Groundworks · · updated
Shed, garden room and hot tub bases: the spec you actually need
Start with the arithmetic — water weighs a tonne per cubic metre — then work out whether you need compacted Type 1 and slabs, or reinforced concrete.
Worth doing yourself
A shed base up to roughly 3m x 3m. Setting out square, digging out the topsoil, membrane, Type 1 in two compacted layers with a hired whacker plate, and either a paved or a timber-edged finish. A weekend for two people, with hire.
Not worth doing yourself
Hot tub and garden room bases. Those are reinforced concrete of a designed thickness, poured in one go, and the loads involved are in a different league — the arithmetic is below. Get it wrong and it cracks, tilts, or takes a building with it.
Almost every failed garden building we get called to has the same base: a few slabs bedded on sand, laid straight onto whatever was there. It holds up a shed. It does not hold up anything with weight, and it does not hold anything level for long.
Work the load out first. Everything else follows from the number.
Do the arithmetic before you choose a base
Water weighs a tonne per cubic metre. A hot tub holding 1,200 litres is therefore 1.2 tonnes of water before anybody is in it. Add the tub itself, add four adults, and you are asking a patch of garden to carry roughly two tonnes, permanently, without settling unevenly by even a few millimetres.
A garden room is lighter per square metre but the load is concentrated along the perimeter where the frame sits, and it has to stay square and level for the doors to keep working. A shed is genuinely light — that is why sheds get away with cheap bases and hot tubs don't.
Slabs bedded on sand spread load poorly and sit on ground that moves seasonally. That is not a base; it's a floor laid on the weather.
When compacted Type 1 with slabs is enough
For a shed, a small summerhouse, a bin store or a bike store: excavate to a firm formation, membrane down, 100–150mm of MOT Type 1 compacted in layers, then flags bedded properly on a full mortar or sand-cement bed — not on five dabs.
Get the base slightly larger than the building footprint so rain running off the roof lands on the base and away, not on the edge of it. Fall the surface away from the doorway.
When it has to be reinforced concrete
Hot tubs, garden rooms, anything with a floor build-up, anything with a heavy tiled or green roof, and anything you will stand a large number of people on.
- Hot tub: a reinforced concrete slab, typically 150mm on a compacted Type 1 sub-base, with steel mesh set within the pour rather than laid on the ground and hoped for.
- Garden room: 100–150mm reinforced slab over compacted Type 1, cast level and square, with a damp-proof membrane under the slab because it is a habitable-feeling space and it will be dry-lined.
- Larger or heavier structures, or poor ground: thickened edges, a deeper sub-base, or a different foundation approach altogether after we've seen what the ground is.
- Anything on soft, made-up or recently disturbed ground: dig it out to something that will carry load. There is no reinforcement that fixes standing on fill.
Falls and drainage around the base
The base itself should be level. The ground around it should not be. Whatever you build wants to sit clear of standing water, or the bottom rail, the cladding and the frame will spend the winter wet.
Fall the surrounding ground away from the base. Where a fence or a boundary means water cannot get away, put in a channel or a land drain along the low side and take it somewhere. A hot tub is also drained down periodically — decide before installation where that volume of water is going, because a couple of cubic metres emptied against a fence line finds the lowest point in your garden and stays there.
Permitted development and building regulations
Garden rooms and outbuildings are usually permitted development, but that depends on size, height, position relative to boundaries and the house, and whether your property has restrictions on it — listed buildings, conservation areas and some new estates. Building regulations are separate again and turn on floor area, use and proximity to a boundary.
Rather than assert any of that, read the current rules on the Planning Portal and check your own property's position with Buckinghamshire Council. We'll build to whatever the answer is, but the answer isn't ours to give.
Sources
A shed base, start to finish
This is the job worth doing yourself. The method below is what we would do for a shed up to about 3m x 3m; the only thing that separates a base that stays flat for twenty years from one that rocks by next winter is compaction, and compaction is a hire item.
Step 1
Set out square with 3-4-5
Knock in a peg at one corner and run a string along the first side. Measure 3m along it and mark. From the same corner, run a second string roughly at right angles and mark 4m. Swing the second string until the distance between the two marks is exactly 5m — that corner is now square. Repeat at each corner, then check both diagonals of the finished rectangle: if they are equal to within 5mm, it is square. Set the base 50–75mm larger than the shed on each side so rain drips clear of the timber.
Step 2
Dig out past the topsoil
Excavate the whole rectangle to 150mm below your intended finished level, and keep going until you are out of topsoil and into firm subsoil, even if that is 250mm. Topsoil is organic, it holds water, and it keeps consolidating for years — anything you leave under the base will shrink and take the corner of the shed with it. The bottom of the hole wants to be flat and firm, not stepped, and no soft spots. If a heel sinks in, dig it out and bring it back up with stone.
Step 3
Membrane, then Type 1 in two layers
Lay a woven geotextile membrane over the formation and lap it up the sides — it stops the stone punching down into the subsoil and stops fines pumping up. Then bring in MOT Type 1 in two layers of roughly 75mm, not one of 150mm. Rake each layer level before you compact it.
