Decking, garden levels and the 300 millimetre rule
The General Permitted Development Order defines a raised platform as one more than 0.3m high, and excludes raised platforms from both Class A and Class E. Above that line you are making an application whatever the deck is made of.
Garden decking more than 300mm high needs planning permission. There is no exemption for it, no size allowance and no distinction between a small platform outside a back door and a terrace covering half the garden. The General Permitted Development Order defines a raised platform as a platform with a height greater than 0.3m, and it then excludes verandahs, balconies and raised platforms from both Class A, which covers extensions, and Class E, which covers garden buildings and other structures within the curtilage.
So the 300mm line is the whole rule. Below it, a deck laid on the ground is usually not something anybody needs permission for. Above it, you are making a planning application whatever the deck is made of.
What a raised platform means in the Order
Part 1 of Schedule 2 to the Order deals with development within the curtilage of a dwellinghouse. Class A permits the enlargement, improvement or other alteration of a house, subject to limits and conditions, and one of those conditions removes from the class anything that consists of or includes the construction or provision of a verandah, balcony or raised platform. Class E, which permits buildings and other structures incidental to the enjoyment of the dwellinghouse, carries the same exclusion.
The definition applies to the platform rather than to the material or the purpose. Timber decking, a composite deck, a paved terrace on a raised sub-base, a raised seating area built off blockwork and a platform supporting a hot tub are all raised platforms if the finished surface sits more than 300mm above the ground. A patio laid directly on a bed of sand at existing ground level is not.
Two practical consequences follow. First, a deck raised to meet the internal floor level of a house with a raised ground floor will almost always exceed 300mm, which is exactly the deck most people want. Second, the exclusion applies anywhere in the curtilage, so moving the deck to the far end of the garden does not help.
Measuring the 300 millimetre height
The Order gives the figure without specifying the datum in detail, and in practice the measurement is taken from the ground level immediately adjacent to the platform up to its finished surface.
That matters on a sloping garden, because the height varies along the deck. A platform that sits at 200mm against the house and 600mm at the bottom of a slope is a raised platform, because part of it exceeds the limit. Authorities assess the highest point.
- Measure to the finished walking surface, not to the joists or the sub-frame
- Measure from the existing ground level adjoining the platform at each point
- On a slope, the highest measured point decides the question
- A step up onto the deck does not count as part of the ground level
- Where the ground is being regraded as part of the work, the pre-existing level is the honest comparator
Regrading is where disputes start. Raising the garden with imported soil and then laying decking on it, so that the deck measures under 300mm from the new level, is a route that has been tried many times. Where the earthworks themselves are an engineering operation, or where the regrading causes material change to levels at a boundary, the authority may treat the whole thing as development. Taking levels before anything is moved is the only reliable protection.
Overlooking from a raised deck
Height is the reason the Order excludes these structures. A deck at 600mm lifts a standing person's eye level to about 2.2m, which is above a normal boundary fence, and the effect on a neighbouring garden is immediate and permanent.
Officers assess three things. The distance from the deck edge to the shared boundary, the extent of the neighbouring garden and windows that become visible, and whether the overlooking is of private space used daily or of a corner nobody sits in. A deck set 3m back from the boundary with planting in front of it is a different proposition from one built to the fence line.
Design moves that answer the objection:
- Set the usable deck area back from the boundary and leave a planted margin
- Reduce the height by stepping the deck down in two levels rather than running one platform out from the house
- Orientate the seating area away from the most sensitive boundary and show that on the plan
- Use a solid balustrade on the flank rather than an open one, where it screens rather than blocks light
- Plant a hedge or install a trellis at the deck edge and offer its retention as a condition
The same logic applies to upper level terraces, and the analysis for Juliet balconies, terraces and overlooking on an extension carries across to a raised garden deck.
Screens, balustrades and the fence rules
A privacy screen on a raised deck is a separate permitted development question, and the arithmetic catches people out. Part 2 of Schedule 2 permits a gate, fence, wall or other means of enclosure up to 2m high, reduced to 1m where it adjoins a highway used by vehicular traffic. The height is measured from the ground level, not from the deck surface.
A 1.8m screen sitting on a 1m deck therefore reads as 2.8m above ground and falls outside the class. Since the deck itself already needs permission, the sensible approach is to include the screen in the same application, with its height dimensioned from existing ground level on the elevation drawing.
