The short answer
There is no single best base. A concrete slab is solid and suits level ground. Ground screws are quick, cause little mess and work well on slopes and near trees. A timber frame on pads or a plastic grid suits smaller, lighter rooms. The ground and the size of the room decide which is right.
The base is the part of a garden room nobody sees, and the part that most often goes wrong when it is rushed. It has to be level, strong enough for the room and everything inside it, and it has to keep the floor dry for decades.
The main garden room base options
Concrete slab
A reinforced concrete slab is the traditional base. The ground is dug out, a layer of compacted hardcore goes down, then a damp-proof membrane and the concrete.
Good for:
- level gardens with good access for diggers and concrete
- heavy rooms, gyms with racks and weights, and rooms with a bathroom
Watch out for:
- a lot of digging, spoil to remove and drying time
- poor fit with steep slopes or tree roots
Ground screws
Ground screws are steel screws wound into the ground by machine, with a frame fixed on top. Each screw can be set to a different height, so the frame comes out level even on uneven ground.
Good for:
- sloping gardens
- gardens with tree roots, where digging a slab would cause damage
- tight access, because the kit is small
- speed, often fitted in a day
Watch out for:
- rocky ground, which can stop a screw going in
- the floor sitting a little higher than on a slab
Timber frame on pads or plastic grid
Lighter rooms can sit on a treated timber frame supported by concrete pads, adjustable feet or an interlocking plastic grid filled with gravel.
Good for:
- smaller, lighter rooms on fairly level ground
- keeping the cost down
Watch out for:
- heavy uses like gyms, which may need something stronger
- soft ground, which can let pads settle over time
How they compare
| Base | Typical cost | Slopes | Speed |
|---|---|---|---|
| Concrete slab | about £80 to £120 per m² | Poor without extra work | Slow, needs curing |
| Ground screws | about £30 to £50 per m² | Good | Fast |
| Timber frame on pads or grid | low | Fair on gentle slopes | Fast |
Price guides put the base for a typical room at about £500 to £2,500, rising towards £4,500 on sloping or clay ground. The builder will usually include the base in the overall quote. Ask for it as a separate line so you can see what the ground is costing you.
Building a garden room on a slope
Plenty of gardens slope, and Sheffield has more than its share. A slope rarely rules out a garden room, but it changes the base.
Gentle slope
Ground screws or adjustable feet take up the difference with little or no digging. This is usually the cheapest answer.
Steeper slope
There are two routes. The room can sit on a raised frame, with steps or a small deck up to the door. Or the builder can dig into the slope to make a level platform, which may need a small retaining wall to hold the bank back. Digging in costs more but keeps the room lower.
Height limits
On a slope, height matters for planning. Permitted development heights are measured from the highest ground next to the building, so raising the room on one side can push it over the limit. Within 2 metres of a boundary, the maximum is 2.5 metres. Our planning guide has the full limits.
Other things that affect the base
Trees and roots
Digging a slab near a tree can damage its roots, and some trees are protected by Tree Preservation Orders. Ground screws or pads disturb far less of the ground.
Drainage and damp
Whatever the base, the floor should sit clear of the ground with air moving underneath. That keeps the frame dry and stops damp rising into the room.
What goes inside
A gym with a loaded rack, a hot tub next to the room or a bathroom all add weight and services. Tell the builder early, so the base is designed for it.
What to ask for in a quote
- The type of base and why it suits your ground.
- Whether the base is included in the price or extra.
- How they will deal with any slope, and what the finished floor height will be.
- What happens to the spoil from digging, if any.
- How the base is kept dry and ventilated.
A good builder will look at the ground before recommending a base, not after. If you’re thinking about a garden room in Sheffield, tell us about the slope in the quote form so the builder comes prepared.