What does a garden room actually cost in 2026?
Real bands, what sits inside them, and the five choices that move the number most.
Start with the honest range
A properly insulated, year-round garden room from a Dorset builder lands somewhere between £18,000 and £45,000 fitted. Below about £15,000 you are almost certainly looking at a summerhouse with a plug socket: fine for storage, cold in February. Above £45,000 you are usually paying for size, glazing or a kitchen and bathroom.
Size is the biggest lever, but not linearly
Doubling the floor area does not double the price. Foundations, delivery, electrics and labour mobilisation are largely fixed, so the cost per square metre falls as the room grows. A 12m² room might work out at £1,700/m² while a 24m² room in the same specification comes in nearer £1,300/m². If you are hesitating between two sizes, the larger one is usually better value.
The five choices that move the number
Glazing. A 4m aluminium slider costs several times a pair of French doors. Cladding. Cedar and thermowood carry a premium over painted redwood. Foundations. Ground screws are usually cheaper and faster than a concrete raft, though a sloping or waterlogged plot changes that. Services. A WC or kitchenette means drainage, which means groundwork. Access. If everything comes through a metre-wide side gate by hand, that adds days.
What should already be included
Insulation to current standards in floor, walls and roof. Certified electrics with a proper consumer unit and enough sockets. Heating. Double glazing. Internal finishes ready to use, not bare board. If a quote is notably cheap, work out which of those five it is missing before you celebrate.
Beware the incomplete quote
The most common source of overspend is not the builder’s greed, it is the quote that stops at the shell. Groundwork, waste removal, electrical certification, decoration and VAT are all routinely excluded. Ask for the total figure delivered and finished, in writing, and ask what would cause it to change.
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