Decking Rules UK: The 300mm, 600mm and 50% Limits Explained
In England a deck stays permitted development if it sits no more than 300mm above ground and covers no more than 50% of the garden. Raised decks need guarding.
Decking looks like the kind of garden job you can just get on with, and often you can. But there are three numbers that decide whether you need planning permission and whether the deck is safe: 300mm, 600mm, and 50%. Get them right and most decks are permitted development. Get them wrong and you risk a deck that has to come down. Here is what each one means.
The three numbers
These are England rules. They come from the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class E. Scotland, Wales and Northern Ireland have their own permitted development orders with different limits, so check yours.
300mm: Class E does not permit "the construction or provision of a verandah, balcony or raised platform" (paragraph E.1(h)), and the interpretation of Part 1 defines "raised", in relation to a platform, as a platform with a height greater than 0.3 metres. So a deck over 300mm is a raised platform, which falls outside permitted development and needs an application. Below 300mm it is not a raised platform and Class E can cover it.
50%: paragraph E.1(b). The ground covered by buildings, enclosures and containers in the curtilage, not counting the original house, must not exceed 50% of the curtilage excluding the ground area of the original house.
600mm: once a deck is high enough that a fall would injure someone (around 600mm or more), it needs guarding (a balustrade). This is a safety matter, not a planning one.
Class E also does not apply at all within the curtilage of a listed building (E.1(g)).
In short: keep it low and not too big, and it is usually permitted development. Raise it, and you are into planning permission and guarding.
The 300mm height rule
The deck surface can sit up to 300mm above the ground without planning permission. A ground-level patio deck is comfortably within this. The moment you build a raised deck (to level out a sloping garden, or to meet a first-floor door) above 300mm, you need planning permission. This is the rule most commonly broken, usually by accident, when a deck is built up to meet a back door.
Worth knowing why the number is 300mm, because it explains what counts. The Order does not set a height limit for decks as such. It excludes verandas, balconies and raised platforms from permitted development altogether, then defines "raised" as a platform higher than 0.3 metres. So this is not a limit you creep over by 50mm and remain broadly compliant. Cross 300mm and the deck is a different category of thing, one permitted development does not cover at all. The same exclusion applies to house extensions under Class A, which is why a raised terrace on top of a single-storey extension always needs an application.
The 50% garden rule
Decking counts towards the same 50% limit as sheds, garden rooms, and extensions: all the additions around your original house together cannot cover more than half the garden. If you already have a large shed or a garden room, a new deck might push you over the line into needing planning permission, even if the deck itself is low.
There is a much tighter limit on designated land. Under paragraph E.2, if your garden is in a National Park, an area of outstanding natural beauty, the Broads or a World Heritage Site, anything sited more than 20 metres from any wall of the house is capped at 10 square metres in total, not 50% of the plot. A low deck down the far end of a large garden in a National Park can therefore need permission when the same deck elsewhere would not. And under paragraph E.3, on article 2(3) land, which includes conservation areas as well as the designations above, nothing may go between a side wall of the house and the boundary.
The 600mm guarding rule (safety, not planning)
This one is about safety rather than planning. A low deck needs nothing. A raised deck high enough to fall from needs a balustrade, generally taken as a drop of around 600mm or more. The guarding must be high enough and the gaps small enough that a 100mm sphere cannot pass through, the same principle as a staircase balustrade.
If you are quoting a raised deck, the balustrade is not optional decoration, it is a safety requirement. A quote for a high deck with no guarding line, or with a wide-gap balustrade a child could slip through, is cutting a corner that matters. Check the quote includes properly specified guarding.
What a fair decking quote includes
- The finished deck height above ground (to check the 300mm rule)
- The structure: posts, footings, joists, and bearers, not just the boards
- The board material (softwood, hardwood, or composite) and fixings
- Guarding (balustrade) where the deck is raised
- Confirmation of whether planning permission is needed
- Any drainage or membrane under the deck
A deck is only as good as its frame. A quote that prices the visible boards but is vague about the footings and joists is the decking equivalent of a driveway with no sub-base; see our driveway cost guide for the same principle. For comparing garden quotes generally, see landscaping quote comparison.
Check your quote
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