Balconies, verandas and decking above 300mm
Decking at 250mm can be permitted development. At 400mm it is a raised platform and needs planning permission, and so does any balcony or roof terrace.
Decking at 300mm or less above the ground is usually fine. Go above that and you have built a raised platform, which is excluded from householder permitted development altogether, so you need planning permission. The same exclusion catches balconies and verandas, which is why you cannot create a roof terrace on a flat roof or add a balcony to a rear elevation without applying. The threshold is a single number in the General Permitted Development Order, and a surprising number of garden projects land the wrong side of it by 100mm.
Councils take this seriously because the rule exists to control overlooking, and overlooking is the complaint that arrives fastest from next door.
Where the exclusion sits
The Town and Country Planning (General Permitted Development) (England) Order 2015 sets out householder rights in Schedule 2, Part 1. Class A, which covers extensions and alterations, excludes any development that consists of or includes the construction or provision of a veranda, balcony or raised platform.
The same exclusion appears in Class B, which covers roof enlargements such as dormers, and in Class E, which covers outbuildings and other structures within the curtilage. So there is no route into any of them.
This is an exclusion rather than a condition, and the difference matters. A condition can be complied with. An exclusion means the right does not exist for that development at all, so the work is unauthorised from the moment it is built and the council can take enforcement action.
What a raised platform is
The interpretation provisions of Part 1 define a raised platform as a platform with a height greater than 0.3 metres. That is the whole definition.
It does not matter what the platform is made of, what it is called on the invoice, or whether anyone can see it. Timber decking, a composite deck, a paved terrace built up on a sub-base, a steel frame supporting an artificial lawn: all of them are platforms.
Veranda and balcony are not defined in the Order, so they carry their ordinary meaning. A veranda is a roofed platform along the outside of a building. A balcony is a platform projecting from a wall with access from an upper floor. Both are excluded regardless of height.
Measuring the 300 millimetres
Height is measured from ground level, and the Order's interpretation of ground level means the surface of the ground immediately next to the structure. Where the ground is not level, the convention is to measure from the highest part of the surrounding ground, which is the more restrictive reading and the one councils apply.
Three practical consequences follow.
A deck built level across a sloping garden will be under 300mm at the top of the slope and well over it at the bottom. The whole structure is then a raised platform.
Building up the ground first does not help. Raising levels is itself an engineering operation capable of needing permission, and an officer assessing the deck will measure from the original ground level rather than the imported material.
The 300mm is to the walking surface, not to the top of the joists or the underside of the boards. Measure what people stand on.
Decking, terraces and flat roof access
A ground-level patio laid on the existing ground is not a platform and is not development in most cases. A deck at 250mm is below the threshold and, provided it satisfies the other Class A or Class E limits, can be permitted development.
A deck at 400mm needs planning permission. So does a raised terrace formed over a garage roof, an external staircase to a first floor terrace, and any arrangement that makes a flat roof usable as amenity space.
That last one catches loft conversions. Adding a door onto a flat roof, fitting guarding, and laying a surface creates a roof terrace, which is a balcony or raised platform in substance and therefore outside Class B. The dormer may be permitted development. The terrace next to it is not, and applying for one while building the other is a familiar enforcement case. Our note on how previous extensions count against your allowance covers the other limitation that often bites on the same project.
Juliet balconies and guarding
A guard rail fixed across a door opening, with no platform outside it, is generally accepted as not being a balcony, because there is nothing to stand on beyond the plane of the wall. The usual reading is that it is an alteration rather than the provision of a balcony.
That is the general position rather than a rule. Where the guarding is set out from the wall far enough to stand on, or where the structure includes a small projecting platform, the argument fails. Councils differ on where they draw the line.
If the design depends on the answer, the safe course is to apply for a lawful development certificate. A certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful, and it is a cleaner answer than an officer's informal email.
Why the rule exists
Overlooking. A platform 600mm high lets someone see over a 1.8m fence into the neighbouring garden and, on a terraced or semi-detached plot, into the neighbour's rear windows.
Local plan policies on residential amenity address this directly, and most councils have a design guide setting out separation distances, screening expectations and what they consider acceptable for elevated amenity space. The tests are qualitative, which means the outcome depends on the particular relationship between the two properties rather than on a formula.
Privacy is also the reason the permitted development rules elsewhere require upper-floor side elevation windows to be obscure-glazed and non-opening below 1.7m. The Order is consistent about it.
Applying when the right fails
A householder application for decking, a balcony or a roof terrace is not difficult, but it needs to answer the overlooking point rather than ignore it.
- Show existing and proposed sections through the garden, with levels, so the height is not in dispute.
- Plot sightlines from the standing position on the platform to neighbouring windows and garden areas.
- Offer mitigation on the drawings: a privacy screen of stated height and opacity, a setback from the boundary, planting, or a smaller usable area.
- Deal with the neighbour's amenity in the supporting statement rather than leaving it to the case officer to raise.
- Expect conditions requiring the screen to be installed before first use and retained.
The design detail that wins these applications is almost always the setback. Pulling a terrace 1.5m back from the boundary edge and screening the flank does more than any amount of argument about the 45-degree line. Where the works also involve principal elevation or curtilage questions, our note on curtilage and the principal elevation is the next thing to read, and the rear extension limits matter where a deck adjoins an extension.
A planning statement that addresses amenity policy directly, with the mitigation already drawn, turns most of these into delegated decisions.
This is general guidance, not legal advice. 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
How high can decking be without planning permission?
A platform higher than 0.3 metres is a raised platform under Part 1 of the General Permitted Development Order, and raised platforms are excluded from householder permitted development. Decking at or below 300mm to the walking surface can be permitted development, provided the other limits in the relevant class are also met.
How is the 300mm measured on a sloping garden?
From ground level immediately next to the structure, and where the ground is uneven the convention is to measure from the highest part of the surrounding ground. A level deck across a slope will exceed the threshold at the low end, which makes the whole structure a raised platform.
Can I build up the ground first to stay under the threshold?
No, and it tends to make matters worse. Raising ground levels is itself an engineering operation that can need permission, and an officer assessing the deck will measure from the original ground level rather than from imported material. Section drawings showing existing and proposed levels are the way to settle it.
Is a Juliet balcony permitted development?
Guarding fixed across a door opening with no platform outside it is generally accepted as not being a balcony, because there is nothing to stand on beyond the wall. That is the usual reading rather than a rule, and councils differ. Where the design depends on it, apply for a lawful development certificate.
Can I turn a flat roof into a terrace under permitted development?
No. Creating usable amenity space on a flat roof provides a balcony or raised platform, which is excluded from Classes A, B and E. The dormer or extension below may be permitted development while the terrace above it is not, and building the two together is a common enforcement case.
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.