How it is built
ICF or timber frame? How a poured annexe differs from a garden room
Both end up as a room in the garden. They are not the same kind of building, and the difference is easiest to see in how the wall is made and how a window gets into it.
The short answer
One is a panel. The other is a poured wall.
A timber-framed garden room is built from studwork, insulated between and around the studs, and clad. An ICF annexe is built from insulated formwork blocks, dry-stacked to the shape of the wall, reinforced with steel and filled with concrete in one pour. What sets is a solid reinforced concrete wall with insulation permanently on both faces.
What ICF actually stands for
Insulated concrete formwork. The insulation is not fitted into the structure afterwards — it is the mould the structure is poured into, and it stays there. That is the whole idea: the formwork never comes off, so the finished wall has continuous insulation inside and out with the structural core between them.
Where the difference shows
Openings
Doors and windows are hung in a poured wall rather than fixed to a panel, with the openings formed in the pour rather than cut afterwards. It is the single clearest difference in how the finished building feels to use — a door in a concrete reveal closes like a door in a house.
Thermal behaviour
A timber frame has studs running through the insulation layer, and every one of them is a small thermal bridge. ICF has continuous insulation on both faces with very little penetrating it, and a dense core that holds temperature rather than following it. In practice that means a small building that stays comfortable rather than one that heats up fast and cools down faster.
Wet trades
The trade-off runs the other way here. A timber frame goes up dry. ICF involves a concrete pour, which is a set-piece day with a pump on the drive. There is one of them, and it is over in an afternoon, but it is real and it needs planning.
Foundations
Ours are ground screws — steel piles wound into the ground with a hydraulic drive head, taking load immediately, with no excavation, no muck away and no slab curing before you can start. Garden rooms typically sit on screw piles or a light concrete pad; a traditionally built annexe usually needs a dug and poured foundation. This is the part we always install ourselves.
Which one is right for the job
If you want a home office, a studio, a gym or a garden bar, a good timber-framed garden room is very likely the right answer. It is lighter, cheaper, quicker, and it can often go up under permitted development because nobody is living in it.
If somebody is going to live in it — a parent, an adult child, a long-term guest — then it is an annexe rather than a garden room. It needs full planning permission and building control sign-off either way, it may be banded for council tax, and it has to perform like a small house for decades rather than like an outbuilding. That is the job ICF is for.
What you build, and what we do
We set out the site, drive the ground screws and level the steel ring beam — the engineering that is difficult to get right and expensive to get wrong. From there the block walls, the pour, the roof frame and deck, the membrane and the windows and doors are yours, or your own builder's. Cladding, electrics, plumbing, heating, kitchen, bathroom and decoration are specified entirely by you.
The blocks key together dry, so the wall goes up without mortar and without specialist trade skills. The ground screws are the one part installed by our approved installer, for safety and warranty reasons.