Cooling a garden room or garden office in Europe
Contents (7)
The garden building has quietly become one of Europe’s most common home extensions. A timber cabin at the end of the lawn — a garden office in Britain and Ireland, a Gartenhaus or Bürohaus in Germany, a tuinkamer in the Netherlands, a studio de jardin in France — buys a spare room without touching the house. It is delivered insulated, wired, painted, and usually with a wall of glazed doors facing the garden, because that is what sells it.
Then July arrives. By two in the afternoon the room that was pleasant in April is at 32 °C and hotter than any room in the house it was meant to relieve. That is not a defect in the building: it is what a small, lightweight, heavily glazed box does in the sun, and it means a garden room needs cooling sized for what it is rather than for its floor area.
Why a garden room overheats faster than the house
Four features stack up, and every one of them is a consequence of the product being small and cheap to deliver.
- The glazing ratio is extreme. A house living room typically has glass equal to 15–20% of its floor area. A 12 m² garden room with 3 m of sliding doors carries 4–5 m² of glass — closer to 40% of the floor. On a clear afternoon each square metre of unshaded glass admits several hundred watts, so the doors alone can deliver more heat than a small electric fire.
- There is almost no thermal mass. Timber frame, insulated panels, a floating floor: nothing to absorb the peak and release it overnight. The cabin responds within minutes in both directions — pleasant in the morning, brutal at four o’clock.
- The roof is flat, dark and close to your head. A near-flat EPDM or felt membrane in full sun runs far hotter than a pitched tiled roof, above an insulated void a few centimetres deep rather than a whole loft — the mechanism that makes top-floor rooms the worst in any building, described in our guide to cooling a room under the roof.
- The internal gains are concentrated. One person, a laptop, two monitors and a router is roughly 250–400 W in 10–15 m². Barely noticeable in a 40 m² living room; here it is a permanent heater under the desk.
The arithmetic below is indicative — orientation, shading and construction change it a great deal — but it shows where the load actually comes from in a 12 m² west-facing cabin on a clear summer afternoon.
| Source | Indicative peak gain | What reduces it |
|---|---|---|
| 4–5 m² of unshaded glazed doors, west or south-west | 800–1,400 W | External blind, awning or pergola — the single biggest lever |
| Roof and walls | 150–400 W | Insulation depth, a light-coloured membrane, an overhang |
| Occupant, laptop, monitors, printer | 250–400 W | Fewer machines, a laptop instead of a desktop |
Totalled, that is roughly 1.3–2.2 kW in 12 m² — of the order of 100–190 W per square metre, against 50–80 W/m² for a decently shaded room inside a masonry house. It is why garden rooms defeat rule-of-thumb tables: by floor area a 12 m² room looks like a 1.5 kW job, and in reality it needs 2.0–2.6 kW. Do the calculation with our guide to sizing an air conditioner in kW, counting the glass rather than the floor.
Shade the glass before buying anything
Every watt intercepted outside the glazing is a watt you never pay to remove, and here the intervention is easy: no façade committee, no scaffold, doors at ground level. An external blind or awning over the glazed doors, a pergola or deciduous tree in front of the west elevation and a light rather than black roof membrane will typically remove a third to a half of the peak load — which changes which machine you need, not merely its running cost. The full sequence is in our guide to cooling a European home without air conditioning.
Which machine actually suits a cabin
| Option | Typical capacity | Verdict for a garden room |
|---|---|---|
| Reversible wall split, outdoor unit on the cabin’s own wall | 2.0–3.5 kW | The default answer. Efficient, quiet indoors, and it heats the building all winter — see our split air conditioner page |
| Fixed monobloc through the wall, no outdoor unit | 2.0–2.6 kW | Useful where an outdoor unit would sit on a shared boundary; noisier inside, two grilles in the cladding, covered in our wall monobloc guide |
| Portable unit with a window hose | 2.0–2.9 kW nominal | The worst fit in an airtight cabin: the hose must leave through a door or a cut panel, and the unit depressurises the room, pulling hot air back through every gap |
| Fan only | — | Two or three degrees of perceived comfort for tens of watts. Enough in a well-shaded cabin in a northern climate, nowhere near enough behind unshaded west glass |
One point decides most of these projects: a garden office is used in February as well as August, and a reversible split is the cheapest heat the building will ever have — 2 kW of warmth at a seasonal COP of 3.5–4.5 draws roughly a quarter of the electricity of the 2 kW fan heater most owners start with. Cooling is what people buy it for; heating is what pays for it.
The electricity supply is the real project
Most garden rooms arrive with lighting and a couple of sockets fed by whatever the supplier could justify. Adding a compressor changes the calculation, and the supply — not the machine — is where the budget and the delay usually sit.
- It needs its own circuit from the house, not an extension lead. A cable trailing across a lawn is a trip and damage hazard, has no proper protection at the far end, and will not satisfy an insurer after a fire.
