Treat the granny annexe as its own small building, not as a spare room on the house system. The decision is whether it has a separate electrical supply, a condensate route, and an outdoor wall that the annexe occupant is allowed to use. Planning for the annexe and for the outdoor unit are both in play. Survey the annexe before you hang a head off the main house multi-split. An F-gas certified company commissions the refrigerant. A new indoor unit on the house outdoor unit is usually not a mix-and-match.
Who owns the annexe wall and the board
The outdoor bracket needs an owner. If you own the house and the annexe, you can choose a wall on the annexe or a nearby house wall, subject to noise and planning. If a relative only occupies the annexe, or you rent it, the wall may not be yours to drill. A party wall with next door is usually the wrong place. Power is separate in the same way. Many annexes have their own consumer unit, fed by a submain from the house. The split wants a dedicated 230 V circuit from that board, not a spur off the annexe ring and not a trailing lead back to the kitchen. If the annexe board is a tiny garage unit with no spare way, the electrical job comes before the cooling. Name who owns the outdoor wall, whether the annexe has its own consumer unit, and how far it sits from the house. Photos of the annexe wall, the proposed outdoor spot, the annexe board and the nearest drain make that ownership visible.
A gully the annexe can reach, and a pipe that may not
An annexe allowed as ancillary to the house can carry conditions about who lives there. The outdoor unit has its own test. In England, check Planning Portal conditions for air source heat pumps: noise, size, and use. A listed house or a conservation setting can take the outdoor spot off permitted development entirely. The quiet problem is the main bedroom in the house, a metre from the annexe condenser. Put the outdoor unit where it serves the annexe without humming through the wall someone sleeps against. One lawful spot has to exist. If the only spot fails the noise or the boundary test, do not force a multi-split from the main house across the garden.
Annexes often lack a soil pipe on the wall you want. Condensate must reach a gully you own, or a pumped line to one, not the neighbour's path and not a flower bed that floods the timber base. A ground-floor annexe can usually fall by gravity. A room over a garage annexe may need a pump down to the gully. Running refrigerant pipes back to an outdoor unit on the main house is a length and height question. If the run exceeds the manufacturer limit, the annexe gets its own pair. As a 2026 illustration, a straightforward annexe single split is often in the low thousands of pounds including VAT. The submain, the pump or a planning drawing is what moves it. Ask for a single split served from the annexe, with model codes, condensate route and a note on Planning Portal conditions. Mention any planning condition on the annexe itself. Do not assume the house multi-split has a spare port.
Frequently asked questions
Can the annexe share the house air conditioning?
Only if the pipe length, the height and a listed combination all work, and you are allowed to use that outdoor unit. Most annexes are a better fit with their own pair and their own supply.
Who is responsible for the outdoor unit if family live in the annexe?
The owner of the wall and the person named on the electrical supply. Agree that before installation, especially if the occupant does not own the building.
Does an annexe outdoor unit need planning permission?
Check the Planning Portal conditions in England, and any condition already attached to the annexe. Flats, listed buildings and some conservation settings need consent even when a simple house wall might not.
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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.