The short answer
The three common garden room bases are galvanised ground screws, a concrete pad, and an insulated timber raft, each chosen to suit your ground and budget. Ground screws are quick, low-disruption and good on most ground; a concrete pad or slab is robust and suits heavier or larger rooms; an insulated raft adds floor insulation for warmth. The base must be level, stable and keep the structure off the damp ground. A poor base is one of the most expensive mistakes to fix, so it is worth getting right. Base type and condition feed directly into your overall cost.
The base is the part of a garden room nobody photographs and everybody forgets to ask about — yet it is the foundation, literally, for everything else. A room is only as level, dry and durable as what it sits on. Get the base right and the room lasts decades; get it wrong and you can face damp, movement and costly remedial work. This guide compares the main options so you know which suits your garden and what to expect on cost.
Base options at a glance
- Ground screws Fast, low disruption
- Concrete pad Robust, heavier rooms
- Insulated raft Adds floor warmth
- Must be Level, stable, damp-free
- Affected by Slope, soil, access
- Get wrong Expensive to fix
The three main base types
Most garden rooms sit on one of three base types. Ground screws are large galvanised steel screws driven into the ground, giving a fast, low-mess base that works on most soils and copes well with slight slopes. A concrete pad or reinforced slab is the traditional, robust option, well suited to larger or heavier rooms but more disruptive and slower to install. An insulated timber raft combines a ground-bearing frame with floor insulation, helping keep the room warm from below — a good match for a year-round office or studio. Your builder should recommend the right base after seeing your ground.
| Base type | Best for | Pros | Watch-outs |
|---|---|---|---|
| Ground screws | Most gardens, slopes | Fast, low disruption, removable | Needs suitable ground |
| Concrete pad / slab | Larger / heavier rooms | Very robust, long-lasting | Disruptive, slower, curing time |
| Insulated raft | Year-round rooms | Adds floor insulation | Needs level, prepared ground |
What affects the base cost
Ground conditions and access drive the cost. A flat, firm, accessible garden keeps base costs low; a slope, soft or made-up ground, tree roots, or access that means barrowing materials through the house all push it up. Removing an old shed or patio first adds cost too. Because the base is hidden once the room is up, some quotes understate or exclude it — always check that groundworks are itemised and included. A vague “base extra” line is a common way for a low headline price per square metre to balloon later.
Damp, drainage and the law
A good base keeps the timber structure clear of the ground and away from rising damp, and manages surface water so it does not pool against the room. If your room includes a toilet or shower, drainage needs designing in from the base stage under Building Regulations Part H. The base itself does not usually need planning permission as part of a permitted-development outbuilding, but the finished room must still meet the height and footprint limits. This is general information; the right base depends entirely on your ground, which a builder should survey before quoting.
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Frequently asked questions
What is the best base for a garden room?
It depends on your ground. Ground screws suit most gardens and slopes with low disruption; a concrete pad suits larger or heavier rooms; an insulated raft adds floor warmth for year-round use. A builder should survey your ground before recommending one.
Do garden room foundations need planning permission?
The base itself is normally part of a permitted-development outbuilding and does not need separate planning permission, provided the finished room stays within the height and footprint limits. Always confirm with your Local Planning Authority.
Can a garden room go on a slope?
Yes. Ground screws or a stepped/raised base can handle a sloping garden, though it adds to the groundworks cost. The room must still be level and stable. A builder should assess the slope as part of the quote.
Is the base included in a garden room quote?
It should be, but not always — a low quote sometimes excludes or underestimates groundworks. Always check the base is surveyed, itemised and included before committing, as it is expensive to fix if done wrong.
Sources & further reading
- GOV.UK Planning Portal — outbuildings and permitted development
- GOV.UK / Building Regulations Approved Document H — drainage and waste disposal
- Trade and industry construction guidance — garden room base and groundworks practice
This is general information, not advice for your specific property or project. The right base depends entirely on your ground conditions, which a builder should survey before quoting — confirm planning with your Local Planning Authority.