A maisonette is a flat on two floors, usually with its own front door. Cooling one floor does not cool the other once doors are shut, so think about an indoor upstairs and one downstairs, often on a single outdoor unit. The lease still governs the condenser, the pipework between floors and any communal wall. Treat planning like a flat until someone shows you it is a house. Noise through the party floor to the neighbour below or above is part of the design.
A hot upper floor and a glazed room below
Heat rises, so the upper floor of a maisonette is commonly the bedroom problem and the lower floor is the glazed living room. One indoor in the lounge leaves the bedrooms warm at night. A multi-split with two or three indoors matches the way the doors actually close. Plan indoors for the rooms you shut at night. One outdoor unit is usual if only one lawful wall exists. Two floors can share it on a multi-split if that position is lawful and the pipe lengths fit the combination chart.
Pipework has to move between storeys in a riser, a cupboard or surface trunking you can tolerate. Chase depths, fire stopping and the neighbour's ceiling below a conversion floor all need care.
The lease, the party floor and the consumer unit
Purpose-built and converted maisonettes are often leasehold. The demise may include the internal face of the wall and not the right to hang plant on the outside. Roof rights, garden rights and the path of a drain can sit with the freeholder or a neighbour. Get lease consent in writing. Written consent should name the outdoor location and the pipe route.
Converted houses transmit bracket vibration into the flat below. Use proper mounts and avoid fixing into a thin party element. The indoor unit's low fan is the smaller issue. The outdoor unit on a shared wall is the larger one. In England, Planning Portal air source heat pump conditions are the planning starting point, and many maisonettes will still need an application or fail a boundary test. Check planning rather than assuming householder rights. F-gas certification covers the refrigerant system, including both indoors on a multi-split. Ask for both model codes, the pipe lengths per floor and the condensate method on each indoor.
Frequently asked questions
Will a lounge unit cool upstairs in a maisonette?
Not reliably. Hot air can drift up an open stair, and closed bedroom doors stop the cooling. Put an indoor on each floor you care about.
Do maisonettes have the same planning rights as houses?
Often no, because they are flats. Check the lease and planning. Do not rely on householder permitted development without reading the Planning Portal conditions against your building.
Can two floors share one outdoor unit?
Yes, on a multi-split, if one lawful outdoor position exists and the pipe lengths fit the combination chart.
Is a neighbour's verbal yes enough?
No. Verbal approval from a neighbour who is not the freeholder will not survive a sale. Written consent should name the outdoor location and the pipe route.
What if the consumer unit shares an old riser?
Converted maisonettes sometimes share ancient risers. A competent electrician should confirm capacity before the cooling quote is treated as firm. The outdoor unit needs a dedicated circuit.
Do listed maisonettes need another consent?
Listed conversions add another consent on top of the lease and the Planning Portal checks. Many maisonettes will still need a planning application or fail a boundary test.
Core buying guides
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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.