If you've ever tried to work in a garden office in July, or quietly abandoned the conservatory by mid-morning, you already know the problem. Glass rooms trap heat brilliantly and shed it badly. The good news is they can be cooled — you just need to go in with the right expectations, because these are genuinely the hardest spaces in any home to keep comfortable.
Why glass rooms are so hard to cool
A normal room gains heat slowly, through walls and a bit of window. A conservatory or heavily glazed garden room gains it instantly: sunlight passes straight through the glass, warms everything inside, and the heat can't easily get back out. It's the greenhouse effect, working exactly as designed — just not in your favour.
Glazing is also a poor insulator compared with a brick wall, and polycarbonate roofs are worse still. So the room heats up faster than anywhere else in the house, and any cooling you put in leaks away faster too. Neither problem is fatal. Both change the maths.
Sizing: glass changes everything
A good installer sizes air conditioning on heat load, not floor area — and glazing pushes the heat load right up. A heavily glazed room can need substantially more cooling capacity than a plastered room the same size, and more again under a glass or polycarbonate roof in full sun.
This matters because an undersized unit in a conservatory doesn't just cool slowly — it runs flat out all afternoon and never quite gets there, which is miserable and expensive. If someone quotes you off floor area alone without asking about the glazing, the roof, or which way the room faces, be wary. A proper survey does the heat-load sums.
Can you run a split to a summer house or garden office?
Usually, yes — and a well-insulated modern garden office is one of the best candidates for a small split system. The question is rarely the unit; it's the power.
A fixed split system needs a proper, dedicated electrical supply. If your garden building is already fed by a buried armoured (SWA) cable from the house with reasonable capacity, you're most of the way there. If it's running off an extension lead or a thin cable clipped along the fence — and plenty are — that needs sorting first. Typically that means a buried armoured cable run from the house consumer unit, often with a small consumer unit in the outbuilding itself, installed and signed off by a qualified electrician. The cost depends heavily on the distance and whether you're happy to dig the trench yourself. Our guide on whether your electrics can handle air conditioning covers the house end of that equation.
One underrated bonus: a modern split is a reversible air-to-air heat pump, so the same unit heats the garden office in winter — usually far more efficiently than the plug-in fan heater most people use out there. For a room you work in year-round, that changes the value calculation considerably. (Note that air-to-air systems sit outside the Boiler Upgrade Scheme, which only covers air-to-water heat pumps — so no grant, but no paperwork either.)
Conservatories: honest expectations
A split system works in a conservatory, and on a genuinely hot day it's the only thing that really does. But be realistic about what "works" means. This will be the highest running-cost room in your house for its size, and on a south-facing afternoon the unit will be working hard against constant solar gain. Expect comfortable, not chilled — especially under a polycarbonate roof.
You can help it enormously with cheap measures: solar-control film or blinds to cut the gain before it arrives, and ventilation on cooler evenings to dump heat for free. Practically, the indoor unit usually mounts on the dwarf wall or the adjoining house wall, and it needs somewhere sensible for the condensate to drain — a decent installer will walk the room and sort this on the survey.
The flip side is the same winter bonus as garden offices: reverse the unit and the conservatory becomes usable in January too, which for many people is the thing that actually justifies the spend.
Your realistic options compared
| Option | Good for | Watch out for |
|---|---|---|
| Fixed split system | The only option that reliably copes with a glass room; heats it in winter too | Needs proper power to an outbuilding; higher running costs in a conservatory |
| Portable unit | Occasional use, renting, or testing the water | Struggles badly against serious solar gain; needs a window for the hose |
| Blinds, solar film, ventilation | Cutting the heat load cheaply — do this regardless | Takes the edge off; won't make a hot conservatory cool on its own |
If you're weighing the first two, our split vs portable guide goes deeper — the short version is that portables lose in exactly the conditions glass rooms create.
What it costs
As a broad guide, a single-room split system usually lands somewhere in the £1,500–£3,000 range fitted, though a glazing-heavy room can push the sizing — and the price — towards the top of that or beyond. Getting power to an outbuilding is on top, and varies too much with cable distance and groundwork to give a useful figure — have it priced on the same survey, along with any house-side wiring work. Running costs depend on your tariff and on how hard the unit has to fight the sun — our running costs guide has the full picture.
One legal point: anyone handling refrigerant must hold F-Gas certification — it is not optional, and it is the first thing to check about any installer.
Getting it done properly
The difference between a glass room that's finally usable and an expensive disappointment is almost entirely in the survey and the sizing. If you'd like that done by someone who's been checked out, tell us about your room and we'll match you with one vetted local installer — one, not a pile of cold calls — for a free, no-obligation quote.