Decking on a slope or uneven ground

In shortA sloping garden is one of the best reasons to deck rather than pave, because the frame does the levelling: posts set in concrete, or adjustable pedestals on firm ground, carry a level platform over the fall. The catch is height. In England and Wales a deck more than 300 mm above the ground at any point needs planning permission, so a level deck on a slope often does, and once the drop passes 600 mm it needs a guard rail too.

Paving a sloping garden means digging, retaining walls and a lot of hardcore. Decking means a frame. The boards sit on joists, the joists sit on posts or pedestals, and the posts are cut to whatever length brings the surface level. That is why decking is so often the answer where the ground falls away or lies in lumps.

The frame is also where the cost and the rules sit. The further the ground drops, the taller the posts, the more bracing they need and the sooner the deck reaches the height at which planning permission and a guard rail come in.

How a deck is levelled over a slope

There are three common approaches, and a single deck often uses more than one.

Posts in concrete. Tall posts at the low end and shorter ones up the slope, set in concrete, with bearers and joists spanning between them. This is the standard method for any real fall. Posts should be UC4-treated timber or metal, and braced once they stand more than a few hundred millimetres tall.

Adjustable pedestals. Where the ground is firm or already paved, screw-adjustable pedestals under the joists take up small differences in level without digging. They suit shallow falls, decks over concrete and roof terraces.

A stepped deck. On a steep garden, one level platform can end up chest height at the bottom of the slope. Two or three smaller decks stepped down the garden keep each one low, look better and may avoid the planning trigger altogether. Steps cost roughly £80 to £220 each.

Laying decking on soil

Decking on soil starts with clearing turf and plants, levelling the worst of the ground and laying a weed membrane topped with gravel, so soil never touches the frame and nothing grows up between the boards. Posts or pads go on or through that layer. Air must move underneath: allow about 150 mm of clearance under the joists and keep the perimeter open or fitted with ventilated skirting, which also keeps rats out. Joists must never rest on the soil.

Where the deck meets the house, the surface should stay around 150 mm below the damp-proof course, or the wall must be protected, so damp cannot bridge into the house.

Decking over concrete

An old patio or concrete base is a good start, provided it is sound and drains. The deck can sit on pedestals, on short bearers fixed to the slab, or on a shallow aluminium frame where the finished height must stay low, for instance under a door threshold. Check the slab falls away from the house and that the deck does not block a drain, an air brick or the damp-proof course. The usual mistake is a deck so low that water sits under it.

The 300 mm planning trigger

In England, permitted development for householders excludes a “raised platform”, and the legislation defines raised as more than 0.3 metres above the ground. The Planning Portal puts it as no more than 30 cm. Wales uses the same figure. So a deck at or below 300 mm needs no application, subject to the other conditions (not forward of the front wall, not covering more than half the garden with other buildings, and not at a listed house or a flat). Above 300 mm, you apply.

Scotland is more generous: its householder rules put the limit at a floor level of 0.5 metres, with their own conditions on position and conservation areas, so read the mygov.scot guidance before relying on it. Northern Ireland follows the England pattern, but we have not confirmed its height figure; check with your council.

On a slope the awkward question is where to measure from. A level deck 200 mm above the ground by the house may stand 700 mm above it at the far edge. Councils generally assess a deck by its greatest height above the ground immediately beneath it, so the far edge is what counts. If you are close to the line, apply for a Lawful Development Certificate, which costs half the fee of a full householder application and gives a written answer. The full rules by nation are in the guide to decking planning permission. Neither that guide nor this note is legal advice.

Guard rails once the drop passes 600 mm

Planning is one set of rules; falling is another. Approved Document K, the building regulations guidance for England, calls for guarding wherever people could fall more than 600 mm, and for external balconies at a dwelling it sets the guarding height at 1,100 mm, with no gap a 100 mm sphere could pass through where small children are about. A free-standing garden deck does not usually need a building regulations application, but a competent installer builds to those figures anyway: a deck 800 mm up with no rail is a hazard whatever the paperwork says.

Balustrade is priced by the metre: about £90 to £200 for composite or metal and £200 to £500 for glass, from published UK price guides, autumn 2026.

What a slope does to the price

A raised deck costs more than the same boards at ground level: published price guides put the uplift at 20 to 60 per cent, depending on height, bracing and access. A 20 m2 raised composite deck with steps and a balustrade runs to about £3,500 to £7,500 all in, against £2,000 to £4,400 at ground level. The decking cost guide has the per m2 ranges and extras, and the note on decking joist spacing covers the frame itself.

When you are ready for quotes, the town pages list installers whose websites we check: all towns.