirm with your Local Planning Authority.
How close to the boundary can a garden room be?
A garden room can be built right up to the boundary under planning rules, but if any part is within 2m of a boundary the maximum overall height drops to 2.5m. There are also separate fire-spread rules under Building Regulations for structures close to a boundary.
What is the maximum height of a garden room?
2.5m to the eaves and 3m overall for a flat or mono-pitch roof, or 4m overall for a dual-pitch roof — but only 2.5m overall if any part is within 2m of a boundary. Height is measured from the highest adjacent ground level.
Does the base count toward the height limit?
Yes. Overall height is measured from the natural ground level, so a raised platform or thick base reduces the height available for the room itself. Keep the base low if you want maximum internal ceiling height within the limits.
Sources & further reading
- GOV.UK Planning Portal — outbuildings: heights, the 50% rule and the 2m boundary rule
- The Town and Country Planning (General Permitted Development) Order (GPDO) — Class E outbuildings
- GOV.UK — permitted development on designated land (conservation areas, National Landscapes, National Parks)
- Your Local Planning Authority — confirmation for your property and any Article 4 restrictions
This is general information, not advice for your specific property or project. Planning rules vary locally and permitted development can be removed by an Article 4 direction — always confirm with your Local Planning Authority before you build.