- Buried armoured cable, sized for the run rather than the current. Over 20–40 m of garden the constraint is voltage drop, so an electrician commonly ends up at 4 mm² or 6 mm² where the current alone would suggest 2.5 mm². Burial depth, warning tape and mechanical protection follow national wiring practice.
- A small consumer unit in the cabin, with RCD protection and separate ways for the air conditioner, the sockets and the lighting, so a tripping kettle does not take the office down.
- The earthing arrangement is a design decision: whether the house earth is exported to the outbuilding or a local electrode is installed depends on the supply type, and it is a question for a qualified electrician.
- It is often notifiable work. In England and Wales a new circuit falls under Part P of the Building Regulations and must be certified; most European countries run an equivalent regime. Keep the paperwork — it is what proves the installation at resale.
Add the loads before choosing the cable: a 2.5 kW class split draws roughly 0.5–1.1 kW in normal running, and a kettle adds 2 kW on top of monitors and a printer. The wider principles are in our guide to the electrical side of an air conditioner.
Permission: two separate questions
The cabin and the air conditioner are consented separately. In England, an outbuilding is normally permitted development within limits worth knowing before you specify a roof: 4 m maximum overall height with a dual-pitched roof or 3 m otherwise, dropping to 2.5 m anywhere within 2 m of a boundary, eaves no higher than 2.5 m, no more than half the land around the original house covered, and no use as separate self-contained living accommodation. Building regulations then generally do not apply below 15 m² of floor area, nor between 15 m² and 30 m², provided there is no sleeping accommodation and the building is at least 1 m from a boundary or built substantially of non-combustible material. Elsewhere the thresholds differ and the logic repeats: in France a garden structure of 5 m² or less needs no formality, 5–20 m² needs a déclaration préalable and anything larger a permis de construire; in Germany the exemption is set by each Land, usually by enclosed volume; in Ireland and the Netherlands the rear-garden shed is exempt within stated area and height limits. Conservation areas and listed buildings override all of it.
The air conditioner is the second conversation, and the geometry of a garden building puts noise at its centre. The outdoor unit ends up at the far end of the plot — a few metres from a neighbour’s boundary and often facing their bedroom window rather than yours, the reverse of a normal house installation. Night-time limits are what bite, so favour the elevation facing your own house, keep the unit off a party fence that will transmit vibration, check the declared sound power before ordering, and give the condensate a soakaway rather than a puddle on the boundary. Our guide to AC noise rules in Europe sets out the levels that apply.
What it costs to run, and the winter dividend
A 2.5 kW class unit holding a shaded cabin at 25 °C draws in the region of 0.4–1.0 kW once the room is down to temperature — roughly 3–6 kWh over a working day, so under a euro to about two euros at prevailing European tariffs. Country prices are in our guide to what air conditioning costs to run in Europe. The winter case is stronger still, and it carries a maintenance argument: an unheated timber cabin closed from November to March is a condensation and mould risk with equipment and paper inside, and leaving a reversible unit on a 12–15 °C setpoint costs little, keeps the fabric dry and means the room is usable in minutes rather than an hour.
Frequently asked questions
What size air conditioner does a garden office need?
Count the glass, not the floor. A well-shaded 10–12 m² cabin in a northern European climate can be comfortable on a 2.0 kW unit; the same cabin with unshaded west-facing sliding doors realistically needs 2.5–3.5 kW. Sizing by floor area alone is what produces the machine that runs flat out at four in the afternoon and never quite catches up.
Can I use a portable air conditioner instead?
It works badly here. The hose has to exit somewhere, and a cabin with one door and one small window usually means a propped door or an improvised panel. Worse, a single-hose portable exhausts indoor air outdoors, so the room sits at negative pressure and draws hot garden air back through every gap in a lightweight building. If it is the only option, seal the opening properly — see our guide to venting a portable air conditioner.
Do I need planning permission to fit an air conditioner on a garden room?
Usually less than for the same unit on a house, since the outbuilding is already consented and the unit is not on a shared façade — but the answer is national and municipal, and heritage zones are the exception everywhere. Noise near a boundary is what generates real disputes. The country-by-country picture is in our guide to permission to install air conditioning in Europe.
Can I run it from a socket in the house?
No. A garden room with an air conditioner needs a dedicated circuit in buried armoured cable, protected and certified, with a small consumer unit at the cabin end. Extension leads across a garden fail on voltage drop, protection and safety at once, and in England and Wales the new circuit is notifiable work.
Sources
Read next
- The room under the roof: cooling a loft conversion in Europe
- Wall, floor console, cassette or ducted: choosing the indoor unit
- Cooling with heat pumps: Europe’s accidental air conditioning
- Heat and health in Europe: how cool a home needs to be, and for whom
- Cooling with an air-to-water heat pump: underfloor loops, fan coils and the dew point