Choosing
9 min readWhat size air conditioning unit do I need?
2.5kW under 12m², 3.5kW to 20m², 5kW above — then adjust for conservatories, lofts and kitchens. Why going bigger is not the safe option, and how oversizing can now cost you permitted development.
The short answer
For a typical British room, start at 2.5kW under 12m², 3.5kW between 12 and 20m², and 5kW above 20m² — then go up one size if the room is a conservatory, a loft room or a kitchen, because glazing, roof gain and appliances add load that floor area doesn’t capture. Do not go up a size “to be safe”. An oversized unit dehumidifies badly, costs more, and in England and Wales since 2026 can push the outdoor unit over the 37 dB(A) noise limit that your permitted development rights now depend on.
Start with floor area
Floor area gets you most of the way for an ordinary room — a bedroom, a study, a living room with a normal amount of glass. This is the base rule, and it is the same arithmetic our quote flow runs.
| Room | Floor area | Capacity | Sold as |
|---|---|---|---|
| Single bedroom, study, small office | Under 12m² | 2.5kW | 9,000 BTU |
| Double bedroom, small living room | 12–20m² | 3.5kW | 12,000 BTU |
| Large living room, open plan | Over 20m² | 5kW | 18,000 BTU |
| Very large open plan, or a knock-through | Over 35m² | 7.1kW | 24,000 BTU |
Then adjust for what the room actually is
Three kinds of room consistently need more than their floor area suggests, and they are the three that generate the most disappointed customers when someone sizes off area alone. Each moves you one step up the ladder.
| Room type | Adjustment | Why |
|---|---|---|
| Conservatory | One size up | Glazing gain. A 15m² conservatory behaves like a 25m² lounge on a July afternoon, and orientation swings it further. |
| Loft room | One size up | Roof gain, usually thinner insulation, and the hottest room in most houses — which is why it's the room people ring about. |
| Kitchen | One size up | Appliance heat. An oven and a fridge are a continuous load that no floor-area rule sees. |
| Upper floor, otherwise ordinary | No change to capacity | It doesn't change the load, but it lengthens the pipe run and may need loft routing — a cost question, not a sizing one. |
So a 15m² conservatory is not a 3.5kW job, it's a 5kW one. A 22m² loft conversion isn't 5kW, it's 7.1kW. These two cases are where sizing by area alone fails most often, and both are common enough that we build the adjustment into the quote rather than leaving it to be discovered at survey.
What size would your room need?
The same calculation our quote flow runs — three questions, and it shows you why the answer is what it is.
Floor area, roughly.
for a medium bedroom on the ground floor
Nothing about this room pushes it off the base figure for its floor area — no glazing gain, no roof gain, no appliance load worth a size step.
Indicative, not a survey. A real load calculation accounts for glazing orientation, insulation and air changes — that happens before we fit anything, and it is included in the price.
Get the price for thisWhat oversizing actually costs you
This is the part the category gets wrong, so it's worth being specific rather than just asserting it. Four things go wrong when a unit is too big for its room, and they get progressively more expensive.
Correctly sized
Reaches temperature and then modulates — running slowly and continuously, which is its quietest and most efficient state.
Oversized
Overshoots, stops, drifts, restarts. It dehumidifies badly, wears faster and is louder in the moments it runs.
Room temperature over time. The unit that never settles is the one that was bought to be safe.
It cools without drying
A cooling coil removes moisture from the air only while it's actually running. An oversized unit hits the target temperature quickly and then throttles right down, so the coil spends much less time wet. The room reads 21°C on the thermostat and still feels clammy — the classic complaint about an oversized system, and one people almost never diagnose correctly, because the number on the display says everything is fine.
It cycles instead of modulating
Inverter units are often described as infinitely variable. They aren't — each has a minimum output, typically somewhere around a third of its rated capacity. Below that floor it can't modulate any lower, so it stops and restarts instead. Every restart is a surge of consumption and a small burst of noise, and the temperature drifts either side of the setpoint rather than sitting on it.
It costs more to buy and more to run
Both, and in that order of significance. The capital difference between a 3.5kW and a 5kW install is real money, and a unit spending its life below its minimum modulation point runs at worse efficiency than a correctly sized one loafing along at 60%. You pay twice for the headroom, and get a less comfortable room in exchange.
