A side return extension is a long, narrow, glazed room beside the original house. The decision is where one indoor unit can see down that slot, because the side wall is mostly glass or the neighbour's boundary. The outdoor unit then has to fit in whatever yard is left without blocking the extension you just built. Survey the glazing and the remaining wall before you buy a kilowatt. An F-gas certified company installs the pair. One zone is usual if the kitchen and the return are open to each other.
Standing in a metre-wide slot under a lantern
Side returns overheat through the roof lantern and the flank glazing, even when the floor area is small. The survey sizes that glass. A unit sized like an old internal kitchen will not hold the return at 3pm. Insulation and solar glass help, and they do not remove the need for cooling if you sit there. Say this is a side return. Photos looking along the extension, the lantern, the solid wall you have left, and the outdoor gap at the end, including the neighbour's windows if they face that gap, are the set that shows the slot.
The end wall that can still see down the return
The indoor unit wants a solid end wall or the original rear wall, throwing along the slot rather than into a metre of flank glass. There is often only one such wall.
- Size the lantern and the flank glass, not the old kitchen alone.
- Throw air along the length of the return.
- Use the remaining solid wall.
- Keep the outdoor unit out of the last bit of path.
The passage that is now the room
The side passage is now the room. The condenser cannot go back where the passage was. It belongs at the far end, on the new end wall if the structure allows, or on the original rear wall, or on a pad that still leaves you a way past the bins. Airflow and service clearance are easy to lose in a one-metre gap. In England, Planning Portal conditions matter here because the neighbour's wall is close. A condenser facing their window across the boundary is the complaint. A multi-split for the rest of the house does not conjure a place in a filled side return. The outdoor unit can sit only in whatever outdoor gap is left beyond the glazing, with airflow and a lawful noise result. It cannot go inside the new room, and it cannot go in a passage that has been built over.
The hob, the steel pier and the gully
If the return is the kitchen, keep the indoor unit off the hob and out of the extractor's immediate path. The extract removes cooking fumes. The split cools the room. They are not substitutes, and they should not blow against each other. Condensate goes to a new gully, not down the inside of the lantern frame. The end wall of a side return is often a lightweight pier or a steel goalpost with little masonry. The bracket has to hit structure. As a 2026 illustration, cooling the open kitchen and return as one zone is often in the low thousands including VAT when a solid wall and a pad both exist. Steel, a pump and a tight neighbour gap are the extras. One unit will cool the old kitchen and the new return if they are one open volume and the unit is placed to throw along that length. The survey should size the glass, not the original kitchen floor area.
Stop when the neighbour's window is the only gap left
If no lawful spot remains, the extension cannot have a split until a spot is found. If a roof lantern occupies the ceiling, a cassette is a fantasy unless a real void exists, which side returns rarely have. A lantern roof usually is not that void. A wall unit on the remaining solid wall is the ordinary answer. Ask for one zone if the kitchen is open into the return, with model codes, where the indoor unit throws, and where the outdoor unit stands without blocking the path. Ask them to say if no lawful outdoor spot remains.
Frequently asked questions
Can the outdoor unit go in the side return?
Only in whatever outdoor gap is left beyond the glazing, with airflow and a lawful noise result. It cannot go inside the new room, and it cannot go in a passage that has been built over.
Will one unit cool the old kitchen and the new return?
Yes if they are one open volume and the unit is placed to throw along that length. The survey should size the glass, not the original kitchen floor area.
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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.