A portable cabin needs a firm, level, free-draining base, not foundations. Existing concrete or tarmac is usually fine as it is. On bare ground, the standard answer is around 150mm of compacted hardcore, concrete pads under the corners, or a slab for the big blocks. Here is what works at each size.
Three jobs. It spreads the cabin's weight so the unit does not settle unevenly, it keeps the floor dead level so doors swing true and water drains the way the plumber intended, and it keeps the underside out of standing water. A cabin on a poor base does not fail dramatically; it racks slowly, doors start catching, and on sanitary units the showers stop draining. Getting the base right costs a fraction of putting it right later.
Portable cabins are surface-mounted, not built in. That matters for planning too: units that sit on the ground and can be lifted away are usually treated as temporary structures, which is covered properly in our planning permission guide.
| Unit | Minimum base that works |
|---|---|
| Steel store or flat pack container | Any firm, level ground; flags under the corners on softer surfaces |
| Single cabin to around 10ft (WC unit, kiosk, gatehouse) | Existing concrete/tarmac, or compacted hardcore; concrete pads or paving flags under the corners also work |
| 16ft to 30ft cabin (site office, toilet block) | Compacted MOT Type 1 hardcore around 150mm deep, or a concrete slab; pads under load points on good ground |
| Large or combined blocks, multi-module buildings | Concrete slab or engineered pad design; we advise per layout on made-to-order buildings |
| Jackleg cabins | The four (or six) legs take point loads on pads or firm ground, and wind down to level the cabin on a modest slope |
If you are starting from grass or soil: strip the topsoil, put down a weed membrane, fill with MOT Type 1 and compact it in layers. For a single cabin that is typically a one-day job and a few hundred pounds in materials, in line with the base budget in our welfare unit cost guide. A full concrete slab only becomes necessary for the larger blocks, or where the cabin is staying for years and you want zero maintenance.
Toilet and shower cabins add two requirements to the base itself. First, a drainage run: the waste outlet needs a fall to a drain, holding tank or sewer connection, so think about which way the pipes leave the unit before you decide its orientation. Second, service trenches for water and power reach the cabin at its connection points; arriving to find the base perfect but the services on the wrong side is a common and avoidable extra cost. Connection details are in the welfare unit cost guide.
The base is only half the ground question; the other half is getting the cabin onto it. A flatbed or HIAB lorry needs firm access to within reach of the base, room to swing the crane where one is used, and no soft verge between the road and the siting spot. The specifics, including when a HIAB is needed and what counts as restricted access, are in delivery and collection. Tell us about overhead lines, gates and slopes when you order; it is far cheaper to plan around them than to discover them on the day.
Honestly, often. If the cabin is going onto an existing yard, car park or slab that is flat and sound, put it straight down. And if the job is a store rather than an occupied cabin, a flat pack container sits on any firm level ground with no preparation, arrives without a crane, and moves again when the site changes. Do not pay for a slab you do not need.
Send a photo of the spot and your postcode and we will tell you what preparation, if any, the unit you want actually needs.