And now: it can cost you permitted development
This is the new one, and it's the reason the old advice has to be retired rather than merely qualified. Permitted development in England and Wales now hangs on a single number: 37 dB(A) at the neighbour's habitable-room doors and windows, calculated under MCS 020(a). Sound pressure falls by roughly 6 dB every time you double the distance from the unit, which means the margin you're working with is often only a few decibels. A larger outdoor unit spends those decibels on capacity you didn't need.
What undersizing costs you
Less subtle, and easier to spot. An undersized unit runs continuously on the hottest afternoons and still doesn't reach the temperature you asked for. It's noisier than it would otherwise be, because it never drops off maximum. And running flat out is the least efficient way a heat pump can operate, so the electricity you save by buying the smaller unit comes straight back.
The failure mode is worse than oversizing in one specific way: it only shows up on the few days a year that you bought the thing for. A unit that is fine in June and useless in a July heatwave is a unit that failed at precisely the moment it was supposed to earn its money.
- Oversized by one step
- Clammy, cyclingPlus the noise margin you may need for planning
- Undersized by one step
- Never quite gets thereOn the handful of days it most needs to
- Correctly sized
- Runs at 50–70%Quietest, driest, most efficient, longest-lived
Heating and cooling are two different loads
Almost every sizing guide in this category sizes for cooling and stops, which made sense when these were summer appliances. If you're going to use it as a heater through the shoulder months — and that's where the running-cost argument actually lives — then the heating load matters too, and it isn't the same number.
In practice a unit sized correctly for cooling a British room will comfortably heat it for most of the season, because our cooling design condition is demanding relative to our mild winters. The exception is the same set of rooms as before: a conservatory sized for July will be working hard in January, when all that glazing runs the other way. If winter use in a conservatory is the point of the purchase, say so at survey — it can change the answer.
Worth reading alongside this: what it actually costs to run sets out why the heating months are the ones that justify the purchase, and an undersized unit run flat out is exactly where those figures stop holding.
What a survey does that this page can’t
Everything above is a rule of thumb, and it's a good one — it will get the right answer for most ordinary rooms. It is not a load calculation, and we'd rather say so than let a table on a website imply a precision it doesn't have.
- Orientation and glazing area — a south-facing wall of glass and a north-facing window are not the same room
- What the walls are actually made of, and what's in the loft above
- Where the outdoor unit can physically go, and how far that is from your neighbour's windows
- The MCS 020(a) calculation itself, which is the thing that either does or doesn't give you permitted development
- Whether the room next door should really be on the same system
The last two are the ones that change answers. Sizing and siting are a single problem in England now: the capacity you choose sets the sound power, and where it can stand sets how much of that reaches next door. We run that calculation for every install and give you the result in writing, because it is the evidence your permitted development rests on — not a formality.
If you want the number for your own room, the quote flow runs exactly the arithmetic on this page and shows its working, including which adjustment it applied and why. It takes about a minute and doesn’t ask for an email address. If you already know the size you need, the prices are published and you can buy without speaking to anyone.
Where this comes from
We’d rather you checked. These are the primary sources behind the claims above, in the order they matter.
- MCS 020(a) — Planning Standards for Permitted DevelopmentSets the 37 dB(A) LAeq assessment at the neighbour's habitable-room openings. The sole accepted compliance route since 28 May 2026, and the reason capacity and planning are now the same question.
- The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2025In force 29 May 2025. Raised the volume limit to 1.5m³ and removed the 1m boundary condition — which is what left noise as the binding constraint.
- CIBSE Guide A — Environmental DesignThe standard UK reference for internal heat gains, glazing gain and design conditions. The basis for treating conservatories, lofts and kitchens differently from ordinary rooms.
- Manufacturer declared performance — current Daikin, Mitsubishi and Toshiba domestic rangesRated capacity, minimum modulation and declared sound pressure. The dB figures quoted here are from the same three units listed on our shop page, so they can be checked against it.
Not yet independently reviewed. This guide was written from the primary sources listed above and is accurate to the best of our understanding on the date shown. It hasn’t been checked by an independent compliance professional, and it isn’t legal advice. Where a decision turns on it, verify with your local authority or with Ofgem directly — and tell us if we’ve got something wrong.