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.
Know your size? Compare garden room quotes
Once you know the size that works for your garden, get three itemised quotes from garden room builders. Free to use, no obligation.
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.