Yes. Every garden room needs a proper base, and getting it wrong is the single most
common reason a garden building fails. What it does not need is the deep trench foundation
of a house extension. There are three sensible options and the ground decides which one.
The three types of base
Concrete slab
A reinforced concrete raft. The most robust option, good for larger buildings and uneven ground.
More groundwork, more spoil to remove, and in a terraced garden with no side access every barrow
goes through the house or down the side return.
Ground screws
Galvanised steel screws driven into the ground, with a timber frame built off them. Fast, almost
no spoil, and they work well on sloping plots. They also leave the ground largely undisturbed,
which matters if there are tree roots you should not be cutting.
Concrete pads
Individual pads at set centres carrying a timber sub-frame. A good middle ground for smaller
buildings on reasonable ground, with less excavation than a full slab.
What decides which one you need
Ground conditions
Much of South London sits on clay, which shrinks and swells with moisture. That movement is what
lifts and twists a badly founded building over a few seasons.
Trees
Mature trees near the build affect both the ground and what you are allowed to do. If the tree
has a Tree Preservation Order, or you are in a conservation area where trees have protection,
excavation near the roots is a problem. Ground screws are often the answer.
Slope
A sloping garden either needs levelling or a base that can be set to level off varying heights.
Ground screws handle this well.
Access
If the only route to the back garden is through the house or a narrow side return, that changes
what is practical. It is one of the first things we look at on a site visit.
What happens if the base is wrong
Doors and windows stop closing properly. Floors develop a slope. Cladding joints open up. In the
worst cases the frame twists enough that the building has to come apart. None of it is repairable
cheaply, which is why the base is not the place to save money.
You can see the range of ground conditions we have dealt with in our garden building project gallery.
Frequently asked questions
What is the best foundation for a garden room?
It depends on the ground. A reinforced concrete slab is the most robust, ground screws are best on slopes, near tree roots or where access is tight, and concrete pads suit smaller buildings on stable ground.
Can you put a garden room on paving slabs?
Not for a permanent insulated building. Slabs laid on sand move, and that movement transfers straight into the frame. They are only suitable for a light storage shed.
Do garden room foundations need building regulations approval?
The base itself does not usually trigger approval if the building is under the size thresholds, but a building over 30 square metres needs building regulations approval, and that includes the structure.
How deep do garden room foundations need to be?
There is no single figure. It depends on ground conditions, the size of the building and nearby trees. That is exactly what the site visit is for.
Thinking about a garden building?
We design and build bespoke garden rooms and studios, purpose built garden offices, garden art and music studios, garden yoga and exercise studios and bespoke garden sheds and storage across East Dulwich, Dulwich, Peckham and the surrounding South London areas. See our garden building project gallery or the project map of every build, then book a free site visit and we will give you an honest estimate with no obligation.