The noise limit for domestic heat pumps and air conditioning changed on 29 May 2025, from 42 dB to 37 dB. That sounds alarming. In practice most installations now pass more comfortably than they did before — and the reason is worth understanding.
The short version. There is a legal noise limit, your installer has to calculate it before fitting anything, and where the unit goes matters far more than which unit you buy.
Air conditioning fitted under permitted development has to satisfy a noise condition. The calculation is set out in a standard called MCS 020 a), and the installation must come out at or below 37 dB(A) at every assessment position: a point 1 m out from each habitable-room window and door of the neighbouring homes, on every side and every floor.
For scale, 37 dB is quieter than a fridge in a quiet kitchen and roughly the level of a library reading room. It is not silence, but it is well below the point at which most people notice a sound as an intrusion.
For air conditioning, MCS 020 a) has been the only accepted route since 29 May 2025: before then, a unit that cooled wasn't permitted development at all. If an installer tells you they use manufacturer data instead, that is not enough for permitted development.
This is the part almost every guide online has wrong. The limit dropped from 42 dB to 37 dB, but the calculation method changed at the same time.
MCS published an impact assessment alongside the change. Under the old 42 dB method, a great many installations passed only marginally — on average by about 1 dB. Under the new 37 dB method, the average margin was over 7 dB. Every installation in their sample that passed the old calculation also passed the new one.
A lower headline number, assessed differently, producing more comfortable passes. If you have read that the new rules make air conditioning harder to install, that is not what the evidence says.
The figure that goes into it is the unit's sound power level, taken from the ErP label or the MCS Product Directory. That is not the same as the decibel figure on the marketing brochure, which is usually a sound pressure measurement taken at one metre in ideal conditions.
From that starting figure, the calculation applies corrections for distance, for any solid barrier between unit and window, and for reflective surfaces near the unit.
This is the change that catches people out. A garden fence used to be accepted as a barrier in the calculation. Under the current standard it counts only if it is solid, with no gaps or cracks, in good repair, and runs at least a metre past the unit on each side. Hedges, trellis and gappy or tired fencing count for nothing.
Where a proper barrier does exist and it completely blocks the line of sight between unit and window, the reduction is significant: 10 dB for a wall or a fence at least 18 mm thick, 5 dB for a thinner solid fence, and half that if the window can be seen from within 25 cm of the top of the unit. But it must genuinely obscure the view. If you can see the neighbour's window from where the unit will sit, the barrier is not doing the job.
Worth separating from the neighbour question. Modern indoor units run at roughly 19 to 26 dB on low speed, which is quieter than most people's expectations of what air conditioning sounds like. The complaint we would expect to hear is not volume but airflow direction — a unit positioned so it blows directly at a bed or a desk. That is a siting decision, not an equipment one, and it is why we agree indoor positions with you at the survey rather than on the day.
We run the MCS 020 a) calculation before quoting, not after, and give you a copy. If the position you would prefer does not pass, we would rather tell you at survey and find one that does, than fit it and leave you to have a conversation with your neighbour about it.
Where a neighbour is likely to be close enough to care, telling them beforehand costs nothing and prevents most problems. It is not a legal requirement. It is just the sensible thing.
General information rather than formal advice. The position described is current as at October 2026, under MCS 020 a) Issue 1.1 (14 April 2025), the version the law names, and the noise condition attached to Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015, as amended. For your own property, ask us at survey.
The noise assessment happens at the survey, so the position is agreed before anything is ordered — not discovered afterwards.
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