How Much Does a Garden Room Cost?
The widest honest spread of any job here, caused by two different buildings being sold under one name.
A garden room built to be used through the winter runs about £1,500 to £2,400 per square metre, so a 12 square metre room is roughly £18,000 to £29,000 and a 20 square metre one £30,000 to £48,000. The same footprint is advertised for a third of that, and the difference is the base, the depth and continuity of the insulation, and whether the roof is a warm deck or a cold deck, none of which can be seen once the building is finished. Because a small detached outbuilding can sit outside Building Regulations control, no inspector and no certificate ever separates the two. The electrical supply from the house is priced by distance rather than by the size of the room, and it is notifiable work that must be tested and certified whether or not the building itself is exempt.
£1,500 to £2,400 per square metre for a building specified to be used through the winter, which puts a 12 square metre room at roughly £18,000 to £29,000 and a 20 square metre one at £30,000 to £48,000. The same footprint is advertised elsewhere for a third of that, and the gap is not a discount.
This is the widest genuine spread of any job on this site, and it is not caused by finish, brand or margin. It is caused by two different buildings being sold under one name, and by there being no inspector and no certificate anywhere in the process to tell you which one turned up.
The fork is in the specification, and it is invisible afterwards
A summer building and an all year room photograph identically on the day they are handed over.
At the bottom of the market the studs are shallow and there is nothing over them, the floor is a thin deck with air moving under it, the roof is a cold deck, the base is paving slabs or an unreinforced pad, and the electrical supply is left as your problem. At the top the studs are at least 140mm and fully filled, a continuous insulated overlay runs across the inside face before the plasterboard, the vapour control layer is taped at every joint, the roof is a warm deck and the base is designed for what the ground is doing.
| Element | Bottom of the market | Built for the winter |
|---|---|---|
| Studs | Shallow, with nothing over them | At least 140mm, fully filled |
| Inside face | Nothing | Continuous insulated overlay, vapour control taped at every joint |
| Roof | Cold deck | Warm deck |
| Base | Paving slabs or an unreinforced pad | Designed for what the ground is doing |
| Electrical supply | Left as your problem | Its own circuit, tested and certified |
Both look like a garden room in a photograph. One of them costs a few hundred pounds a month to hold at a usable temperature in February, and the other does not. Because a small detached outbuilding can sit outside Building Regulations control, nobody official ever compares the two.
The three items that carry the difference
Almost the whole of the spread lives in the base, the wall build up and the roof.
The base is a foundation and not a patio. A reinforced ground bearing slab with insulation beneath it, concrete pads with a joisted deck over, or screw piles wound down past the movement zone all cost differently, and which one is right is decided by the ground rather than by the budget. Around Colchester that usually means shrinkable clay and often a mature tree within reach of the building, so what the clay does through a season sets this line item before anyone talks about cladding.
The wall is the cheapest place to save money and the most expensive place to have saved it. Deeper studs, a continuous overlay and a properly taped vapour control layer are a few hundred pounds of material and a couple of days of labour while the frame is open. Once the linings are on, none of it can be added without stripping the building back out.
The roof is a straight choice between a warm deck, with the insulation above the structural deck, and a cold deck with a ventilation path that will eventually block. The warm deck is dearer and it is the one that is still dry in ten years.
Why the rate is worse on a small building
Very little about a garden room scales with its floor area.
Setting out, the base plant and its delivery, the door set, the consumer unit, the supply trench and the days each trade spends getting there are much the same on a 9 square metre building as on an 18. A small room rarely costs half of a large one, and the marginal square metre is usually the cheapest one in the whole project. It is worth pricing two sizes before shrinking the plan to save money.
The supply run is priced by distance, not by the size of the room
This is the item most often missing from a headline figure, and it does not care how big the building is.
A garden room needs its own circuit: armoured cable from the house consumer unit, buried at proper depth with warning tape above it, sized for the load and for the volt drop over the distance, terminating in a small consumer unit inside the building. A duct for network cable goes in the same trench. A 10 square metre room at the far end of a long garden can carry a bigger supply cost than a 20 square metre one sitting near the house.
That work is notifiable under Part P of the Building Regulations where the supply comes from the dwelling, whether or not the building itself is exempt, and it should be tested and certified by a competent electrician who issues an electrical installation certificate. Anyone telling you an exempt building needs no certificate is wrong, and the point at which that gets discovered is usually when the house is sold.
What sits outside the rate
Budget separately for VAT, the supply run and its trench, network cabling, heating equipment, floor finishes, a cloakroom and its foul drainage, the path down the garden, tree work, landscaping afterwards and any application fee. Check the VAT position before comparing anything: a supplier’s advertised price frequently includes it and a builder’s quotation frequently does not, and twenty per cent of this budget is thousands of pounds.
Four questions that make two prices comparable
- What is the stud depth, and is there a continuous insulated overlay across the inside face?
- What is the base, and what was it designed against?
- Is the roof a warm deck or a cold deck?
- Is the electrical supply from the house included, and does the price include the certificate?
Four plain answers, in writing, will separate the two products faster than any amount of looking at photographs. A supplier building the better one will answer all four without hesitating, because those answers are the reason for the price.
Turning the range into a figure
What the room is for changes the specification more than the size does. A gym needs a floor built for dropped weights and far more ventilation than the floor area suggests; a music room needs mass in the walls; an all year office needs the glazing oriented so it is not a greenhouse at two in the afternoon. Say it at the survey and it costs very little. Say it afterwards and it is a rebuild. If the plan is a room to work in, the comparison with building onto the house is worth making before you commit to the garden.
These figures are indicative for the Colchester area at the review date below. Use them to sense check a price rather than as one.
Last reviewed August 2026.
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