There is no single maximum floor area for a garden room under permitted development. The size of garden room you can build without planning permission depends on three things: how much of your garden you have already built on (the 50% rule), the height of the building, and whether your property sits on designated land. Get all three right and you will not need to apply for planning permission, no matter how large the footprint.
These rules come from Class E of the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended). Below, we break down each rule with the exact measurements, practical examples, and a look at which of our garden room sizes fit comfortably within the limits.
The 50% Rule: How Much of Your Garden Can You Cover?
Under permitted development, the total area of ground covered by all buildings, extensions, outbuildings, sheds, greenhouses and containers within your curtilage (your property boundary) must not exceed 50% of the total curtilage area, excluding the footprint of the original house.
“Original house” means the house as it was first built, or as it stood on 1 July 1948 if it was built before that date. This is important because any extensions added by previous owners count towards your 50% allowance, even if they were there when you bought the property.
What counts towards the 50%?
- Rear and side extensions (including conservatories)
- Existing sheds, summerhouses, and greenhouses
- Any outbuilding you plan to add, including the new garden room
- Garages (unless they were part of the original house)
A practical example
Say your total curtilage is 300 square metres and the original house covers 80 square metres. That leaves 220 square metres of land around the house. You can cover up to 50% of that 220 square metres, which is 110 square metres, with buildings. If you already have a 15 square metre shed and a 20 square metre rear extension, you have used 35 square metres. That leaves 75 square metres of allowance for a garden room, which is more than enough for even our largest standard model.
For a smaller property with a total curtilage of 150 square metres and an original house of 60 square metres, the calculation gives you 45 square metres of allowance. After subtracting an existing 8 square metre shed, you still have 37 square metres available. Our Multi+ at 31.5 square metres would fit comfortably.
Height Limits for Garden Rooms
Height is where many people get caught out. The rules set three different maximum heights depending on roof type and how close the building sits to the boundary.
The three height limits
- Within 2 metres of a boundary: The maximum overall height is 2.5 metres. This applies to the highest point of the building, not just the eaves. In practice, this means a flat roof or very shallow mono-pitch roof.
- More than 2 metres from a boundary, with a dual-pitched roof: Maximum overall height of 4 metres, with eaves no higher than 2.5 metres.
- More than 2 metres from a boundary, with any other roof type: Maximum overall height of 3 metres, with eaves no higher than 2.5 metres.
Most garden rooms use a flat roof or shallow mono-pitch design, which keeps the height well within the 2.5 metre limit even when placed close to a boundary. All of our standard garden rooms are designed to comply with these height restrictions, so placement near a boundary is straightforward.
Other Permitted Development Conditions
Beyond size and height, Class E sets several other conditions your garden room must meet.
Position on the plot
The building cannot sit on land forward of the principal elevation (the front) of the original house. In simple terms, garden rooms must go in the rear garden or, in most cases, to the side.
Single storey only
The garden room must be single storey. No mezzanines, no loft rooms, no upper floors.
No raised platforms or balconies
Any veranda, balcony, or raised platform attached to or forming part of the garden room must not exceed 0.3 metres (300mm) above ground level. A low deck or step is fine, but a raised terrace is not.
Use and purpose
The building must be used for purposes incidental to the enjoyment of the house. A home office, gym, studio, hobby room, or entertaining space all count. What does not count is self-contained living accommodation, which means a garden room with its own kitchen, bathroom, and sleeping area intended as a separate dwelling will need planning permission.
Designated Land: Conservation Areas, AONBs, and National Parks
If your property is on designated land, the rules are tighter. The legislation groups these areas under the term “Article 2(3) land” and applies additional restrictions.
Conservation areas (and all Article 2(3) land)
On any Article 2(3) land, including conservation areas, Areas of Outstanding Natural Beauty (AONBs), National Parks, the Broads, and World Heritage Sites, you cannot place a garden room to the side of the house. It must be in the rear garden only.
