A shipping container garden room starts with a structure that is already weatherproof, already rigid and cheap to buy, which is a real advantage. What it also is, is a steel box with a very high thermal conductivity, a fixed set of dimensions, and a delivery problem. The conversions that work take those three seriously, and the ones that do not end up cold, wet inside and stuck on a driveway.
Insulate on the outside if you possibly can
Steel is an excellent conductor and every point where it bridges from inside to outside is a cold spot where moisture condenses. Insulating internally eats the limited width and leaves the steel cold behind the insulation, which is exactly where you do not want condensation. External insulation with a cladding over it keeps the steel warm and dry, and it is why the good conversions do not look like containers.
Condensation is the failure mode
A sealed steel box with people in it produces water that has nowhere to go, and it appears on the coldest surface, which is the underside of the roof. Controlled ventilation, a properly continuous vapour control layer on the warm side of the insulation, and no bare steel exposed inside are what prevent it. This is the part of a container conversion that is most often done badly.
Delivery is a crane conversation
A container arrives on a lorry and is lifted into place, which needs a vehicle that can reach the property, room to set up, and clear space above the garden. Overhead cables, mature trees, a narrow lane or a house between the road and the garden can each make the delivery the most expensive line in the project or rule it out entirely.
Dimensions are fixed and narrower than you think
Internal width in a standard container is under two and a half metres before any insulation goes on, and internal height is limited unless you buy a high cube. Once an insulated build up is added to walls, floor and ceiling, the usable space is noticeably smaller. Draw the furniture into the real internal dimensions before committing.
Questions people ask about shipping container garden room
Is a container garden room cheaper?
The shell is cheap and the conversion is not. Once external insulation, cladding, proper glazing, lining, electrics and the crane are added, a good container conversion is often close to a good panel building, and the case for it is durability and security rather than price.
Does it need planning permission?
The same outbuilding tests apply, and a container is judged on height, position and coverage like anything else. In practice the visual impact of an unclad container makes a planning objection more likely, which is another argument for cladding it.
Will it rust?
Corten weathering steel is designed to form a stable surface layer, and the places that corrode are the cut edges, the welds and anywhere water sits. Sealing penetrations properly and keeping the base off the ground with drainage around it are what determines how long it lasts.