Air conditioning for a garden room or garden office

A garden room is the only room on the property with no heating system and no shade. It went up for a home office, a studio or a gym, cost £12,000 to £30,000, and spends a good part of the year unusable at both ends of the thermometer.

Early September is when owners notice. The room was 31C and abandoned through July. In six weeks it will be too cold to sit in.

Why garden rooms swing further than the house

A brick house with cavity walls and a tiled roof holds its temperature. It takes half a day to warm up and half a night to cool down. A garden room does neither.

The build is a lightweight timber frame with almost no thermal mass. The glazing is usually a full bifold wall pointed at the garden, which in most UK plots means south or west. The roof is flat, dark, and takes sun from mid-morning until early evening. A 4m by 3m room with that spec climbs past 30C by lunchtime in July and sits at 8C on a November morning.

The usual answer is a 2kW oil-filled radiator in winter and a fan in summer. The fan does nothing above 27C. The radiator costs 48p an hour at the 24p per kWh Ofgem cap, or £2.50 a day.

One unit covers both seasons

A split system in heating mode delivers around three units of heat for every unit of electricity it draws. Producing the same 2kW of warmth costs roughly 16p an hour rather than 48p, a third of the price of the radiator it replaces. We ran the full figures in winter warmth from an air conditioner.

Cooling costs 18p to 24p an hour on a hot day for a room this size, set out by room in our cost per hour guide.

There is no wet pipework from the house, no radiator to plumb and no extending the boiler across the garden.

Size it for the glass, not the floor area

The rough rule for a normal room is 1kW of cooling per 8 to 10 square metres. Garden rooms break it, because the heat arrives through the glazing and the flat roof rather than the floor plan.

A 12 square metre garden room with a bifold wall needs 3.5kW where the same floor area in a bedroom would take 2.5kW. Glass on two sides, or use as a gym, pushes it higher again.

Undersizing shows up worse in winter than in summer. Heat output falls as the outdoor temperature drops, so check the data sheet for the heating figure at 2C or -7C rather than the nominal rating. A unit quoted at 3.5kW may give 2.6kW on the coldest morning of the year, exactly when you need it. Our BTU sizing guide sets out the calculation a good installer runs.

The electrics usually decide the job

Most garden rooms are fed by a single armoured cable from the house consumer unit, on a 16A or 20A breaker, feeding a small board inside for lights and sockets.

A 3.5kW split draws around 1.2kW cooling and closer to 2kW when it starts from cold in heating mode. That fits on paper. It stops fitting once the same cable is carrying a heater, a kettle, two monitors and a gym fan. Installers want a dedicated circuit and an isolator beside the outdoor unit, and the existing supply often has no room for it.

If the cable was sized for lighting and a couple of sockets, a heavier armoured run means a trench across the garden. Budget £400 to £900 depending on distance and whether the ground is lawn or paving. Anyone still building the garden room should oversize that cable now, while the trench is open. Our guide to air con and your electrics covers what installers check.

Where the condenser goes

Mounting the outdoor unit on the garden room’s own wall is cheapest and tidiest. The pipe run is one to three metres, straight through the frame, with nothing buried.

Putting the condenser on the house and running refrigerant lines down the garden is possible in ducting, but it adds several hundred pounds and buried joints are a problem nobody wants in five years.

Garden rooms sit close to boundaries, so noise matters. A domestic condenser runs at 45dB to 52dB at one metre. Point it away from the neighbour’s patio and bedroom window, fit anti-vibration mounts and use night mode. Our guide to air con noise has the full numbers.

In heating mode the outdoor unit sheds water. Left to drip onto decking it turns it slippery in autumn and icy in January, so pipe it to a border or a soakaway.

Planning permission

The garden room itself was built under permitted development in most cases. The air conditioning unit is judged separately, under the rules for domestic outdoor plant, which limit unit size, position relative to the boundary, and how many units a property can have.

Conservation areas, listed buildings and flats sit outside those allowances. Check with the council before the installer books a date. Our planning permission guide explains where the lines fall.

What it costs in 2026

A single split in a garden room runs £1,800 to £2,800 fitted, the same bracket as a bedroom or home office on our prices page. Expect the lower end where the condenser hangs on the garden room wall and the pipe run is short, and the upper end for a glazed room needing 5kW or more.

Add the electrical work if the supply will not take it. Most garden room installs are a one-day job.

September is the right month to book it

Installer diaries clear once the heat breaks. A job booked now gets fitted in weeks rather than the months quoted in July, and the unit earns its keep from the first cold week of October rather than sitting idle until next summer. We set out why in autumn air con installs.

Get quotes from installers who have fitted garden rooms before

Tell us on the quote form the rough size of the garden room, how much of it is glazed, and how it is fed from the house. Installers can then quote the unit and any electrical work together rather than surprising you on the day. Three fixed quotes back within 24 hours from vetted North West installers.

Or check the prices page for a rough bracket first, or email us a question if you want to talk it through before requesting quotes.