Raised platforms, decking and the 300mm trigger

Decking and raised platform rules catch more householders than any other part of the permitted development regime, because the trigger is only 300mm. Part 1 of Schedule 2 to the General Permitted Development Order 2015 defines a raised platform as a platform more than 0.3 metres high, and both Class A, which covers extensions, and Class E, which covers outbuildings, exclude verandas, balconies and raised platforms from permitted development. Build a deck at 310mm above the adjacent ground and you need planning permission for it, however modest it looks.

That is the whole rule in one paragraph. The difficulty is measuring it, because gardens slope and "adjacent ground" is not always where you think.

Why a raised platform is treated differently

Height changes what people can see. A patio at ground level looks into a neighbour's garden from the same angle the lawn always did. Lift the standing position by half a metre and sightlines over a 1.8m fence open up, which is why the order treats platforms as a separate class of harm rather than as hard surfacing.

The same reasoning explains why verandas and balconies are excluded outright with no height test at all. Anything that creates a usable raised standing area attached to the house sits outside Class A.

Hard surfacing at ground level is different again. Class F allows a hard surface within the curtilage of a house for a purpose incidental to its enjoyment, with the front garden permeability condition attached, so a patio laid on the existing grade is normally fine. The moment it is built up on a frame, the 300mm question arrives.

Measuring the 300mm from the right ground level

Measure from the surface of the ground immediately next to the platform, and where that ground is not level, from the highest part of it. Not from the lowest. Not from an average. Not from the finished floor level inside the house.

Three measurement mistakes recur:

1. Measuring to the deck boards but ignoring a sub-frame or sleepers that sit proud at the edge
2. Taking the level from a path or step that was itself built up, rather than from the original ground
3. Measuring in the middle of a sloping garden where the platform is lower, when the test applies along its whole extent

The practical implication is that the height varies around a platform on sloping ground, and the relevant figure is the greatest height measured from the ground next to it. Our note on measuring height under Part 1 from adjacent ground works through the general rule, which applies to outbuilding and extension heights too.

Guard rails and balustrades are not part of the platform height for the 300mm test, though they are plainly relevant to the privacy judgement if an application becomes necessary.

Decking on sloping gardens

A garden that falls away from the house is the classic case. You want a level sitting area outside the back doors, the ground drops 600mm over five metres, and the only way to get a flat surface is to build up.

Options that keep you inside permitted development, in rough order of practicality:

• Cut into the slope rather than building out from it, so the finished deck sits within 300mm of the adjacent ground along its length
• Terrace the garden in two or three steps, each retained and each platform below the trigger, with the retaining structures treated as walls rather than platforms
• Keep the raised area small and set it away from the boundary, accepting that if any part exceeds 300mm you need permission for the whole thing
• Lay a ground-level patio next to the house and put the level change further down the garden

Retaining walls bring their own limits. A wall over 1m next to a highway, or over 2m elsewhere, needs permission under Class A of Part 2, and a dug-out terrace can also be an engineering operation in its own right.

Steps, ramps and landings

Steps and a landing giving access to a door are not usually treated as a raised platform where they are no larger than reasonably needed for access. A broad landing with room for a table is a different matter, and officers do look at the footprint.

Ramps for accessibility sit in the same territory. Where a ramp and landing are provided for a disabled occupant, the works may attract a planning fee exemption if permission is needed, and some councils take a pragmatic view on enforcement. Do not rely on that: get it confirmed in writing.

If you want certainty before building, apply for a lawful development certificate for proposed works under section 192 of the Town and Country Planning Act 1990. It costs a fee and takes eight weeks, and it is the only formal way to confirm that works are lawful.

Balconies above a single storey extension

This is where people come unstuck most expensively. A flat-roofed rear extension built under Class A is permitted development. Putting a door onto that roof and using it as a terrace is not, because the result is a balcony or raised platform and both sit outside the class.

The roof itself can be accessible for maintenance. What matters is whether it has been laid out for use: a door or French windows at first floor level, a balustrade, a floor finish. A rooflight and a hatch do not create a terrace. A set of doors and glass balustrading plainly does.

Flats are outside Part 1 altogether, which applies to dwellinghouses only. Any terrace, balcony or roof garden on a flat needs planning permission regardless of height.

And if the house sits in a conservation area or is covered by an Article 4 direction, check what has been withdrawn before assuming anything. Our explainer on how a council makes an Article 4 direction sets out how to find one.

Overlooking objections where permission is needed

Once an application is required, the case turns on privacy and overlooking rather than on height in the abstract. Councils apply local separation distances, a 45-degree test in some householder design guides, and a judgement about whether the raised use would allow direct views into habitable rooms or over private garden areas close to a neighbour's house.

Measures that help, and that officers accept:

• A privacy screen of 1.7m or 1.8m along the boundary side of the platform, shown on the drawings rather than offered in the covering letter
• Setting the usable area back from the boundary by a metre or more
• Reducing the platform to the area actually needed, so the standing position is not along the shared fence
• Obscure glazed balustrading rather than open rails where the deck sits high

The planning statement is where the privacy argument gets made, with reference to the council's own householder design guidance if it has one.

Showing levels on the section

Drawings win these applications. A plan alone cannot show a height, and an officer who cannot find the 300mm dimension will ask for more information, which adds weeks.

Submit an existing and proposed section through the garden at the critical point, with existing ground level, proposed deck level and the difference dimensioned. Add spot levels on the plan at each corner of the platform, and show the screen in elevation with its height from the deck surface.

Where the works have already been built, the enforcement position matters: time limits for action against unauthorised building works in England changed under the Levelling-up and Regeneration Act 2023, and the current position is set out in our note on enforcement time limits for permitted development breaches. Do not assume the old four-year rule still applies.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it, and remember that a lawful development certificate is the only formal way to confirm that works are lawful.

Enter your postcode on PlanScope to see which documents an application for a raised terrace would need.

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