A concrete base for a garden room is doing three jobs at once: spreading the building's weight onto ground that can take it, staying level for decades, and keeping moisture out of the floor. Most base failures are not concrete failures. They are the result of somebody assuming the ground rather than looking at it, and the fix afterwards is worse than the base.
Depth is decided by the ground, not by the building
A slab sitting on firm undisturbed subsoil behaves very differently from one on made ground, on soft clay, or on a filled garden. The excavation goes down to something that can carry the load, and how far that is cannot be read off a drawing. Where trees are nearby, clay shrinkage under a shallow slab is a real movement risk and the foundation may need to go substantially deeper than the building would otherwise need.
The membrane matters as much as the concrete
A damp proof membrane under the slab, lapped and turned up at the edges, is what stops ground moisture rising into a floor that is going to have insulation and a finish on top of it. Insulation under or over the slab is what stops the floor being cold to stand on. Neither is expensive and both are invisible once it is poured, which is why they are the first things to disappear from a cheap base.
Levels, falls and where the water goes
The base has to be level for the building and the ground around it has to fall away from it. A slab set flush with a lawn is a slab that sits in water every winter. Think about where roof water will discharge before the base is poured, because a soakaway or a gully is far easier to dig at the same time as the foundation.
The alternatives, and when they are better
Screw piles and concrete pad and beam systems are quicker, need far less spoil taken away, and cope better with a sloping or root filled garden. They cost more per square metre than a simple slab on good ground and less than a deep one on bad ground. On a difficult site they are frequently the right answer rather than the cheap one.
Questions people ask about concrete base for garden room
How thick should a garden room slab be?
There is no single figure, because thickness follows the load and the ground. A slab is designed rather than chosen, and on anything other than good firm ground the sensible route is to have the design done rather than to copy a number from a forum.
Can I build on an existing patio?
Rarely without work. Patios are laid on a bed that was never intended to carry a building, and the levels and falls are wrong. It is sometimes possible to build a new base within or over one, but it starts with somebody lifting a slab and looking.
Do tree roots matter?
A great deal, on clay. Nearby trees draw moisture out of clay soil and the ground shrinks and swells with the seasons, which moves shallow foundations. Distance, species and soil all matter, and where there is any doubt it is a question for somebody who can look at the site.