The noise limit for domestic heat pumps and air conditioning tightened in May 2026, 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 the assessment position — broadly, your neighbour's nearest habitable room window.
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.
Since 28 May 2026, MCS 020 is the only accepted route. The previous allowance for equivalent acoustic methods has closed, so if an installer tells you they use manufacturer data instead, that is no longer sufficient 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 has to meet an acoustic specification — ordinary featherboard or panel fencing no longer attenuates anything for compliance purposes.
Where a proper barrier does exist and it completely blocks the line of sight between unit and window, the reduction is significant — up to 10 dB. 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 calculation before quoting, not after. 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 July 2026, under MCS 020(a) Issue 1.0 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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