Most garden rooms are sold as a building, not as a job. The quote covers the frame, the cladding, the doors and the insulation, and somewhere near the bottom there is a line saying the base is the customer's responsibility. That line is where a lot of these projects come unstuck, because the base is the one part nobody can put right afterwards.
The base is rarely in the price
Check the quote before you sign it. Some garden room suppliers will lay a base as an extra. Plenty of others will only install onto a level base that is already sitting there on the day. If the ground is not ready when the lorry turns up, you pay for a wasted delivery and you go to the back of the fitting queue.
Ask the supplier early: what base type will they install onto, what footprint and tolerance do they need, and do they want services run in before the floor goes down. Get the answers in writing. A base built to the wrong dimension by 50mm is a real problem when the building has been made in a factory to fixed sizes.
Slab, pads or screws
Three approaches cover most jobs.
A full concrete slab is the traditional answer. Dug out, filled with compacted hardcore, damp proof membrane, reinforcement mesh, then concrete poured level across the whole footprint. It spreads weight well, it gives you a solid floor for a heavy fit-out, and it does not move. It is also the wettest and slowest option, it produces the most spoil to cart away, and it needs curing time before anything is built on it.
Concrete pads carry the building on a series of footings with a timber or steel frame spanning between them. Less digging, less concrete, and air can move under the floor. It suits ground that falls across the plot. It only works if the setting out and the spans are right, so it is less forgiving of guesswork than a slab.
Ground screws are steel piles wound into the ground to a set depth. They go in fast, often inside a day, they make very little mess, and they handle sloping or awkward gardens. They are not the answer in very loose or sandy ground, and somebody still has to look properly at what the ground is before committing to them.
None of the three is automatically right. It depends on what the ground is doing, how heavy the building is, and how much fall there is across the plot.
What goes into a concrete base
If you are having a slab, the concrete is the last thing that happens and the least interesting part of it.
Topsoil comes off first. Topsoil is organic, it rots down and it compresses, so it has to be out of the way before anything structural goes on top. Under the slab you want a properly compacted sub-base, whacked down in layers rather than tipped in and levelled in one go. Then the damp proof membrane, then mesh sat up on spacers so it ends up inside the concrete rather than lying on the floor doing nothing.
Get the level right and get the size right. Most suppliers want the slab the same size as the building or a fraction smaller, so rain runs off the cladding and drips clear instead of sitting against the bottom rail.
Where the water goes
A garden room puts a new roof in your garden. That roof sheds water somewhere, and unless you plan for it, the water goes straight into the ground at the edge of the building.
The ground should fall away from the base, not towards it. Rainwater off the roof wants guttering and a proper outfall, usually a soakaway set far enough away that it is not softening the ground under your new floor. Sending it onto a neighbour's land is a good way to start an argument you do not need. On heavy Cambridgeshire clay, water with nowhere to go tends to sit, and sitting water at the edge of a slab is how you end up with a damp floor and a rotten bottom rail two winters later. Our guide to drainage and soakaways covers this in more detail.
Planning and building regulations
Most garden rooms go up under permitted development, but the limits are specific. An outbuilding is capped at 2.5 metres in height anywhere within two metres of a boundary. Further in from the boundary you get four metres with a dual pitched roof or three metres for any other roof, with a maximum eaves height of 2.5 metres. Nothing forward of the principal elevation of the house. No balconies or raised platforms over 0.3 metres. And no more than half the land around the original house covered by buildings.
Building regulations are a separate question. A detached outbuilding under 15 square metres of floor area with no sleeping accommodation does not normally need approval. Between 15 and 30 square metres it is still normally exempt, provided there is no sleeping accommodation and it is either at least a metre from any boundary or built substantially of non-combustible materials. Sleeping in it changes the answer. So can a conservation area, a listed building or an Article 4 direction, so check with your district council rather than assuming.
Drains, trees and boundaries
Before you fix the position, find out what is under the lawn. A public sewer crossing the garden restricts what you can build over or close to, and it needs the water company's agreement first. We wrote about that in build over agreements.
Trees matter as well. Put a shallow base near a mature tree on shrinkable clay and the ground will move as the tree draws moisture out of it through the summer. The base moves with it.
Getting the ground right first
The building is the easy part. Everything that decides whether it stays level, stays dry and lasts twenty years happens before it arrives, and it happens in the ground.
We build garden room bases across Newmarket, Cambridgeshire and Suffolk, from a straightforward reinforced slab through to pads, drainage and the connections that go with them. If you have a garden room on order, or you are just working out what the ground work will involve, call us on 07916 033276 or request a quote and we will come and look at it. Have a look at our foundations and footings page for what else we take on.