Sussex, Surrey & Kent · 6 min read
Ground Screws vs Concrete Base for a Garden Room
Both bases work when they suit the ground. Here is how clay, trees, slope and access decide between ground screws and a concrete raft.
Garden room base being excavated and levelled on Weald clay in a West Sussex garden
Short answer: choose ground screws on sloping plots, near trees or where a concrete pour cannot reach. Choose a reinforced concrete raft on level, stable ground, or when the room will hold a bathroom, sauna or heavy fit-out. On the shrink-swell clay across much of Sussex, Surrey and Kent, either works only if it is designed around what a trial hole finds.
Ground screws and concrete compared
| Factor | Ground screws | Reinforced concrete raft |
|---|---|---|
| Guide cost, typical room | Around £2,500 to £6,000 | Around £4,000 to £9,000 |
| Digging and spoil | Very little | Full excavation and muck away |
| Near trees | Usually the better option, minimal root disturbance | Often ruled out by root protection areas |
| Sloping ground | Height set per screw, no retaining needed | Needs cut, fill or a stepped slab |
| Tight access | Hand-held kit fits through a side gate | Concrete must be pumped or barrowed |
| Bathrooms and heavy loads | Possible, needs careful design | The simpler choice |
| Time on site | Often a day | Several days plus curing time |
Please treat every figure here as an indicative guide range based on projects we have completed, not a quotation. The final price depends on access, ground conditions, levels, drainage and specification, and is only confirmed after a site visit.
Why clay soil changes the answer
Weald clay around Haywards Heath, Horsham, Crawley and Tonbridge swells in a wet winter and shrinks in a dry summer. A shallow base sitting in that top layer moves with it, which is how doors stop closing and floors pick up a slope. Ground screws need to reach firmer ground below the active layer, and a concrete raft needs proper excavation, a compacted sub-base and reinforcement so it moves as one piece rather than cracking.
When ground screws are the right call
- A mature oak, willow or poplar within influencing distance of the building
- A garden that falls away, where a slab would need retaining walls
- Access only through a narrow side gate or through the house
- A wish to keep digging, spoil and lorry movements to a minimum
When a concrete raft is the right call
- A level, stable plot with good access for a pump or barrow run
- A room with a shower, WC, sauna or heavy stone flooring
- A building tied into new paving at the same level as the floor
Whichever base you choose, check these
- A trial hole, and a written note of what the ground was
- Screw depth and load test results, or slab reinforcement and membrane spec
- Where roof water goes, and a soakaway sized for the roof
- Whether spoil removal is in the price
If you are not sure which base your garden needs, get in touch and we will look at the ground before recommending either.
Planning your next step
If this guide has helped shape your plans, see how we approach it in practice on our Garden rooms page, or browse every article in our garden rooms hub. Homeowners reading this often go on to How Much Does a Garden Room Cost in Sussex? and Oak-Framed vs Contemporary Garden Rooms: Which Suits a Sussex Home?. When you are ready to talk through your own garden, get in touch with the Fresh Air Landscapes team.
Common questions
- Are ground screws better than a concrete base for a garden room?
- Neither is better everywhere. Ground screws suit slopes, tree roots and tight access because they need little digging. A reinforced concrete raft suits level, stable ground and rooms with a bathroom or heavy fit-out. A trial hole decides it.
- Do ground screws work on clay soil?
- Yes, when they are driven below the zone where clay shrinks and swells and are load tested on site. Short screws set only in the top layer of clay will move with it, so depth matters more than the screw itself.
- Which is cheaper, ground screws or a concrete base?
- As a guide, ground screws for a typical garden room fall around £2,500 to £6,000 and a reinforced concrete raft around £4,000 to £9,000. Access and spoil removal move both figures more than the choice itself.
Where we work
We design and build gardens, garden rooms and pergolas across Mid Sussex and the surrounding towns.
More garden rooms guides
- How Much Does a Garden Room Cost in Sussex?A line-by-line look at where garden room budgets go in Sussex, and the specification choices that separate a ten-year building from a thirty-year one.
- Oak-Framed vs Contemporary Garden Rooms: Which Suits a Sussex Home?If you are drawn to oak-framed buildings but need the performance of a contemporary garden room, this guide compares the two styles on looks, planning and longevity.
- Garden Rooms: The Complete Buyer's GuideGarden rooms range from a shed with a window to a fully insulated, year-round building. This guide explains the differences that actually matter before you spend anything.