Your air source heat pump, cleared to install.

For a domestic heat pump, where the unit goes affects the outcome more than any other single decision, and it is the cheapest variable to change. The MCS 020 calculation applies corrections based on the installation position: a unit standing clear on an open elevation, one against a single wall, and one in a corner formed by two walls give materially different results from identical equipment.

Distance to the assessment position compounds it. The relevant position is a habitable room window or door of the nearest neighbouring property, and gaining a few metres of separation, or moving the unit to an elevation that faces away from that window, will often turn a marginal failure into a comfortable pass without spending anything.

This is why we would rather look at a proposal before installation than assess one afterwards. Once the unit is fitted, the pipework run is committed and the options narrow to acoustic housings, barriers or replacement, all of which cost more than a different position would have.

MCS 020 exists to demonstrate compliance with the noise condition attached to permitted development rights. Where those rights do not apply, the calculation is not the relevant test. Listed buildings and conservation areas restrict permitted development, and a great many installations in those settings need a full planning application regardless of how quiet the unit is.

Scale takes a scheme outside it too. Multiple units, commercial installations and multi-dwelling schemes are assessed under BS 4142 against the measured background, which is a considerably more demanding test in a quiet area than a fixed 42 dB limit. We have seen schemes pass MCS 020 comfortably and indicate a significant adverse impact under BS 4142 on the same plot, because the night-time background there sat in the twenties.

The practical advice is to establish which test applies before selecting equipment. Choosing a unit on the basis of a 42 dB limit that turns out not to apply is an expensive way to discover the difference.

Meeting MCS 020 means the installation satisfies the noise condition attached to permitted development. It does not mean nobody will hear the unit, and it does not prevent a statutory nuisance complaint. Those are separate tests: one is a planning threshold, the other is a judgement by the council about whether the noise unreasonably interferes with the enjoyment of a neighbouring home.

The gap matters most in quiet areas. A unit at 42 dB in a location where the night-time background is 25 dB is clearly audible, and audibility is what drives complaints. This is the single most common source of disappointment we encounter: an installation that is entirely lawful and still generating an argument between neighbours.

Where that risk exists, we would rather flag it before installation than assess it afterwards. Usually the answer is a quieter unit, a better position, or an operating schedule that avoids the quietest hours, and all three are easier to arrange before the system is commissioned than after a complaint has been made.