There is no universal maximum height difference between an indoor unit and its outdoor unit. Small splits often fall in the 10 to 20 m class, and that is only a class. The quote must use the manual for that outdoor unit. Some manuals allow a different rise when the indoor unit is above the outdoor unit than when the outdoor unit is above the indoor unit. A loft bedroom down to a basement yard can leave a small model's class even when the pipe length in metres still looks acceptable.
A loft bedroom and a condenser in the front area
A three-storey townhouse with the indoor unit in the roof and the condenser in a front area can be near or past a small unit's class. Storey height is a rough check only. Measure the actual vertical between the two units. A loft bedroom down to a basement yard can leave the class even when the pipe length in metres still looks acceptable.
Oil return and two lines in the manual
Oil has to return to the compressor. A tall rise changes that, which is why the manufacturer prints a maximum height difference and sometimes a different figure for each direction. The installer applies the line that matches your house, not the more generous number from a different model. Often 10 to 20 m for small splits. That is a class, not your limit. Use the manual for that outdoor unit. There is no universal maximum to publish.
Length can pass while height fails
A run can pass the height check and fail the length check, or the reverse. Bends, a long loft crossing and the vertical stack all sit in the same design. Extra refrigerant, where the manual requires it for a long run, does not buy extra height. Diameter, height and length all come from the same manual. A fatter pipe chosen on site does not raise the allowed height difference. Commissioning stays with an F-gas certified company.
Basement wells and tall-house gardens
Flats that want the condenser in a basement lightwell, and bedrooms that want it in the garden of a tall house, are the usual near-misses. A ground-floor living room with the outdoor unit just outside is rarely a height problem. Say which floor you are on when you ask for a price. Say which unit is higher.
Stop when either limit in that manual is past
If either limit fails, change the outdoor position or the model. A short vertical drop onto a forbidden wall is not a solution. The outdoor position still has to be lawful. In England, Planning Portal heat pump conditions cover noise, boundaries and the rest.
Frequently asked questions
Is 10 m or 20 m the UK maximum height difference?
Neither. Those figures describe a common class for small splits. Your maximum is in the manual for that outdoor unit. There is no universal maximum.
Does it matter which unit is higher?
Yes. Many manuals set different limits for an indoor unit above the outdoor unit and for the outdoor unit above the indoor unit. The price should use the line that matches the house. Direction can change the allowed figure.
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.