Decking, terraces and the 300 millimetre platform test

Raised decking is one of the few garden projects where permitted development runs out almost immediately. Build a deck more than 300 millimetres above the ground and it stops being permitted development, because Schedule 2 Part 1 of the General Permitted Development Order 2015 excludes verandas, balconies and raised platforms from both Class A (extensions to a house) and Class E (outbuildings and other structures in the garden). A raised platform, for the purposes of that Part, is a platform higher than 300 millimetres. Three hundred millimetres is roughly the depth of a brick on edge plus a joist. It goes quickly.

So the decking planning permission height question has a short answer. Under 300 millimetres and you are usually looking at hard surfacing rules instead. Over it, you need an application, whatever the garden looks like.

Why a raised platform falls outside Class A and Class E

Class A covers the enlargement, improvement or other alteration of a house. Class E covers buildings, enclosures, swimming pools and similar structures within the curtilage, used for a purpose incidental to the enjoyment of the house. Both classes carry a list of things they do not permit, and verandas, balconies and raised platforms appear on both lists.

The reason is straightforward. A platform lifts people up, and a person standing 600 millimetres higher sees over a 1.8 metre fence into the garden next door. Parliament took the view that this is a matter for a case-by-case judgement rather than for a blanket right.

Note that the exclusion applies to the structure, not to the use. Laying a timber deck at ground level is not a raised platform even if you sit on it. Lifting that same deck on posts is.

Measuring the 300 millimetres

Measure from the ground to the walking surface, not to the top of the handrail or the top of the balustrade. The height of a guard rail does not convert a low deck into a raised platform, though it may bring other rules into play if the deck sits near a boundary.

What counts as the deck surface is the finished level you stand on, including the decking boards themselves. Builders sometimes quote the joist height and then add 25 millimetre boards on top, which is how a deck designed at 295 millimetres arrives on site at 320.

Sloping gardens and where the ground is taken

This is where most disputes start. The interpretation provisions in Part 1 take the height of a structure from the highest part of the surface of the ground next to it. On a garden that falls away from the house, a deck built level with the back door can be 200 millimetres above ground at the house wall and 900 millimetres above it at the far end.

Read strictly, measuring from the highest adjoining ground is generous to the applicant. Read the way many councils do, the deck is a raised platform because part of it stands well clear of the ground beneath. I have seen the point argued both ways, and I would not advise a client to build on the strict reading alone. The honest position is that this is unsettled at the margins, and a Lawful Development Certificate application under section 192 of the Town and Country Planning Act 1990 is the only way to get a binding answer before you build.

There is a practical alternative on a sloping plot. Terrace the garden with retaining walls and lay the deck directly on the made-up ground at each level. A retaining wall is a means of enclosure under a different Part of the Order, and ground that has been built up becomes the ground for measurement purposes once it is in place. That sequencing matters, so take advice before relying on it.

Steps, ramps and handrails

Steps serving a deck are usually treated as part of the same operation rather than as separate works. If the deck needs permission, draw the steps on the same plan.

A ramp for wheelchair access has its own route. Part 1 of Schedule 2 includes a class permitting the provision of a ramp or other means of access for disabled people, and that right is not cut across by the raised platform exclusion in the way Class A is. For anything beyond a modest ramp, check the current text of the Order rather than relying on a summary.

Guard rails become a Building Regulations question rather than a planning one. A change in level of more than 600 millimetres generally calls for guarding under the Building Regulations, and the structural adequacy of posts, bearers and fixings is a separate matter from whether you need permission.

Overlooking, and why it decides the application

Once an application is in, the deck is assessed on amenity. Officers look at three things: the distance from the deck edge to the neighbouring boundary, the eye level of someone standing on it compared with the top of the existing boundary treatment, and whether the view is into a garden close to the house or into the far end of a long plot.

Local plan amenity policies carry the test, and most authorities have a householder design guide that gives indicative separation distances. Those distances vary, so read the adopted document for your authority rather than borrowing a number from elsewhere.

Common refusal reasons are specific and avoidable:

• A deck platform within a metre or two of a shared boundary with no screening.
• A deck at first floor level reached from a rear door, which reads as a balcony and overlooks two gardens rather than one.
• Screening proposed as planting rather than as built structure, where the planting takes years to work and could be removed.
• A deck that raises the usable garden level above the neighbour's patio, so noise carries differently.

The fix is usually geometry rather than argument. Pull the platform away from the boundary, drop its level, or set a solid privacy screen on the overlooking side and show it on the elevation.

Designing to stay under the threshold

If the garden is close to level, staying inside permitted development is often simpler than applying. The approach is roughly this:

1. Survey the existing ground levels across the footprint before anything is designed.
2. Set the finished deck surface at or below 300 millimetres above the highest adjoining ground, measured to the top of the boards.
3. Excavate rather than build up where the ground needs to come down, keeping made ground away from tree root protection areas.
4. Keep the structure clear of the 50 per cent curtilage calculation that applies to outbuildings, since the 50 per cent rule for garden buildings can be tight on a small plot.
5. Photograph the levels as built and keep the survey, so you can evidence the position if a complaint arrives years later.

At or below the threshold, a timber deck sitting on the ground is generally treated as a hard surface, which brings in the separate rules on surfacing within a curtilage. Those rules matter most at the front of a house, where the permeable surfacing requirement under Class F applies above a certain area. In a rear garden the surfacing right is wider, but drainage still has to go somewhere other than next door.

Designated land changes the picture again. In a conservation area, a National Landscape, the Broads, a National Park or a World Heritage Site, several householder rights are narrowed, and an Article 4 direction can remove them entirely for a defined area. Check both before you assume a right exists.

When an application is the only route

For a deck over 300 millimetres, the route is a householder planning application. The documents are modest: location plan, block plan, existing and proposed levels, elevations showing the deck against the boundary treatment, and a short Planning Statement explaining the amenity position.

That statement is where applications are won. Address the overlooking directly, give the separation distance, show the screen on the drawing and say what the deck replaces. A deck that takes the place of a crumbling raised patio in the same position is a different proposition from one that creates a new raised area, and the officer will not know that unless you say so.

If the deck forms part of a wider garden project with an outbuilding, think about whether the two read as one development. A large deck leading to a garden room can start to look like a separate dwelling or a self-contained unit, which is a different and harder application. Read up on the separate planning unit problem with garden offices before you combine the two.

Enter your postcode on PlanScope to see which supporting documents a householder application for raised decking is likely to need.

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 any of it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.