How do I put a shed on a sloping garden?

Cut-and-fill versus a stilted base, why filled ground is the one you must not build on, and getting a level platform on a slope that stays level.

Difficulty: intermediate
Time: 8 hr
Read: 5 min
Safety: warning

Short answer

Cut into the slope rather than building out from it — filled ground settles and a shed built on it drops within two years. Take a level platform out of the bank, retain the cut face, and set the base on the undisturbed ground you have exposed. Where the slope is too steep to cut, build a level timber frame on posts founded in concrete instead.

Every sloping-garden shed problem is the same problem: half the base is on ground that was always there, and half is on ground that you or somebody else put there. Undisturbed ground is stable. Fill settles, and it settles for years. The whole method below exists to keep the shed off the fill.

Safety

Cutting into a slope creates a face that can collapse, and any retained height above about 600mm is a retaining structure that needs designing rather than improvising — a failed retaining wall with a shed on top of it is a serious matter. Do not undermine a neighbouring boundary, a fence line or the foundations of any other structure. Check for buried services before excavating.

What you'll need

Spade, mattock and a wheelbarrow
Long spirit level and a straightedge, or a laser level
Pegs and string line
MOT type 1 or hardcore
Plate compactor, hired
Concrete or paving slabs for the base
Sleepers or blocks for retaining the cut face (optional)
Land drain and gravel (optional)

Step by step

Measure the fall across the shed footprint before deciding anything.Peg the four corners, run a level string from the high side, and measure down at each corner. A fall of under about 150mm across the footprint is a small cut-and-fill job. Over 400mm and you are into retaining, which changes the whole approach.
Cut into the slope, do not build out from it.Take the platform out of the bank so the base sits on undisturbed ground across its whole area. Everything you dig out goes elsewhere in the garden, not under the low side of the shed.
Dig the platform level and slightly oversize.300mm bigger than the shed on every side. You need room to work, room for a gravel margin, and room for the retaining detail on the high side.
Retain the cut face.Below about 600mm of retained height, sleepers or concrete blocks bedded and backfilled with free-draining gravel will do. Above that, or on made ground, get the wall designed — this is not a place to improvise, and a wall failing under a shed takes the shed with it.
Deal with the water that now runs at the cut face.Cutting into a slope intercepts water moving down through the ground, and it will now arrive at the back of your platform. A gravel-filled trench with a land drain along the cut face, falling to daylight or a soakaway, is the standard answer and the step most often skipped.
Build the base on the undisturbed platform.Compacted MOT type 1 in thin layers, then slabs, a concrete pad or a timber frame on pads — as you would on flat ground. The point of all the excavation was to give you flat ground.
Where you cannot cut, build a level frame on posts.Treated posts founded in concrete on the low side, a level timber frame across them, the shed on the frame. Post depth is set by the ground, but a rule of thumb for a small building is a third of the exposed height in the ground with a minimum of 600mm. Brace the frame diagonally in both directions — an unbraced post frame racks.
Keep the shed clear of the retained face.A gap of at least 300mm between the shed wall and the retaining wall, gravel-filled and drained. A shed wall touching a retained bank is permanently wet.
Check the platform again after the first heavy rain, before the shed goes on.Water will show you where it wants to go. It is far easier to add a drain now than under a loaded shed.

Tips

Barrow the spoil somewhere useful before you start, not into a heap where you will need to stand. Digging a platform generates far more material than people expect.
Level a long slope with a hosepipe water level or a hired laser rather than a spirit level on a straightedge. Over 3 metres, small errors compound badly.
On heavy clay, dig the platform in dry weather. Working wet clay smears the surface into a layer that will not drain.
If a retaining wall would be needed at all, price a stilted timber frame as well. On a steep slope it is often cheaper, quicker and structurally simpler.

Common mistakes

Levelling by building up the low side — Fill settles under load for years. The shed follows it down, the frame racks, the door stops shutting and the only fix is to lift the building and start again.
Retaining a bank with the shed wall — A shed wall is cladding on a light frame. It is not a retaining structure, it will not resist soil pressure and it will be permanently wet on the buried face.
Forgetting the drainage at the cut face — You have created a low point in the middle of a slope. All the water that used to run past now arrives at the back of the shed.

If it doesn't work

The low side of the base drops within a year

Cause: It is bearing on fill rather than undisturbed ground. — Fix: There is no repair that does not involve taking the load off and rebuilding that side onto firm ground, either by digging down to it or by installing posts or piers founded below the fill.

Water runs through the shed after rain

Cause: The cut into the slope intercepted a flow path and there is no drain at the back of the platform. — Fix: Excavate a trench along the high face, lay a perforated land drain in gravel with a fall, and take it to a soakaway or daylight well clear of the shed.

The retaining wall is bulging

Cause: Water pressure behind it, no free-draining backfill, or a wall never designed for the height it is retaining. — Fix: Stop using the shed and get the wall assessed. Bulging is the stage before failure and there is no DIY patch for a retaining wall in that state.

Questions people ask

How much slope is too much for cut-and-fill?

There is no single number, but once the cut face is over about 600mm you are building a retaining wall, and that should be designed rather than guessed. At that point a posted frame is usually simpler, cheaper and safer.

Can I use the excavated soil for the base?

No. Excavated topsoil and subsoil placed back as fill will not compact to a stable bearing layer with garden tools. Use it in borders and buy MOT type 1 for the base.

Do I need permission to cut into a slope?

Excavation itself is not usually a planning matter on a domestic garden, but a retaining wall near a boundary, an outbuilding's height measured from the new ground level, and anything affecting a neighbour's land all can be. Ask the local authority with a sketch showing levels.