A UK rear extension is often the hottest room in the house: glass, a roof lantern, bi-folds and an open-plan kitchen. A single 3.5 kW wall unit sized like a Victorian reception frequently fails. The cheapest time to install is before plaster, with the outdoor position on the drawings. After handover you are in retrofit territory with trunking and making good.
Why extensions overheat
The last decade of kitchen-diner extensions produced an open-plan room that overheats: thin new fabric, lots of glass, sometimes a dark kitchen against a west garden. Cooking heat arrives just as solar gain peaks. A wall unit on the old house wall may not throw to the bi-folds.
The open-plan kitchen diner guide covers the occupied room. This page is the build sequence and the outdoor siting that builders forget until second fix.
- First-fix pipe and condensate before plaster if you can.
- Lanterns and bi-folds are load, not decoration.
- The outdoor unit still needs a legal, quiet wall or slab.
Formats that work
A high-capacity wall unit on the coldest internal wall can work if glass is modest. Bulkhead ducted with two supplies, kitchen end and sofa end, is the tidy answer. Two indoors on a multi-split is honest when the extension is really two loads. Floor-standing units help winter heating in tall glazed rooms and eat floor next to the doors.
Do not put the indoor intake over the hob. Grease kills coils. The hidden air conditioning guide covers bulkheads. The 5 kW guide covers the next size up from 3.5 kW.
Planning, neighbours and the builder
The condenser is still a planning, noise and sometimes party-wall conversation. A new extension does not grant a free pass for a humming box by a neighbour's window. Read the planning permission guide and the outdoor unit position guide.
If the builder is still on site, get sleeves, a pad or brackets, and a drain route into their programme. Drilling a finished render two months later is how snagging starts. New-build warranties are a cousin of this problem. The new-build cooling guide covers developer process.
Brief for a quote
Drawings if you have them, photos of lanterns and the proposed outdoor elevation, hob fuel, and whether the room is already plastered. Mention evening entertaining if that is when it fails.
Use the quote form. Say it is a rear extension so you are not introduced as a single bedroom split.
Frequently asked questions
Should I install air conditioning while the extension is being built?
Yes if you already know you will want it. First fix is cheaper and tidier than chasing plaster. You can still delay the indoor fascia until you pick a series.
Will one wall unit cool an extension with bi-folds?
Sometimes, if the survey agrees. Often you need more capacity or a second supply. Close the glass if you want the system to win. Cooling the garden is not the brief.
Do I need planning permission for the outdoor unit on a new extension?
Same tests as any house: siting, listed status, conservation area, noise. New brickwork does not skip those tests.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
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.