Annexe air conditioning cost in the UK
A self-contained annexe is lived in all year, so the same unit needs to cool in summer and heat in winter. Here's what that costs to install, what drives the price up, and how we match you with F-Gas certified local engineers for free.
The short answer
Most annexes are heated and cooled by a single wall-mounted split system, which runs £1,500–£3,000 installed, with typical jobs landing around £1,800–£2,500. A larger annexe with a separate bedroom and living space usually wants two indoor heads on one outdoor unit — a multi-split — which pushes it to £2,800–£5,000. The unit heats as well as cools, so for an annexe with no gas connection it often replaces electric panel heaters and works out cheaper to run.
What annexe air conditioning costs
Prices are indicative ranges for a supply-and-fit job by a certified installer, including the outdoor unit, indoor head(s), pipework, brackets and commissioning. The final figure depends on how far the indoor and outdoor units sit apart and how tidy the pipe run needs to be.
| Annexe type | Typical system | Installed cost |
|---|---|---|
| Studio annexe (one open room) | Single split, 1 indoor head | £1,500–£2,600 |
| One-bed annexe (living space + bedroom) | Multi-split, 2 indoor heads | £2,800–£4,500 |
| Larger or two-bed annexe | Multi-split, 2–3 heads | £4,000–£6,500 |
| Detached annexe, long pipe run to house | Single or multi-split + extra pipework | add £300–£800 |
Ranges reflect 2026 UK supply-and-fit prices for R32 split systems. Your quote will vary with unit brand, output (kW), access and the length of the refrigerant run. Always get an itemised quote.
The single biggest swing on an annexe job is the distance between the indoor head and the outdoor condenser. In a converted garage or a room over the garage that share a wall with the house, the pipe run is short and the price sits at the bottom of the range. For a detached garden annexe at the end of the plot, the engineer may need a longer, buried or trunked run and sometimes a small amount of electrical work — that's where the extra few hundred pounds comes from.
One unit that heats and cools
Annexes are usually well insulated modern conversions, so they don't need much. A 2.5kW indoor head handles a room up to roughly 25m²; step up to 3.5kW for an open-plan living space or a room with a lot of glazing.
The heating side is what makes air conditioning make sense in an annexe. A modern air-to-air unit is a heat pump running in reverse — it has a SCOP of around 3.5–4, meaning it gives back three to four units of heat for every unit of electricity. If the annexe has no gas supply (most don't), that beats panel heaters or a plug-in oil radiator comfortably. Run the numbers at the October 2026 price cap and a 2.5kW unit costs roughly 18–23p an hour to run flat out, less once it's up to temperature and modulating. Electricity is capped at about 26p per kWh from 1 October, with the cap for a typical home rising to £1,723 a year.
If you're weighing this against the alternative, our running costs guide breaks down the pence-per-hour figures, and the garden room page covers a similar detached-building scenario if your annexe sits away from the house.
Planning, building regs and the legal bits
For most annexes an external condenser is permitted development, but there are catches worth knowing before you commit. The unit generally has to sit at least a metre from the boundary, keep below a noise limit at the neighbour's window, and not go on a wall facing a road. Detached annexes, flats and anything in a conservation area or a listed building are treated differently — check our planning permission guide before you order anything.
Two things aren't optional. Any installer handling refrigerant must be F-Gas certified — it's a legal requirement, not a nice-to-have, and it's the first thing to check on a quote. And if the annexe is genuinely self-contained accommodation, the installation is likely to fall under building regulations, so use an engineer who'll notify the work.
On grants: air-to-air systems don't qualify for the £7,500 Boiler Upgrade Scheme, which is for air-to-water heat pumps only. From 28 April 2026 a narrower £2,500 air-to-air route opened for homes heated only by direct electric — an annexe with just panel heaters might fit, but the rules are tight and need an MCS installer and product, so treat it as a maybe and confirm the current position with Ofgem. Separately, fixed air-to-air installations mostly carry 0% VAT until 31 March 2027 under the government's energy-saving materials relief; portable and cooling-only kit doesn't.
Single split or multi-split?
It comes down to how the annexe is laid out and whether you can live with a unit you can hear in the bedroom.
A studio annexe with one open space only needs a single split — one head, one outdoor unit, lowest cost and simplest to service. Once there's a separate bedroom, you've got a decision. A multi-split runs two indoor heads off one outdoor unit, so each room has its own control and you're not cooling an empty bedroom all evening. It costs more up front but it's the sensible choice for anywhere someone sleeps, because you can turn the living-room head down overnight and keep the bedroom quiet.
Bedroom noise is the thing people underestimate. A good wall unit runs at around 19–24 dB(A) on its lowest fan speed — quieter than a fridge — but a cheap one drones. If the annexe is for an elderly relative or a light sleeper, tell the installer it matters and spend a little more on a quiet model. Our flat and apartment page has more on choosing units where noise and shared walls are a concern.
Annexe air conditioning: common questions
A single split for a one-room annexe is typically £1,500–£2,600 installed. A one-bed annexe with two indoor heads on a multi-split runs £2,800–£4,500, and a larger two-bed setup £4,000–£6,500. A long pipe run from a detached annexe back to the house can add £300–£800.
Yes. Air-to-air units are reversible heat pumps, so the same system that cools in summer heats in winter. With a SCOP of around 3.5–4 it gives back three to four units of heat per unit of electricity, which usually beats electric panel heaters — handy for an annexe with no gas supply.
For most annexes the outdoor unit is permitted development, provided it meets the conditions on boundary distance, noise and position. Detached annexes, flats, conservation areas and listed buildings are treated differently, so check the rules first — our planning permission guide covers the detail.
Generally yes, for heating. A panel heater is 100% efficient; a heat pump is 350–400% efficient, so it delivers the same warmth for roughly a quarter to a third of the electricity. At the October 2026 cap of about 26p per kWh, a 2.5kW unit costs around 18–23p an hour flat out, and less once the room is warm.
Air-to-air systems don't qualify for the £7,500 Boiler Upgrade Scheme, which is air-to-water only. A narrower £2,500 air-to-air route opened on 28 April 2026 for homes heated solely by direct electric, using an MCS installer and product — an annexe on panel heaters might be eligible, but the rules are tight, so confirm the current position with Ofgem. Fixed installations mostly carry 0% VAT until 31 March 2027.
A studio annexe only needs a single split. Once there's a separate bedroom, a multi-split with two heads on one outdoor unit is worth the extra — you get independent control in each room and can keep the bedroom quiet overnight rather than cooling the whole annexe from one head.
A good wall unit runs at around 19–24 dB(A) on its lowest setting, quieter than a fridge. Cheaper units can drone, so if the annexe is for an elderly relative or a light sleeper, ask the installer to quote a low-noise model and mention which room it's going in.
Yes. Anyone handling refrigerant must hold F-Gas certification — it's a legal requirement. We only match you with F-Gas certified local engineers, and self-contained annexe work usually falls under building regulations too, so use someone who'll notify the installation.
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