Heating a garden office with air conditioning
A garden office is one room, used in working hours, too cold in January and too hot in July. That happens to be the job a wall mounted heat pump is best at. Here is the arithmetic, and the three things to sort out before anybody fits one.
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The short answer
For an insulated garden office that is used most working days, a wall mounted heat pump is usually the cheapest way to heat it, and the only common option that cools it in summer as well. An electric heater turns each unit of electricity into one unit of heat, so at the current price cap a kWh of heat costs 26.32p. A heat pump with a published seasonal efficiency of 4.61 delivers the same kWh for about 5.7p. Planning is usually not an issue: the permitted development rules for air source heat pumps cover one fitted on a building in the garden of a house, with the same limits as one on the house. The one to watch is the count: on a semi or a terrace only one outdoor unit across the house and garden is permitted development, so if the house already has one, the garden office unit needs planning permission. What decides whether it works is the building itself: it needs to be properly insulated, and it needs an electrical supply big enough to run it.
What a kWh of heat costs
The comparison that matters in a garden office is against an electric heater, because it is the simplest thing to plug in out there, and so it is the one to beat.
| Heating | Efficiency | Cost per kWh of heat |
|---|---|---|
| Electric panel, fan or oil filled heater | 1.0, always | 26.3p |
| Daikin Comfora 2.5kW heat pump | 4.61 seasonal (SCOP) | 5.7p |
| Daikin Stylish 2.5kW heat pump | 5.15 seasonal (SCOP) | 5.1p |
At the Ofgem price cap of 26.32p per kWh for 1 October to 31 December 2026. Heat pump figures are Daikin's published seasonal efficiencies for average climate. Estimates, not quotes: what you pay depends on your tariff and how the building holds heat.
Why a garden office suits it so well
Most of the honest limits of heating with air conditioning do not apply out here. It only heats the room it is in, and a garden office is one room. It does no hot water, and a garden office needs none. It is not a replacement for a whole house heating system, and nobody is asking it to be.
It also warms a room in minutes rather than the best part of an hour, because it blows heated air straight into the space. In a building you walk into at nine and leave at six, that matters more than the efficiency figure. You heat it when you are in it, not all day on the off chance.
And then there is July. A timber building with a lot of glass gets very hot in a London summer, and the same unit on the wall cools it. An electric heater does nothing about that.
Sort these out first
A heat pump is the right answer to a well built garden office. It is not a fix for a badly built one, and these are the things that decide which you have.
Insulation and glazing
Heat leaves a thin walled building fast, and no heating system, however efficient, keeps up with a shed. If the walls, floor and roof are insulated and the glazing is at least double, a heat pump has an easy job. If they are not, insulate first, because it is the cheaper half of the problem.
The power supply
The unit needs a dedicated electrical supply, and the cable out to the garden has to be big enough for the unit and everything else that runs out there. Running a complete new circuit is notifiable electrical work under Part P of the Building Regulations in England, so it is done by a registered electrician or signed off by building control. We check what is already there at the survey.
Where the outdoor unit goes
The outdoor unit can go on the garden building or on the ground beside it. The permitted development rules for air source heat pumps cover one fitted on a building within the garden of a house, so the limits are the same as for one on the house itself: a maximum size, a limit on how many, and the MCS planning standard. The count covers the house and the garden together: two on a detached house, one on any other, so a unit already on the house uses up a semi's allowance. Our planning guide sets them out.
Where the water goes
In heating mode the outdoor unit makes water as it defrosts, and in cooling mode the indoor unit does. Both need somewhere to drain, which in a garden is usually straightforward but is worth planning so it does not end up on a path that freezes.
Sizing it for winter, not summer
Most sizing advice is written for cooling. A garden building loses heat through all six faces, including a floor that sits over cold air, so in winter it can need more heat than its floor area suggests. We size these on the heating load and check the cooling fits, rather than the other way round.
The reassurance is in Daikin's published figures. Its 2.5kW Comfora and Stylish units still deliver around 2kW of heat at minus ten degrees outside, which is far colder than a normal London winter night. A small, properly insulated garden office does not need a large unit.
Leave it ticking over
A garden building that is allowed to go fully cold overnight can suffer condensation on cold surfaces when it is warmed up again. Setting the unit to hold a low background temperature rather than switching it off completely is cheap with a heat pump, and kinder to the building and to anything electronic left inside it.
The ranges this applies to
Every price is fully installed and includes the unit, the installation, the electrical work and VAT.
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