Building
How a garden room is built, from the base up
In short
A garden room is built in six stages: a level base on ground screws or a slab, a timber frame, a roof deck with a waterproof membrane, insulation and a vapour layer on all six sides, doors and windows, then cladding outside and lining, power and finishes inside. A builder does it in one to three weeks on site. The base and the insulation decide whether it lasts and whether you use it in winter.
Most garden rooms look simple from the lawn: a box with a lot of glass. The difference between one you use every day and one that smells damp by its second winter is almost all in parts you never see once it is finished. This is the order a garden room goes together in, what happens at each stage, and the points where it is worth slowing down.
Before anything is built
Two things come first, and both are on paper.
Height and position. If any part of the room is within 2 m of a boundary, the whole building must be no more than 2.5 m high under permitted development. That 2.5 m has to hold the base, the floor build-up, the ceiling and the roof. Settle the position before the design, because moving a room 2 m away from the fence can add 50 cm of allowed height. Our planning guide covers the other limits.
Size and use. Under 15 m² of floor area and nobody sleeping in it, a garden room is exempt from the building regulations. Between 15 and 30 m² it stays exempt only if it is at least 1 m from every boundary or built mostly of non-combustible materials. Many rooms are designed to 14.9 m² for exactly this reason.
Stage 1: the base
Everything that goes wrong later tends to start here. A base has to be level, it has to stay level through wet winters and dry summers, and it has to keep the timber floor clear of the ground so air can move underneath.
There are three common choices:
- Ground screws. Galvanised steel screws driven into the ground with a machine, usually in a grid at about 1.2 to 1.5 m centres, then a timber or steel frame bolted on top. Quick (often half a day), little digging, easy to level on a slope, and nothing to cure. The best answer on most lawns.
- Concrete pads. A small concrete pad under each support point. Cheap, but they depend on the ground under each pad being firm, and on clay they can move.
- A slab. A reinforced concrete slab, usually 100 to 150 mm thick on compacted hardcore. The heaviest option and the slowest, as it needs about a week to cure, but the right one for a large room, soft ground or a room with a heavy floor load such as a gym.
On heavy clay that shrinks in summer, the base needs to reach below the layer that moves. Longer screws or a slab taken deeper do that; shallow pads often do not.
Stage 2: the floor and frame
The floor is a grid of treated timber joists on the base, with insulation between the joists and a deck of moisture-resistant board on top. A breathable membrane under the joists stops wind and vermin getting into the insulation.
The walls are a timber stud frame, typically 75 to 100 mm deep, which sets how much insulation fits. The frame goes up quickly: a builder will often have the walls standing and the roof joists on in a day or two. This is the point to check door and window openings against the drawing, because changing them later means taking cladding off.
Stage 3: the roof
Most garden rooms have a flat roof with a slight fall, about 1 in 60, so rain runs to a gutter at the back. The roof deck is covered in EPDM rubber or a single-ply membrane, both of which last decades if they are laid in one piece without seams. A pitched roof suits a room 2 m clear of the boundary, where the height limit rises to 4 m.
The membrane goes on as soon as the deck is down. From that point the shell is dry, which matters: insulation and boards that have been rained on before they are sealed in are a common cause of damp later.
Stage 4: insulation and the vapour layer
This is the stage that decides whether a room is used in January.
A good specification for year-round use is 75 to 100 mm of PIR foam board in the floor and walls and 100 to 150 mm in the roof, all six sides, with no gaps at the joints. The joints are taped. On the warm (inside) face, a vapour control layer stops moist room air reaching the cold outer layers, where it would condense. Leave that layer out, or puncture it with careless fixings, and the result is mould behind the lining within a couple of winters.
Builders usually leave a ventilated cavity between the outside of the frame and the cladding, so any moisture that does get in has a way out.
Stage 5: doors, windows and power
Double glazed doors and windows go into the prepared openings, packed, fixed and sealed. Sliding and French doors are the standard choice; bifolds open up a whole wall but cost more and need a stiffer head above them.
The electrics come next. A garden room needs its own circuit from the consumer unit in the house, fed through armoured cable buried at a safe depth across the garden. This is notifiable work under the building regulations, so it must be done by an electrician registered with a competent person scheme, who will issue a certificate. Keep it; a buyer’s solicitor will ask for it. If you want wired internet, lay a data cable in the same trench at the same time.
Stage 6: cladding and fit-out
Outside, the cladding goes on over battens: larch and cedar are the usual timbers, with composite boards for low maintenance and charred timber or metal for a contemporary look. Corner trims, fascias and the gutter finish it.
Inside, the walls are lined in plasterboard or birch ply, the floor is laid (engineered wood, vinyl plank or carpet tiles), and the electrician returns to fit sockets, switches, lights and a heater. A typical room takes one to three weeks on site from base to handover, depending on size and finish. Most of the elapsed time in a garden rooms Leeds project is actually the wait for a start date in spring, not the build.
Doing it yourself
A self-build can save a third or more of the cost, since the materials for a well-insulated 4 × 3 m room typically come to £8,000 to £12,000. The parts that most often go wrong are the base (not level, or not deep enough on clay), the vapour layer (missing or punctured), and the roof membrane (seams in the wrong place). The electrics must be done or signed off by a registered electrician whoever builds the rest. If you are weighing the two routes, our price calculator shows what a built room usually costs for the same size.