Garden room construction sounds like one thing and is at least four. Timber frame built on site, factory made panels craned or carried in, structural insulated panels, and masonry are different products with different wall thicknesses, different bases underneath them and different amounts of time in your garden. The method also decides how much internal space you get from the same footprint.
Building a garden room in timber frame on site
A studwork frame put together in the garden, insulated between and often over the studs, sheathed, wrapped and clad. It is flexible, copes with awkward shapes and levels, and needs the longest on site. The quality is entirely in the workmanship and the detailing, which is why it varies more than any other method.
Factory panels: the commonest garden room installation
Walls, floor and roof made in a workshop and assembled on the base in a day or two. Consistency is the advantage, and the constraints are access and tolerance: panels have to reach the garden, and the base has to be right within millimetres rather than roughly. This is the commonest way a supplied garden room is built.
Structural insulated panels for a fully insulated garden room
A rigid insulation core bonded between two boards, so the panel is the structure and the insulation at once. It gives a thin wall for a given performance, which means more internal floor from the same footprint, and it is very airtight, which makes ventilation something to design rather than assume.
Masonry
Blockwork or brick on a proper foundation, which is a building rather than a structure in a garden. It is the most expensive and the slowest, it needs the deepest base, and it is the one that reads as permanent to a valuer and to a neighbour. It also brings the design closer to the point where building regulations apply.
Foundations and the floor
Three approaches dominate. A reinforced concrete slab is the most robust and the most disruptive to install, and it suits masonry and heavy uses. Concrete pads or piers with a timber ground frame spanning between them is the common answer for a timber building, uses far less concrete and copes with a slight slope. Screw piles are driven into the ground and carry a frame, which suits difficult access, wet ground and tree roots and is quick. Whichever is used, the floor build up above it needs insulation and a damp proof membrane: an uninsulated floor over a ventilated void is one of the commonest reasons a garden room is cold underfoot in winter.
Flat, pent or a pitched roof garden room: why flat dominates
A flat roof, usually with a small fall hidden behind a fascia, keeps the ridge height down, which matters because the permitted development height limit is lower within two metres of a boundary. A pent roof falls one way visibly and does the same job with a simpler fascia detail. A pitched roof looks more like a building and eats the height allowance quickly. Whatever the shape, it is a warm deck problem: insulation above the deck, waterproofing above that, and a continuous vapour control layer below, because there is no loft to inspect.
Garden room cladding, and how it looks in ten years
Cedar and larch weather from honey to silver grey unless they are oiled on a cycle, and the change is uneven where rain hits differently, which surprises people who expected the brochure colour permanently. Composite and fibre cement boards hold colour and cost more. Painted or stained softwood is cheapest and needs the most maintenance. Rendered and masonry buildings last longest and cost most. In every case the cladding wants a drained and ventilated cavity behind it rather than being fixed flat to the sheathing, because that batten space is what keeps the structure dry.
Questions people ask about garden room construction
Which method is warmest?
Any of them can be, because warmth is a specification rather than a method. What differs is how much wall thickness a given performance costs you, and structural insulated panels usually win on that measure while masonry usually loses.
Which is fastest?
Factory panels, by a wide margin, once the base is ready. The base is the part that takes the time on every method, and it is worth remembering that a two day installation still follows a week or more of groundworks.
Does the method change whether I need approval?
Not directly. Planning and building regulations follow the size, position and use of the building rather than how the walls are made. What the method does change is how easy it is to meet a standard if approval does apply.
What foundation does a garden room need?
A concrete slab, concrete pads with a timber ground frame, or screw piles, chosen on the ground, the access and the weight of the building. All three need an insulated floor build up with a damp proof membrane above them.
Does cedar cladding need maintenance?
Only if you want to keep its colour. Left alone it silvers, unevenly where rain hits differently. Oiling on a cycle keeps the tone, and that is a maintenance commitment rather than a one off.