Single split or multi split air conditioning?

The basic difference

A single split is one indoor unit joined to one outdoor unit. A multi split is two or more indoor units joined to one larger outdoor unit, each with its own pipes back to it. Inside the rooms they look the same. The difference is what happens outside, and that matters a great deal in London.

	Single splits	Multi split
Outdoor units	one per room	one for several rooms
Fault on outdoor unit	one room affected	all connected rooms affected
Modes	each room heats or cools independently	all rooms in the same mode
Efficiency	usually best when well matched	can drop with mismatched rooms
Outdoor space	needs room for several boxes	needs one spot
Permitted development	one unit only on most houses	one unit fits the rule
Adding rooms later	add another system	only if spare capacity and ports

Why planning pushes London homes towards multi splits

In England, a heat pump outdoor unit can be permitted development under Class G of the planning rules if it meets the conditions. Since May 2025 those conditions allow:

• one heat pump on a house that is not detached
• two on a detached house
• one on a whole block of flats

Each unit must also be used for heating, not solely for cooling, must pass the 37 dB(A) neighbour noise calculation, and must sit within a size limit of 1.5 cubic metres on a house. The full list is on the Planning Portal.

Most London houses are terraced or semi detached. If you want three bedrooms cooled with single splits, that means three outdoor units, two of which fall outside permitted development and need a planning application. A multi split gets all three rooms onto one outdoor unit that can stay within the rules, which is why it is so common on terraces in Wandsworth, Fulham and Islington.

When single splits are the better choice

Single splits still make sense in several London situations:

• Detached houses, where two outdoor units are allowed and rooms are far apart
• Rooms with very different loads, such as a small bedroom and a big glazed kitchen extension
• Separate uses, such as a home office that needs cooling every day while bedrooms only need it at night
• Resilience, where you would rather one fault did not take out every room
• Phased budgets, where you want one room now and another next year

If planning permission is needed anyway, for example because the units are for cooling only or the house is listed, the one unit rule matters less, and single splits can be chosen on their merits.

Efficiency and sizing

A multi split outdoor unit is sized for the realistic peak load of the house, not the sum of every room at full power, because the sun does not hit every room at once. That is sensible. The risk is mismatch: one small indoor unit running on a large outdoor unit at low demand can make the compressor work outside its sweet spot. Manufacturers publish which combinations of indoor units work with each outdoor unit and how many can be connected. Stick to those combinations.

For reversible systems, look at the seasonal efficiency ratings, SEER for cooling and SCOP for heating, for the actual combination being installed, not just the best case in the brochure.

Pipe runs and disruption

Every indoor unit on a multi split needs its own pair of copper pipes, a cable and a drain route back towards the outdoor unit. In a terrace, that can mean pipes through the loft, down an internal wall and out at the back, or in neat covers on the rear wall. Single splits often have shorter runs because each outdoor unit sits close to its room, but you need wall or roof space for each one.

If you are converting a loft or refurbishing, run the pipes before the plaster and floors go down. It is cheaper and neater than retrofitting.

Cost

A multi split usually costs more than one single split but less than several separate systems, because there is one outdoor unit and one electrical supply. The Energy Saving Trust gives a typical figure of around £3,700 for a three bedroom semi detached house with an air to air system (Energy Saving Trust). The real price depends on the number of rooms, pipe lengths, how the pipes are hidden and access for the outdoor unit.

A worked example

Take a two storey Victorian terrace in Earlsfield with a loft conversion. The owners want the loft bedroom, the main bedroom and a back bedroom used as an office cooled, and they would like some heating in spring and autumn. There is a flat roof over the kitchen extension.

With single splits, they would need three outdoor units, and only one could be permitted development. With a multi split, one outdoor unit on the extension roof, set back 1 metre from the edge and behind the parapet, serves all three rooms. The loft pipes come down through a cupboard, the other two run along the rear wall. All three rooms heat or cool together, which suits a family house. That is the typical answer on our Wandsworth area page.

For more on running a whole house this way, read our guide to whole house air conditioning with a multi split, or see the multi split service page.