Decking, raised platforms and the 300 millimetre rule

Decking is permitted development up to 300 millimetres above ground level and needs planning permission above it. That single figure comes from the definition of a raised platform in Part 1 of Schedule 2 to the General Permitted Development Order 2015, which describes a platform with a height greater than 300 millimetres. Both the householder extension rights in Class A and the outbuilding rights in Class E exclude a veranda, balcony or raised platform, so a deck that crosses the line loses the benefit of both.

What the Order means by a raised platform

The definition is short and it does the work. A raised platform is a platform with a height greater than 300 millimetres, and the exclusion applies whether the platform is attached to the house or sits in the garden.

Two consequences follow that surprise homeowners.

A ground level patio is not a raised platform at all, so paving a garden raises no issue under this rule, although drainage and permeability may raise others.

And the exclusion applies to the whole structure rather than to the part that exceeds the height. A deck that is 200 millimetres high across most of its area and 400 millimetres high at one corner is a raised platform.

Timber, composite, concrete and steel are treated the same way. The Order describes what the thing does, not what it is made of.

Why 300 millimetres is the dividing line

The figure is a proxy for overlooking. Standing on a deck raises the eye level by whatever the deck adds, and a small increase changes what you can see over a typical garden fence.

A fence at 1.8 metres screens a neighbouring garden from someone standing on the ground. Add 300 millimetres and most of that screening survives. Add 600 and it does not, because the sight line now passes over the top of the fence and across the neighbouring garden at close range.

That is the whole logic of the rule, and it explains why councils take the point seriously even on modest structures. The objection is not about the deck. It is about where people stand on it and what they can see.

Balconies and verandas are outside permitted development entirely

Height does not help here. Class A and Class E exclude a veranda, balcony or raised platform without qualification, so a first floor balcony, a Juliet balcony with a projecting platform, or a roof terrace formed over a flat roofed extension all need planning permission regardless of dimensions.

This catches loft conversion schemes in particular. A dormer built under Class B may itself be permitted development, and cutting a door out to a terrace on the flat roof beside it is not.

The same applies to converting an existing flat roof into a usable terrace. The roof already exists, so nothing is being built, and the change is still development requiring permission because it creates a raised platform and a new use of that surface.

Where a house sits in a conservation area or a National Landscape, or where an Article 4 direction is in force, the permitted development baseline may be narrower still. Owners in those areas should first check which permitted development rights have been withdrawn.

Sloping gardens and how the height is measured

Height is measured from the surface of the ground immediately adjacent to the platform, which makes a sloping garden the most common source of trouble.

Build a level deck out from the back door across a garden that falls away, and the structure may sit at 100 millimetres by the house and 500 millimetres at the far edge. The deck is a raised platform, and the fact that most of it sits low is irrelevant.

Three practical responses work.

Step the deck down in sections, each finishing below the threshold relative to the ground beneath it, with the steps between them.

Excavate rather than build up, cutting into the slope so the finished surface sits closer to natural ground level, subject to what that does to drainage and to any retaining structure.

Or accept the position and apply for permission, which for a garden deck is usually a straightforward householder application.

Do not measure from the original levels if the ground has since been raised. If topsoil has been imported and the garden has been built up, the council will look at whether that engineering operation was itself development.

Overlooking and privacy when an application is needed

Once an application is required, the assessment is about the relationship with neighbours rather than about the 300 millimetre figure.

Councils look at the height of the platform, its distance from the boundary, the length of boundary it runs along, the difference in ground levels between the two properties, and whether the deck looks into a neighbouring habitable room window or only into a garden.

Most residential design supplementary planning documents deal with raised terraces explicitly, and many set an expectation that a raised platform close to a boundary will be screened. Find the adopted guidance for your authority, because the wording differs and it is what the officer will apply.

Where a previous scheme has already been refused on neighbour impact, the reasons for that refusal are the starting point for the redesign rather than something to argue with. The same logic applies here as in reworking a rear extension against the stated amenity reasons.

Screening, obscure glazing and planting

Three mitigation measures come up, and they are not equal.

Solid screening to the sides of a raised deck is the most effective and the most commonly conditioned. Specify the height above deck level, the material and the position on a plan, since a condition requiring screening in accordance with the approved drawings needs a drawing to work from.

Obscure glazing to a balustrade helps where the balustrade itself is the sight line, and does nothing where the problem is a person standing behind it.

Planting is the weakest, because it takes years to establish and can be removed by a future owner. Officers know this, and a scheme relying on a hedge as its only mitigation tends to attract a screening condition instead.

One point in the applicant's favour. A deck used occasionally is different from a raised terrace directly outside a kitchen door, and describing the intended use honestly can help an officer reach a proportionate view.

When a retrospective application is the sensible route

Plenty of decks go up before anyone reads the Order. If a neighbour has complained or the council has written, the choice is to remove the structure, to alter it so it falls within permitted development, or to apply for permission for what has been built.

An application under section 73A of the Town and Country Planning Act 1990 regularises development already carried out. It is assessed on the same planning merits as any other application, and the fact that the work is finished carries no weight either way.

Two points to weigh. The council can decline to determine repeat applications in defined circumstances, so a considered application is better than a hurried one. And enforcement time limits in England were extended by the Levelling-up and Regeneration Act 2023, so the older assumption that an unchallenged structure becomes immune after four years should be checked against the current position rather than relied on.

Where the deck is borderline on the measurements, a lawful development certificate under section 192 for proposed works, or section 191 for existing ones, is the only formal way to establish that the position is lawful. It costs a fee and produces a document a purchaser's solicitor will accept, which a builder's assurance does not.

A short Planning Statement setting out the levels, the distances and the screening does more for a borderline case than a long letter about the family's use of the garden.

Enter your postcode on PlanScope to see which reports a householder application on your property is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.