AONBs, National Parks, the Broads, and World Heritage Sites
These designations carry an extra restriction beyond the side-of-house rule. Any outbuildings positioned more than 20 metres from any wall of the house must not exceed 10 square metres in total ground coverage. This does not apply to buildings within 20 metres of the house, and it does not apply to conservation areas on their own.
Listed buildings
If your home is a listed building, permitted development rights for outbuildings do not apply at all. You will need both planning permission and listed building consent. Our team can help you with this process. Please get in touch to discuss your options.
Article 4 Directions
Your local council can also remove permitted development rights through an Article 4 Direction. This is common in some conservation areas. Always check with your local planning authority before starting any project. For a detailed overview of these rules, see our planning permission and building regulations guide.
Building Regulations: A Separate Set of Rules
Planning permission and building regulations are two different things, and it is worth understanding both. Even when your garden room does not need planning permission, building regulations may still apply depending on the floor area and position.
- Under 15 square metres: Generally exempt from building regulations.
- 15 to 30 square metres: Exempt from building regulations provided the building contains no sleeping accommodation and is either positioned at least 1 metre from any boundary, or constructed substantially from non-combustible materials.
- Over 30 square metres: Building regulations approval is required. The building must meet current standards for structure, fire safety, insulation, and electrical work.
Our garden rooms are built to meet or exceed building regulations standards regardless of size. We use high-performance insulation, proper electrical installations, and fire-rated materials as standard.
Which Garden Room Sizes Work Without Planning Permission?
To put all of this into context, here are our four standard garden room models with their footprints, and how they sit against the rules.
- Midi (4.0m x 3.0m = 12 square metres): Under 15 square metres, so exempt from building regulations entirely. Fits easily within the 50% rule on nearly any property.
- Maxi (5.0m x 3.5m = 17.5 square metres): Falls in the 15 to 30 square metre bracket. Exempt from building regulations if placed 1 metre or more from the boundary.
- Multi (6.0m x 4.0m = 24 square metres): Also in the 15 to 30 square metre bracket. A very popular size for home offices and studios with plenty of space for a desk, meeting area, and storage.
- Multi+ (7.0m x 4.5m = 31.5 square metres): Just over the 30 square metre threshold, so building regulations apply. Still well within permitted development rights for planning purposes on most properties.
All four models are single storey and designed to sit within the 2.5 metre height limit, meaning they can be placed close to a boundary without issue. If you need a different size, we also build bespoke garden rooms to any dimensions.
Getting a Lawful Development Certificate
While not required, we strongly recommend applying for a Lawful Development Certificate (LDC) from your local council before you build. This is a formal confirmation that your garden room qualifies as permitted development. It typically costs around £100 and takes a few weeks to process.
An LDC is especially useful when you come to sell your property, as it provides proof to buyers and their solicitors that the building was erected lawfully. Without one, you may face questions during conveyancing that could delay a sale.
Quick Summary: Garden Room Size Rules at a Glance
- No fixed maximum floor area, but total site coverage (all outbuildings and extensions) must not exceed 50% of your curtilage excluding the original house.
- Maximum height of 2.5 metres if within 2 metres of a boundary.
- Maximum height of 4 metres (dual-pitch roof) or 3 metres (other roofs) if more than 2 metres from a boundary. Eaves always capped at 2.5 metres.
- Must be single storey, in the rear garden (not forward of the front of the house), and used for purposes incidental to the house.
- On designated land, additional restrictions apply. Check with your local planning authority.
- Building regulations are separate from planning permission. The thresholds are 15 and 30 square metres.
If you are unsure about any of these rules or want to discuss which size works best for your garden, give us a call or drop us a message. We have been building garden rooms across South East London, Kent, Surrey, and Sussex since 2004, and we are happy to visit your property and talk through the options.
Save £5,000 with Our Ambassador Programme
If you are considering a garden room, ask us about the Ambassador Programme. We offer a discount of up to £5,000 to customers who are happy for us to use their build as a local show garden for a short period after installation. It is a great way to reduce the cost while helping other homeowners in your area see the quality of our work first hand.