Decking and raised platforms in a garden

Garden decking needs planning permission once it becomes a raised platform, and Part 1 of Schedule 2 to the General Permitted Development Order 2015 defines that as a platform more than 300 millimetres high. Below that figure a deck is usually treated as a hard surface and the householder rights can cover it. Above it, the permitted development classes that would otherwise apply specifically exclude verandahs, balconies and raised platforms, so there is no route to build it without an application. That 300 millimetre line does most of the work in this subject, and it is measured from the ground, not from the top of your existing patio.

Flats are a separate case, and they have no householder rights at all.

The raised platform definition and the 300 millimetre threshold

Part 1 defines a raised platform as a platform with a height greater than 300 millimetres. There is no allowance for a handrail, no concession for a sloping garden and no distinction between timber, composite and stone.

Height is taken from ground level. Where the ground is not level, Part 1 measures from the highest part of the surface of the ground next to the structure, which on a sloping plot means the uphill side. That catches a deck that steps down the garden: the lowest corner might stand a metre clear of the ground while the measurement is taken from the top. Read the interpretation provisions rather than assuming the deck is measured where it is tallest.

Measure before you buy the timber. A deck built at 320 millimetres because the joist depth came out larger than drawn is an unauthorised development, and the remedy is either removal or a retrospective application.

Why decking is excluded from several classes

Three classes could in principle cover garden decking, and the raised platform exclusion appears in two of them.

Class F is the one that helps. It permits the provision or replacement of a hard surface within the curtilage for a purpose incidental to the enjoyment of the dwellinghouse. Where decking sits at or below 300 millimetres, treating it as a hard surface is the ordinary reading. Where it is in front of the principal elevation and faces a highway, Class F attaches a condition about porosity or run-off to a porous area, which matters for a decked front garden.

Class E remains useful for the structure beside the deck. A garden room is capable of being permitted development even where the deck in front of it is not, so the two should be assessed separately rather than as one project.

Overlooking from a raised deck

This is the planning issue that decides applications. A deck at 600 millimetres puts a standing adult's eye level roughly two metres above the neighbouring lawn, which is higher than a 1.8 metre fence, and the privacy loss is real rather than theoretical.

Officers look at three things. The sightline from the nearest part of the deck into neighbouring habitable room windows. The sightline into the useable part of the neighbouring garden, particularly the part immediately behind the house. And whether the overlooking is of a kind that will be used, meaning a seating area rather than a corner nobody stands in.

Four design moves work, in roughly this order of effectiveness:

1. Lower the deck. Dropping from 600 to 300 millimetres often removes the objection and the need for the application at the same time.
2. Set the deck back from the boundary, so the sightline over the fence falls short of the neighbour's patio.
3. Put the seating area at the house end of the deck and leave the boundary end as planting.
4. Screen with an obscure or louvred panel at the boundary edge, sized against the sightline rather than chosen from a catalogue.

The same analysis applies to terraces and balconies at first floor, and the approach to overlooking from a terrace or balcony carries across directly.

Balustrades, screens and privacy

A balustrade does not count towards the 300 millimetre platform height, but it does count visually and it does count under the Building Regulations. Approved Document K requires guarding where there is a fall of more than 600 millimetres, so a deck much above the threshold will need a handrail, which then becomes the thing the neighbour sees.

Screens have their own planning position. A screen or fence on top of a deck is measured from the deck surface for the purposes of the fence classes in Part 2, which is why a two metre screen on a 700 millimetre deck is not permitted development in its own right. Design the deck, the guarding and the screening as one proposal and show them together on a section drawing.

A note on materials. Smooth composite boards and glass balustrades read as a built structure from a neighbouring garden in a way that a low timber deck does not, and officers respond to that. Timber, planting at the edge and a lower profile all help the case.

Retaining structures and levels

Changing garden levels is not decking, and it is not covered by the decking analysis. Significant cut and fill is an engineering operation and therefore development in its own right, and a retaining wall is subject to the height limits in the fence, wall and gate class, which are lower where the structure adjoins a highway.

So a deck built level with the house across a sloping garden can involve three separate questions: the platform height, the engineering operations to form the ground beneath it, and the retaining structure holding the soil back. Applications that address only the first of these get queried.

Where the deck sits near a boundary wall or a neighbour's structure, the Party Wall etc. Act 1996 may apply to excavation close to their foundations. That is civil, not planning, and it needs a notice served before work begins.

Drainage from a hard deck

Rain falling on a solid deck has to go somewhere, and in a small urban garden that somewhere is often the neighbour's border. Gaps between boards do not make a deck permeable if there is a membrane and a concrete slab beneath it.

Deal with it by specifying a permeable sub-base, a gravel or soakaway strip at the low edge, and a fall away from boundaries and from the house. For a front garden the Class F condition on porosity or run-off to a porous area makes this a legal requirement rather than good manners. Where the garden already has drainage problems, the thinking behind rainwater handling on a householder extension applies to decking too.

Trees, roots and protected trees

Decking is often the least damaging way to build in a garden with a large tree, because a deck on screw piles or pads can span a root protection area with very little excavation. That is a genuine advantage and worth saying in an application.

Where the tree is protected by a Tree Preservation Order or stands in a conservation area, any work within the root protection area needs care, and the no-dig construction principles in BS 5837:2012 are what an arboriculturist will apply. An Arboricultural Impact Assessment is a report councils usually expect from a suitably qualified author where protected trees are affected, and professional review is strongly advised.

Applying where the threshold is exceeded

A householder application is the route. The documents that make it straightforward are a site plan with the deck outlined, existing and proposed sections showing ground level, deck level and any guarding or screening, an elevation from the neighbour's side of the boundary, and a short Planning Statement addressing privacy, overlooking and drainage directly rather than waiting to be asked.

If the work is already built, a retrospective application or an application for a certificate of lawfulness of existing use or development under section 191 of the Town and Country Planning Act 1990 is the way to regularise it. A lawful development certificate is the only formal way to confirm that works are lawful.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to check whether your garden sits in a conservation area or near a protected tree before you set the deck height.

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