There are five arrangements a British home realistically uses, and the choice between them is made by the building rather than by preference. What decides it is where the indoor unit can go, whether an outdoor unit is permitted, and whether there is a void to hide anything in. Working through those three questions narrows five options to one or two before anybody quotes.
High wall head
The default. It mounts near the ceiling, throws air along it and lets it fall, drains by gravity to an outside wall and is easy to service. It suits almost any room with a suitable high mounting position, and its drawback is that it is visible. Most domestic systems in Britain are this, and for good reasons.
Ceiling cassette
The indoor unit sits within a ceiling void with only its face showing, throwing air in two or four directions. It gives even distribution in a square room and needs less depth than full ductwork. It requires a void and an access route, so it suits a suspended ceiling, a flat roof void or a first floor with a loft above.
Ducted
The unit lives entirely out of sight and only grilles appear in the room. It is the neatest result and the most demanding installation, needing a void, duct routes, insulation, a drain and service access. Best specified before ceilings go up, and best zoned so rooms can be treated separately.
Low level console and monoblock
A console sits under a window like a radiator and suits full height glazing or sloping ceilings where nothing can be mounted high. A monoblock puts the whole circuit in one wall mounted case ducted through the wall, with no outdoor unit at all, which is the answer where a lease, a listing or a conservation area rules a condenser out.
Questions people ask about types of air conditioning
Which type is cheapest to install?
A single high wall split, in a room on an outside wall with the condenser close by and a spare way in the consumer unit. Everything else is that job plus something: a void, a bulkhead, longer duct, joinery, or access equipment.
Which type is quietest in the room?
Ducted, because the machine is not in the room at all, provided the ducts are sized properly. A badly ducted system can be noisier than a good wall head, because undersized duct makes the fan work and the air whistle.
Can different types share one outdoor unit?
Usually yes, within the manufacturer's compatibility rules. A multi split can commonly run a wall head in one room and a cassette or ducted unit in another, which is how a house ends up with visible units where it does not matter and hidden ones where it does.
What about evaporative or portable coolers?
Neither is a fitted system and neither is covered here. A portable vents through a window and pulls warm air back in around the seal, and an evaporative cooler adds moisture to the air, which works poorly in a humid British summer. This section is about systems an engineer installs.