Short answer: a garden sauna or cold plunge needs a firm, level, well-drained base that can take its full weight. That usually means a concrete slab, paving slabs on a compacted sub-base, or a level timber frame, as the manufacturer specifies. A filled plunge tub is one of the heaviest things you can put in a garden, so work out its weight before you choose. Every product page lists the base the manufacturer requires, and that always comes first.
1. Why the base matters
- Saunas: a base that isn't level twists the frame. Doors stick, seals gap and barrel staves open up. Most of these problems start underneath.
- Plunge tubs: an uneven or soft base strains the tub, leaves the water line off-level and can sink under the load.
- Warranties: manufacturer warranties usually require installation as their instructions say, including the base.
2. Work out the weight
Water weighs about 1 kg per litre. For a plunge tub:
Filled weight = litres of water + the tub's empty weight + the people in it
As an illustration, assuming a tub that weighs 100 kg empty and one 90 kg adult:
| Tub size | Water | Tub and one adult (assumed) | Filled weight |
|---|---|---|---|
| 300 litres | 300 kg | 190 kg | 490 kg |
| 400 litres | 400 kg | 190 kg | 590 kg |
| 500 litres | 500 kg | 190 kg | 690 kg |
Then think about where that weight lands. If the 500-litre tub stands on a footprint of 1 m by 1 m, that's about 690 kg on a single square metre. A smaller, round footprint concentrates it further. Use the real empty weight and footprint from the product page.
Saunas weigh less for their size, but the load isn't always spread evenly. Barrel saunas rest on cradles, so the weight comes down along a few narrow strips, and every cradle needs full support at the same height. The stove and stones add weight in one spot. The product page gives the weight.
Decking, balconies and upper floors are not usually built for a load like a filled tub. Check the load limits, ideally with a structural engineer or builder, before you put a plunge on one.
3. Choosing a base
| Base | Good for | Watch out for |
|---|---|---|
| Concrete slab | Plunge tubs and saunas; the strongest option | Digging and setting time; the top must be level; plan any cable duct before it's poured |
| Paving slabs on a compacted sub-base | Most saunas; plunge tubs where the manufacturer allows it | Every slab needs a full, even bed so none rocks or sinks under a cradle or tub |
| Timber frame or deck | Saunas where the manufacturer specifies one; sloping gardens | Check the load before a filled plunge goes on; keep timber off the soil; height limits (see section 6) |
| Ground screws | Sloping or awkward ground, with less digging | Only if the manufacturer accepts them; they need a level frame on top; check for buried pipes and cables first |
If the manufacturer specifies a base, use that one. If you're unsure, a builder or landscaper can design a base for the weight and your ground.
4. Getting it level
- Check with a long spirit level in both directions and across the diagonals, not just along one edge.
- Level means the whole footprint. Barrel cradles and tub feet all need equal support.
- Don't rely on offcuts or wedges as a permanent fix. Correct the base instead.
- Check again just before delivery, and after the first few weeks, especially on new paving or timber.
The manufacturer's instructions give the tolerance for each product.
5. Drainage and siting
- Avoid hollows and spots that hold water after rain. Standing water softens the ground and keeps timber damp.
- A gravel margin around the base helps rain drain away and cuts splash-back onto the timber.
- Plan for emptying the plunge. Tubs need a water change from time to time. Follow the manufacturer's draining instructions. Don't empty water containing sanitiser into a pond, stream or wildlife area, and ask your water company if you're unsure where it can go.
- Leave clearance for ventilation, maintenance, the door swing and the chiller's airflow. Wood-fired saunas also need safe clearance from buildings, fences and overhanging trees.
- Connect the pair. If you have a sauna and a plunge, put them close together with a firm, non-slip path between them.
- Think about the cable. Ask your electrician whether to lay a duct before the base goes down (see our electrics guide).
6. Planning rules for the base (England)
In England, decking and other raised platforms are permitted development, with no planning application needed, if they are:
- no more than 30 cm above the ground;
- together with extensions and outbuildings, covering no more than half the garden;
- not forward of the wall forming the principal elevation, usually the front of the house.
Listed buildings and designated land have extra limits. Our planning guide covers the sauna itself. Different rules apply in Wales, Scotland and Northern Ireland, so check with your local planning authority there.
7. Access for delivery
Saunas and plunge tubs usually arrive kerbside on a pallet, and you move them from there. Build the base before delivery day, so the parts can go straight into position. Measure the route from the kerb to the base, including gates, side passages and steps. Our pallet delivery guide explains what to expect.
Before delivery: a base checklist
- Manufacturer's base specification read on the product page.
- Filled weight worked out, for a plunge tub.
- Base built, level in both directions and fully set.
- No standing water, and a plan for emptying the plunge.
- Cable route or duct agreed with your electrician.
- Route from the kerb to the base measured and clear.
FAQs
Can I put a barrel sauna straight on grass or gravel?
Not on grass or bare soil: it settles unevenly and the timber stays damp. Loose gravel on its own rarely stays level. Use the base the product page specifies.
Can a cold plunge go on decking?
Only if the deck is built for the load. In our example, a filled 500-litre tub weighs about 690 kg. Have a structural engineer or builder check the deck first.
Do I need planning permission for a sauna base?
In England, a raised deck or platform more than 30 cm above the ground needs planning permission. Our planning guide explains the rules for the sauna itself.
This is general guidance. Always follow the manufacturer's base specification, and get a builder, landscaper or structural engineer to check anything that carries a heavy load.
Sources
- Planning Portal, decking (planning permission): https://www.planningportal.co.uk/permission/common-projects/decking/planning-permission
- Planning Portal, outbuildings (planning permission): https://www.planningportal.co.uk/permission/common-projects/outbuildings/planning-permission