Balustrades required for safety are a Building Regulations matter rather than a planning one, and guarding is expected where there is a fall of more than 600mm. Where the planning case depends on keeping the structure low, check the guarding requirement before settling the height, because a deck at 650mm acquires a balustrade and with it a further 1.1m of visual height.
Trees, drainage and flood flows
Three technical issues come up often enough to plan for.
Decking built over the root protection area of a tree can be designed to avoid harm, using a piled or screw-pile sub-frame with no excavation for footings and a permeable surface above. Where the tree is protected by a Tree Preservation Order or stands in a conservation area, that design needs to be shown, and a Tree Survey with an arboricultural method statement is the normal requirement.
Surface water is the second. A large deck over an impermeable membrane and a concrete base changes how rainfall reaches the ground, and on a small urban plot the run-off has to be dealt with on site. Keeping the sub-base permeable, or draining to a soakaway rather than to the boundary, deals with the point before a neighbour raises it. The approach to green roofs and rainwater on a householder extension applies to decking for the same reason.
The third is flood risk. In Flood Zone 2 or 3, a raised solid platform can obstruct flood flows across a garden and displace water elsewhere. An open deck on slender piles has a far smaller effect than a deck on a solid plinth, and the design should reflect that.
Conditions on use and what they can require
Authorities grant permission for raised decks with conditions more often than they refuse them, and the conditions are usually about the things that caused the objection.
Expect requirements to install a screen or planting before the deck is first used, to retain it thereafter, to build only to the approved levels, and occasionally to limit the deck to the area shown on the approved plan. Conditions restricting the hours of use are sometimes proposed and are hard to justify, since the private use of a garden is not easily controlled, so they are worth resisting with a reasoned response during the application rather than after.
A short Planning Statement explaining the design decisions and offering the screening condition is usually enough on a householder application of this kind. It is a two page document, and it saves a round of correspondence.
Recording ground levels before you start
Every argument about decking is an argument about levels, and the evidence always has to be reconstructed after the fact.
Before anything is built, take spot levels across the garden against a fixed datum such as a damp proof course or a manhole cover, photograph the garden from each boundary with a staff or measure in shot, and keep the survey with the house papers. If the deck is built under 300mm and a neighbour later complains, that record is what settles it. Without it, the burden of showing the platform was not a raised platform sits with the owner, years after the ground has moved.
Where you want certainty in advance, an application for a Lawful Development Certificate for proposed development under section 192 of the Town and Country Planning Act 1990 gives a formal determination that the work needs no permission. The fee is lower than a full application and the certificate goes in the file for the next sale.
This is general guidance, not legal advice. A Lawful Development Certificate is the only formal way to confirm that works are lawful, so check the position with your local planning authority or a qualified professional before relying on it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Do I need planning permission for decking?
Yes if the finished surface sits more than 300mm above the adjoining ground. The General Permitted Development Order defines a raised platform as one higher than 0.3m and excludes verandahs, balconies and raised platforms from Class A extensions and from Class E garden structures. A patio laid at existing ground level normally needs nothing.
How is the 300 millimetre height measured?
From the ground level immediately adjacent to the platform up to its finished walking surface, not to the joists. On a sloping garden the height varies along the deck and the authority assesses the highest point, so a deck at 200mm against the house and 600mm down the slope is a raised platform.
Can I raise the garden level so the deck measures under 300mm?
It is a route that rarely works. Where the regrading is itself an engineering operation, or materially changes levels at a boundary, the authority may treat the whole scheme as development. The honest comparator is the pre-existing ground level, which is why taking spot levels before any soil moves matters.
Does a privacy screen on the deck need permission?
Usually yes. Part 2 of Schedule 2 permits a means of enclosure up to 2m high, measured from ground level rather than from the deck surface, so a 1.8m screen on a 1m deck reads as 2.8m and falls outside the class. Include it in the same application with heights dimensioned from existing ground level.
How do I prove later that my deck did not need permission?
Record spot levels across the garden against a fixed datum before building, photograph each boundary with a measure in shot, and keep the survey with the house papers. For certainty in advance, apply for a Lawful Development Certificate for proposed development under section 192 of the Town and Country Planning Act 1990.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.