Short answer: Outdoor unit noise is assessed at the nearest neighbouring door or window. In England and Wales, a calculated level of 37 dB(A) or less under MCS 020(a) meets the permitted development condition. Careful positioning, anti-vibration mounts and choosing a quiet unit usually achieve this comfortably.
How the calculation works
It starts from the unit's published sound power, then allows for distance, reflecting surfaces such as walls and corners, and any barrier. Moving a unit a few metres or out of a corner can make a noticeable difference.
Positioning
We avoid placing units under neighbours' bedroom windows, in enclosed corners that amplify sound, or on lightweight walls that transmit vibration. Ground stands and anti-vibration feet help.
Being a good neighbour
Telling neighbours before installation goes a long way. Night modes that cap outdoor fan speed are useful for bedrooms.
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Reviewed October 2026. Rules and prices change; check the date and ask us if anything is unclear.