Choosing the right foundation for a timber cabin

Why the base does more work than the frame

A timber cabin is only as good as what sits beneath it. Get the foundation right and the structure will stay square, the doors will keep closing, and the floor will stay dry for decades. Get it wrong and you will spend your summers re-levelling posts and fighting damp that creeps up through the bearers.

The job of a foundation is straightforward: transfer the weight of the cabin into the ground without moving, and keep the timber clear of soil and standing water. In the UK that means dealing with shrinkable clay, frost heave, and the small matter of our weather. A base that would be perfectly adequate in a dry summer can heave 20mm in a wet winter if it is too shallow.

The right choice comes down to three things: what your soil is doing, how level the site is, and how heavy the finished cabin will be. Answer those and the material picks itself.

Reading your ground before you spend a penny

Dig a test hole about 600mm deep in the spot where the cabin will sit. You are looking for a few specific things.

• Clay swells when wet and shrinks when dry. It is the single biggest cause of foundation movement in the UK, and it gets worse near trees and hedges, which pull moisture out of the soil for metres around them.
• Sandy or gravelly soil drains well and compacts predictably. It is the easiest ground to build on.
• Soft spots, made ground or old roots need digging out and replacing with compacted hardcore, or a deeper foundation that reaches past the problem.
• Slope matters as much as soil. A gentle fall of 1 in 20 can be handled with stepped pads. Anything steeper usually wants screw piles or a properly engineered solution.

If the site holds water after rain, fix the drainage before you build. A French drain or a shallow channel uphill of the cabin will do more for its lifespan than any upgrade to the base itself.

Gravel and slab bases: quick, cheap and surprisingly capable

For a small cabin, a summerhouse or a glamping pod, a gravel base is often all you need. Excavate 150mm, lay a woven geotextile membrane to stop the subsoil pumping up into the aggregate, then fill with 100mm of compacted MOT Type 1. Compact it in two passes with a plate compactor — this is the step most people skip, and it is the one that matters most.

On top of the Type 1 you can lay paving slabs on a full mortar bed at the corners and along each bearer line, or simply build up a further 30–40mm of compacted 10mm gravel and sit the cabin's bearers directly on that.

The advantages are cost, speed and drainage. Water falls straight through. The limitation is load: gravel bases suit cabins up to roughly 3m × 4m with light contents. Put a log-fired hot tub in one and you will see it settle.

Concrete: slabs, pads and plinths

Concrete is the traditional answer and still the best one for heavier cabins, garden rooms with heavy furniture, or anything intended to last 30 years.

A concrete slab is the simplest option for a flat site. Excavate 200mm, add 100mm of compacted Type 1, then pour 100–150mm of C25 concrete reinforced with A142 mesh, with the mesh sitting mid-depth in the slab. Leave a 150mm apron of gravel around the edge so splashback does not rot the bottom rail.

Concrete pads are better on uneven ground and cheaper than a full slab. Space them no more than 1.2m apart under every bearer, dig each one down to firm subsoil, and pour pads of at least 300mm × 300mm × 300mm. Frost depth varies across the UK — in Scotland you may need to go deeper than in Devon — but 450mm is a sensible minimum in most of the country.

Plinth walls of brick or block lift the cabin well clear of the ground and look smart, but they need a proper trench-fill foundation beneath them and they must have ventilation gaps so air can move under the floor.

Ground screws and screw piles: the modern favourite

Screw piles — steel shafts with a helical blade, driven into the ground with a torque driver — have become the go-to for glamping sites and sloping gardens. They need no excavation, no concrete and no curing time, so a cabin can be up the same week.

Typical domestic ground screws are 550–800mm long and carry 1–3 tonnes each. They work brilliantly in firm clay, sand and chalk. They struggle in soft peat, loose fill or ground full of boulders, where the installer cannot achieve the torque that proves the load capacity.

• Best for sloping sites, tree-root protection areas, and anywhere you want to avoid digging.
• Adjustable heads let you level the frame precisely after installation.
• Cheaper than concrete once you factor in excavation and skip hire.
• Removable, which matters if planning permission is temporary or the cabin may move.

Get the installer to provide a torque log for every pile. If they cannot, walk away.

Matching the base to the cabin

A few rules of thumb worth taping inside your toolbox.

• Under 6m², gravel or slabs will usually do.
• 6–15m² on flat ground, concrete pads or a slab.
• Any size on a slope, or anywhere near mature trees, go for screw piles or a designed foundation.
• Anything with a hot tub, a woodburner and a tiled floor needs a slab or piles, full stop.

Whichever you choose, three details protect the cabin. Keep timber at least 150mm clear of soil. Use pressure-treated bearers running across the joists, not along them. And let air move freely underneath — a cross-ventilated void below the floor is what stops condensation rotting the underside of your beautiful new build.

Spend a weekend getting the base right and you will forget all about it. Cut corners and it will remind you every spring.