Air condition the room that overheats while you are actually in it. That is often the main bedroom at night, or a loft office in the afternoon, not the largest room on the floor plan. The first job should also be the room with a workable wall, a drain and one lawful outdoor spot. Survey that room before you spend money on a multi-split for the others. An F-gas certified company fits the pair once the room is chosen.
Where you are when the heat bothers you
A south-facing living room can be the hottest space at 4pm and empty by 9pm, while the bedroom is where you lose sleep. If nights are the complaint, the bedroom is first. If you work at home all day in a roof room, that room is first even if it is small. A big kitchen you only use to cook is a later project. Write down the hours. Floor area is how you size the unit after you have chosen the room, not how you choose the room.
The easy ground-floor route and the outdoor unit for that room
A ground-floor room beside a side passage is the cheap install, and it is the wrong first room if you never sit there. Use the easy route when it belongs to the room you care about. If the bedroom is on the second floor and the only condenser position is a long way off, check the pipe length and height before you fall back on the dining room out of convenience. Glazing, a lantern and a room in the roof raise the duty. The survey measures that. Do not skip a hot loft office because a wall split looks simpler in the spare room. Do skip a room where consent, a lease or a missing drain means the unit cannot be fitted this year.
Place the outdoor unit for the room you are doing, with enough airflow and a noise result you can live with. In England, Planning Portal conditions are the check. Leaving a huge multi-split outdoor unit in that spot so the other rooms can join is only sensible when those rooms have already passed the pipe test. If two rooms suffer at the same time they may share a multi-split, but only if pipe length, height and one lawful outdoor spot all work. If one of two equally bad rooms has no lawful outdoor route, do the one that does. If both routes work, compare one multi-split against two singles using pipe length and height, not habit.
As a 2026 illustration, the first straightforward room is often in the low thousands of pounds including VAT. Choosing the wrong room does not save that money. It spends it where you are not sitting. It can be relocated later at a cost, and the refrigerant circuit has to be redone by an F-gas certified company. Better to put the first unit in the room you will still care about in five years.
A reverse-cycle unit in the room the radiators miss
A reverse-cycle unit, as Energy Saving Trust describes air-to-air heat pumps, also gives you heat in a room the radiators never quite reach. Rank the rooms by when you suffer, not by size. Name the first room clearly, with a photo of its wall, the outdoor elevation you can use, and the consumer unit. Mention the second room only as a later question. Ask for a surveyed single split for the first room, with model codes, pipe length and drain. Invite a multi-split price only if you already know the other rooms can reach the same lawful outdoor spot.
Frequently asked questions
Should I start with the lounge because it is the biggest?
Only if that is where you are when the house is too hot. Floor area is how you size the unit after you have chosen the room, not how you choose the room.
What if two rooms are equally bad?
Survey both. If one has no lawful outdoor route, do the one that does. If both routes work, compare one multi-split against two singles using pipe length and height, not habit.
Can the first unit be moved later?
It can be relocated at a cost, and the refrigerant circuit has to be redone by an F-gas certified company. Better to put the first unit in the room you will still care about in five years.
Do two rooms that suffer at different times have to be bought together?
No. Two rooms that suffer at different times still need two indoor units if both doors shut. They do not need to be bought on day one. Do the second room only after the first is a known success.
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.