Step 4
Compact each layer with a hired plate
A wacker plate — a vibrating plate compactor, about £40–£50 a day — is what does this. Run it over each layer in overlapping passes in two directions until the surface stops moving and rings hard under the plate. Boots, a sledgehammer and a fence post do not compact Type 1; they dent the top of a loose layer and leave the rest of it to settle later, which is exactly why so many shed bases rock.
Step 5
Level, or give it a fall
A shed base can be dead level, and for a floor that is easiest to work to. If the shed is open-fronted or you want water off it, put a fall of about 1:80 in one direction — 25mm over 2m. Check with a long spirit level or a straight edge and level across both diagonals as well as the sides, not just one way.
Step 6
Edge it so it can't spread
Compacted stone with nothing holding the perimeter will creep. Either bed a course of block paving or concrete flags on a 30–40mm sharp sand screed with the outer course haunched in concrete, or frame the base in treated 150 x 47mm timber pegged every 600mm and screwed to the pegs. Paved is better if the shed is heavy or houses a mower you wheel in and out.
Step 7
Check before anything goes on it
Straight edge in both directions and both diagonals: no daylight over 3mm. Diagonals equal to within 5mm. Walk the whole area and feel for any give. Fix it now — you cannot fix it once there is a shed on top.
Tools, hire and time
- Spade, fork, mattock, wheelbarrow, rake, stiff brush
- Pegs, string line, tape, 1.8m spirit level or straight edge, rubber mallet
- Hired vibrating plate compactor — roughly £40–£50 a day plus deposit
- Woven geotextile membrane, MOT Type 1, sharp sand, edging (blocks or treated timber)
- A skip or a grab bag for the spoil — a 3m x 3m dig at 200mm is over 2 tonnes
Two people, a weekend. Day one: set out, dig, muck away. Day two: membrane, stone, compact, edge, finish.
Estimator
Shed base: stone, sand and spoil
Put in the footprint you're building and the depths from the method above. Order about 10% over — stone bulks down as it compacts and nobody wants a second delivery.
- Base area
- 9.0 m²
- Dig depth
- 175 mm
- MOT Type 1
- 2.8 tonnes
- Sharp sand
- 0.4 tonnes
- Membrane
- 10.4 m²
- Spoil to remove
- 2.5 tonnes
Tonnages assume Type 1 at 2.1 t/m³ compacted, sharp sand at 1.6 t/m³ and spoil at roughly 1.6 t/m³ loose. Spoil bulks up once dug — size the skip on the loose figure, not the hole.
Check services
Before you put a spade or a bar in the ground
Water, gas, electricity and telecoms runs from the house often sit shallower than people expect, particularly around new estates and anywhere a garage or extension has been added later. Find the service entry points, work out where the runs are likely to head, and dig the first spit by hand. Never swing a mattock or drop a breaker into ground you haven't proved.
The checklist is a one-page PDF — open or download it below.
Download the "Before anybody digs" checklist (PDF)Open it full size (opens in a new tab)
Where this method stops: hot tubs and garden rooms
A compacted stone base is right for a shed and wrong for the other two, and the reason is arithmetic rather than caution.
Water weighs a tonne per cubic metre. A mid-size hot tub holds 1,200–1,500 litres, the shell and cabinet add a couple of hundred kilos, and then you put four to six adults in it. That is comfortably over two tonnes standing on a footprint of about four square metres, and it is a live, sloshing load that never comes off. A garden room adds its own dead load, a roof that catches wind, and — unlike a shed — it has to stay square enough for doors and glazing to keep working.
Those need reinforced concrete of a designed thickness on a compacted sub-base, poured in one continuous pour with steel in the right place and the right cover, or in some cases piled or trench-fill footings. That is not a hire-shop weekend: it is a set-out and shuttering job, a same-day concrete delivery you cannot pause, and a finish you get one attempt at. Get the thickness or the steel wrong and it cracks across the middle, and a tub or a building on a cracked slab moves for the rest of its life.
Get the base specified for the thing you're actually buying
Tell us the model, the filled weight or capacity, and roughly where it's going, and we'll come and tell you what the ground can take and what the slab needs to be. Do this before the tub or the room is ordered — the delivery date is a bad time to find out the access won't take it.
Read next
- Laying turf on Aylesbury Vale clay: what has to happen first
Why turf laid straight onto clay dies, what the failure looks like at three weeks and at three months, and the preparation that actually works.
- Your new-build garden is waterlogged. Here's who's responsible.
Why gardens on new estates hold water, how to tell compaction from a drainage fault, and the order to escalate it in — developer first, warranty provider second.
- Price calculator
Put your dimensions in and get the range a base of this spec sits in.
- How we build
Excavation, compaction in layers and falls — the same principles, whatever is going on top.
- Your new-build garden is waterlogged
How to tell compacted ground from a drainage fault before you build on it.