Decking Rules UK: The 300mm, 600mm and 50% Limits Explained
UK decking is permitted development if it sits no more than 300mm above ground and covers under 50% of the garden. Decks above 600mm need guarding. Here is what your quote must cover.
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
300mm: decking is permitted development only if the deck surface is no more than 300mm above the natural ground level. 50%: the decking, together with other extensions and outbuildings, must not cover more than half the area of land around 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). Listed buildings and conservation areas have tighter rules and often need permission regardless.
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.
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.
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
If you are holding a decking or garden quote, upload it and we check whether the structure, the guarding, and the planning position are covered, or whether it is a missing item that turns a cheap deck into an expensive problem. It takes under a minute.
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