Extension Air Conditioning Cost UK (2026)
A new single-storey extension is usually the brightest, most glazed room in the house, and by July it can be the one nobody wants to sit in. Bifolds, a roof lantern and a south-facing garden aspect turn it into a greenhouse. The good news: an extension is the easiest room to air condition well, because you can build the system in while the walls are still open. Here is what it costs in 2026, and why the timing of the decision matters as much as the kit.
Quick answer
Air conditioning for a house extension typically costs £1,800–£4,500 supplied and installed in 2026. A small single-storey extension on one wall unit sits at the lower end; an open rear extension with a ceiling cassette, or a wrap-around on two heads, sits at the top, and a large L-shape can pass £5,000. Deciding before the plaster goes on is where the real money is saved.
What extension air conditioning costs
All figures are 2026 supplied-and-installed prices from F-Gas certified engineers, covering the outdoor unit, pipework and commissioning, with VAT.
| Extension and layout | Typical installed cost | Notes |
|---|---|---|
| Small single-storey extension, one wall unit | £1,800–£2,800 | Up to ~20 m²; a single split sized for the glazing, not just the floor area |
| Open rear extension, ceiling cassette designed in | £2,200–£4,000 | Needs a ceiling void; four-way airflow reaches both ends of a long room. See the cassette guide |
| Wrap-around or L-shaped extension, two heads on one outdoor unit | £3,000–£5,000 | Multi-split pricing; a head at each end keeps the whole space even |
Air Con Surrey 2026 baselines for Surrey and the South of England, VAT included. Whole-floor ducted systems (£7,000–£15,000+) are covered in the ducted guide.
The working range for most extensions is £1,800–£4,500. An extension usually costs a little more than a living room of the same size because of the extra glazing and the longer pipe run back to a sensible outdoor-unit position. For how a quote is put together line by line, see the installation cost guide.
Design it in, don't bolt it on
This is the one point that separates an extension from every other room: you get a window, while the build is open, to hide the whole system. Miss it and you pay to put it right later.
If the extension is on the drawing board, get the builder and an air conditioning engineer talking before first fix. A pre-cored pipe route through the new blockwork, a spare fused spur, a planned spot for the condenser and a ceiling void where a cassette can live all cost very little to leave in and a fair bit to add afterwards. Even if you fit the units a year later, roughing in the pipework now is worth doing. Retrofitting a completed room isn't a disaster, plenty of our engineers do it every week, but it usually means surface trunking, wall units rather than a flush cassette, and a bit of touching-up paint.
Glazing is what sets the capacity
Extensions punch above their floor area for cooling load, and the reason is nearly always the glass.
A modern extension is well insulated in the walls and roof, so on paper it looks easy. Then you add three metres of bifolds, a roof lantern and a west-facing garden, and the solar gain on a July afternoon dwarfs anything a bedroom of the same size ever sees. Add in what the room is used for, most extensions become a kitchen-diner or a family living space, and the numbers climb again. An open kitchen extension carries the appliance and cooking heat covered on our kitchen and open-plan page; a family-room extension behaves more like an oversized living room.
The practical upshot is that a good engineer surveys the glazing, aspect and use rather than reaching for a rule of thumb. A typical open rear extension lands around 3.5–5 kW; under-size it and it will cool nicely in May and then lose the battle on the two weeks in summer you actually bought it for. This is the same trap we flag for conservatories and glazed loft conversions.
Layout, the outdoor unit, planning and heating
Wall unit or cassette. A single wall unit is the cheaper, retrofit-friendly choice and suits most single-storey extensions if there is a sensible wall to mount it on and blow along the room. A ceiling cassette gives you clean walls and even four-way airflow, but it needs a void, which is why it is a decision for the build stage. For long or L-shaped wrap-arounds, two heads on a multi-split usually beat one hard-working wall unit; our wall vs ceiling comparison weighs it up.
Siting the condenser. The outdoor unit normally goes on an outside wall of the extension or nearby in the garden. On most homes that is permitted development, provided it sits within the size and siting limits and isn't on a wall facing a highway; conservation areas and listed buildings need consent, so check first. Our planning permission guide has the current rules, and a position away from patio seating and a neighbour's window saves grief later.
Heating in the shoulder months. A glazed extension is often the coldest room from October to April. A modern unit is an air-to-air heat pump with a SCOP of 3.5–4+, so the same system that cools it in July takes the chill off on a March morning far more cheaply than plug-in heaters. It does not qualify for the Boiler Upgrade Scheme, which is for air-to-water systems; the air conditioning vs heat pump guide explains the difference. Running costs are modest, a 2.5 kW unit draws roughly 18–23p an hour at about 26p/kWh, with worked examples on the running costs page.
Sources & methodology
- Installed prices are Air Con Surrey 2026 baselines from engineer quotes across Surrey and the South of England, including VAT; glazing, access, pipe run and brand tier move any individual quote.
- Electricity at ~26p/kWh under the Ofgem cap in force from July 2026 (£1,663/yr typical dual fuel); heat-gain commentary reflects standard cooling-load practice, and a survey replaces rules of thumb.
- Permitted development limits for outdoor units follow the current Town and Country Planning rules for England; installation must be carried out by an F-Gas certified engineer, which is a legal requirement.
FAQs
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