Cool the rooms you close. One wall split serves one room properly, and a little beyond an open door. Several rooms need several indoor units, either as separate singles or as a multi-split on one outdoor unit. The multi-split is the wrong saving when the pipe length, the height difference or the only lawful outdoor spot cannot carry every room. Survey those limits before you choose. An F-gas certified company handles the refrigerant either way.
Cooling the room whose door stays shut
A single split holds a setpoint in the room where it is mounted. With the door open it takes the edge off the landing. With the door shut, the next room is on its own. If the room you close on a hot evening is one bedroom, price one indoor unit and one outdoor unit. Energy Saving Trust describes the reversible machines as air-to-air heat pumps, so the same pair can heat that room in spring without a second system.
As a 2026 illustration, a straightforward single-room reverse-cycle split in an ordinary house is often quoted in the low thousands of pounds including VAT once the wall, the drain and the electrics are simple. Those figures move when you need a new circuit, a condensate pump, scaffold or making good on a solid wall. Treat any number from a forum as a prompt for a survey. The comparison that matters is two written quotes with model codes for the same rooms.
More than one room, and the outdoor spot that has to serve them
A multi-split puts two or more indoor units on one condenser, each with its own control. It suits a house that has one good outdoor place and pipe runs that stay inside the manufacturer's length and height limits. Bedrooms on the same floor, close to that wall, are the easy version. A second indoor unit on a multi-split is an extra, not a doubling, and a second complete single is usually more than that extra because it brings a second outdoor unit.
If a planner, a freeholder or a conservation area limits you to one elevation, that constraint decides how many rooms the condenser can reach. Ask a written quote for a single-room price and, only if the survey shows the pipes and the height will work, a multi-split price for the extra rooms. Photos of each indoor wall and of every outdoor spot you are allowed to use are what make those two prices about your house.
Frequently asked questions
Will a bigger unit cool the rooms next door?
It cools the room it is in, more aggressively. Closed doors stop that air. Size each shut room on its own after a survey.
Is a multi-split always cheaper than two singles?
Often on outdoor units and on one electrical point, and only when pipe length, height and a single lawful outdoor spot all work. If they do not, the cheaper drawing is the wrong system.
Can I cool one room now and decide the rest later?
Yes. Fit the room you use. A later room can be a second single or, if a multi outdoor was specified for it and the manufacturer allows the later indoor, a second head. Do not assume a spare port without that written.
Is an open-plan kitchen and living room one zone?
Usually yes. Size that whole volume as one zone. A hallway head is not a whole-house system, and it does not hold a setpoint in rooms you close.
What if one room is a long way from the only outdoor spot?
The multi-split fails when one room is a long way from the only spot you are allowed to use, or when the drop from an attic room to a yard exceeds the height limit. Two single splits need two lawful outdoor positions. Forcing every room onto one outdoor unit is how the furthest room never performs.
What if the firm cannot name one lawful outdoor position for all the rooms?
Take the single room now. If the firm cannot name one lawful outdoor position that reaches all of them, the multi-split price is not a comparison you can use.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.