Start with the ground, not the cabin

It is tempting to get excited about cladding, glazing and the log burner, but the part of a garden room you will never see again is the part that decides whether it still feels solid in ten years' time. A base that is out of level by even 20mm across a 4m span will show up as sticking doors, gaps in the wall panels and a floor that creaks when you walk across it. Get it right once and everything afterwards is straightforward.

Before you dig anything, check the installation guide or warranty terms for your chosen cabin. Most manufacturers specify a maximum tolerance for the base, and some will not honour a claim if the base falls outside it. If your garden room is a heavy one, or you plan to put a hot tub, a piano or a full bookcase inside, that weight needs to be accounted for at ground level.

Choose the right base for your site and cabin

There is no single correct answer here — the best option depends on the size of the structure, the soil you are working with and how much time you want to spend. The four approaches below cover almost every domestic project.

• Concrete slab. The most robust option and the right choice for large cabins or anything heavy. Typically 100–150mm of concrete reinforced with A142 mesh, poured over a compacted sub-base with a damp-proof membrane underneath. It is permanent, so be certain about the position before you pour.
• Paving slabs. Well suited to small and medium cabins. Lay 600x600mm slabs on a compacted sub-base, bedded on a mortar mix or sharp sand, and check each one with a level as you go. Cheaper than concrete and easy to adjust later.
• Timber bearers. Pressure-treated joists laid across a slab or paved base, usually at 400mm centres. They raise the cabin clear of the ground and allow air to circulate underneath, which is an excellent defence against damp.
• Ground screws or adjustable feet. Quick, tidy and ideal on a sloping site where digging out would mean a lot of spoil. They need a rigid cabin floor frame to spread the load, so confirm suitability with the supplier first.

Clear, level and drain the site

Mark out your footprint with pegs and string, allowing at least 300mm beyond the cabin on every side for access and airflow. Strip the turf and remove the soft topsoil to a depth of around 150mm. Organic material left under a base will rot and settle, taking your level with it.

Think about water before you think about timber. Check where the ground naturally drains and make sure the finished base sits at least 50mm above the surrounding soil so surface water runs away from it, not towards it. If the site is known to be wet, a French drain or a shallow soakaway filled with gravel along the uphill edge is a small job that prevents a very large problem later.

Set out accurately and check your levels properly

A square, accurately marked base makes everything else simpler. Use the 3-4-5 method on the corners, or simply measure both diagonals — they must be identical. Peg and string your lines, then check them again the following morning before you commit to any digging or pouring.

For levels, a short spirit level on its own is not enough. Use a long straightedge and a level together, or hire or borrow a laser level, and take readings across the full width and length of the area, plus both diagonals. Sighting a level by eye across a 4m base will catch far more mistakes than trusting a 600mm spirit level placed in a few spots.

Build the base layer by layer

Whatever base type you choose, the preparation underneath is broadly the same. Spread MOT Type 1 hardcore to a depth of 100–150mm and compact it in 50mm layers using a plate compactor, damping it lightly with water as you work. Rushing this stage is the single most common cause of a base that dips in one corner a year later.

For a concrete slab, lay a damp-proof membrane over the compacted sub-base, position the mesh on small spacers so it sits mid-depth, then pour and tamp to a flat, level finish. Keep it flat rather than introducing a fall — the cabin needs level, and drainage should be handled by the ground around it. Allow at least three days to cure before building on it, and longer in cold weather.

With slabs, bed each one firmly and tap it level before moving on, checking frequently against your string lines. With timber bearers, run a strip of damp-proof course along the top of the base beneath each joist, and leave a clear 50mm gap underneath for airflow.

Final checks before the cabin arrives

Leave the base to settle for at least 48 hours, then go back over it with your level one more time. Sweep it clean, lay a weed membrane if you are using bearers or slabs, and trim back any vegetation from the edges. Confirm that water runs away freely on all four sides and that nothing is bridging the damp-proof layer.

Do those checks properly and the exciting part — the build itself — will be a straightforward weekend's work rather than an exercise in shimming and swearing.