What size garden room can I have?
The short answer
Take your garden area behind and beside the house, subtract 25 m² and any sheds you're keeping: that's your largest garden room, up to a cap of 30 m² for all garden buildings together. A 6 × 7 m terraced garden (42 m²) allows 17 m². Enter your own measurements below to see it drawn to scale.
A guide to the size rules only. The room must also sit behind the front of the house, stay within the height limits (3 m, or 4 m with a tiled or slated pitched roof) and not be lived in or used for a business. Use the planning checker for those.
Common Irish garden sizes
The largest garden room each of these gardens allows without planning permission, with no existing sheds:
| Garden | Area | Largest room |
|---|---|---|
| Small terraced garden 5 × 6 m | 30 m² | 5 m² |
| Typical terraced garden 6 × 7 m | 42 m² | 17 m² |
| Semi-detached, 1990s estate 8 × 9 m | 72 m² | 30 m² |
| Semi-detached, larger garden 9 × 12 m | 108 m² | 30 m² |
| Detached or corner site 12 × 15 m | 180 m² | 30 m² |
Once a garden passes 55 m², the 30 m² cap becomes the limit rather than the open space rule.
How the size rules work
Two rules decide how big a garden room can be without planning permission. Whichever is smaller wins:
- The 30 m² cap. All sheds, garages, greenhouses and garden rooms on the property count together. An existing 8 m² shed leaves 22 m².
- The 25 m² open space rule. At least 25 m² of private garden must remain behind or beside the house after you build.
See the open space rule for a worked example.
Choosing the shape
- Depth of 3 to 4 m suits most uses. Much less than 2.5 m feels like a corridor.
- Wide and shallow rooms get more light and a better view of the garden.
- Standard sizes are cheaper than custom ones. Round down to the nearest size a supplier makes.
See prices by size for scale plans of what fits in each size.
Common questions
What size garden room can I build without planning permission?
Up to 30 m², as long as at least 25 m² of garden is left behind or beside the house afterwards, and the 30 m² includes any sheds or garages you already have. In a small garden the 25 m² rule usually limits you first: subtract 25 from your garden area to find your maximum.
Does my side garden count towards the 25 m²?
Private open space behind or beside the house counts. A front garden does not. If your side passage is private and usable, include it.
Does the garden room have to be a certain distance from the boundary?
Not under the garden room (Class 3) exemption. In practice, leave enough space to maintain the walls and cladding, usually at least 0.3 to 0.5 m, unless the room is designed for a tight fit. A Class 3A garden home must be at least 0.6 m from any boundary.
Does a patio or decking count as open space?
The rule is about private open space left for the occupants, and a lawn or patio at ground level is open space. Raised decking has its own rules, so if a large part of your remaining garden is decked, ask your council how they treat it.
Sources
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