England: 37 dB(A) since 29 May 2025. Scotland, Wales and Northern Ireland: 42 dB(A). Both numbers are real. They come from two different standards, and most of the arguing online is people quoting one of them at people living under the other.
Figures on this page last checked 30 September 2026. Each carries its own receipt with its own date.
Look up the noise rules for an air source heat pump and you will find installer blogs that disagree on the one number that matters, each of them quite sure. They are usually both reading a genuine MCS standard. Just not the same one. Here is which standard applies where, what the number is measured against, and the sum itself, so you can run it on your own garden. Nobody is going to ring you.
Claim check
The heat pump noise limit is 42 dB
England, since 29 May 2025
The limit in England is 37 dB(A) under MCS 020 a). The 42 went with the old standard, and the law now names MCS 020 a) and nothing else.
Receipt: MCS 020 a) Issue 1.1, SI 2025/560
Scotland, Wales and Northern Ireland
They still use the older MCS 020, and its limit is 42 dB(A). MCS says that stands until further notice.
Receipt: MCS consultation response, MCS, 29 May 2025
In England an air source heat pump is permitted development only if the MCS 020 a) sum puts it at or below 37.0 dB(A) at every assessment position. An assessment position is a point 1 metre outside the middle of a door or window to a habitable room of a neighbouring property. The sum is rounded to one decimal place before it is compared.
37 dB(A)
sound pressure level from the heat pump, LAeq over 5 minutes, at every assessment position
Covers 29 May 2025 onwards.
Source: MCS, MCS 020 a) Issue 1.1 · checked 2026-09-30 · JSON
The older standard, MCS 020, sets the permitted development noise limit at 42 dB(A). It is the one Scotland, Wales and Northern Ireland still use.
42 dB(A)
permitted development noise limit under MCS 020, the standard Scotland, Wales and Northern Ireland still use
The source page carries no date. Read 30 September 2026.
Source: MCS, consultation response on MCS 020 · checked 2026-09-30 · JSON
MCS says installations in England must meet MCS 020 a) from 29 May 2025, and installations in Scotland, Wales and Northern Ireland must keep meeting the older MCS 020 until further notice. The same change let English heat pumps go within 1 metre of the property boundary, which the old rules did not allow. MCS 020 a) applies whether or not the installer is MCS certified.
A lower limit sounds like a tougher rule. MCS's own testing says it mostly is not, because the sum changed at the same time as the number.
The old MCS 020 sum, checked against 42 dB(A), had a step that assumed 40 dB(A) of background noise. Where a heat pump came in quieter than that, the installer had to use a figure higher than the heat pump's own. MCS removed that step and set what it calls a corresponding limit of 37 dB(A). MCS tested 48 real assessments both ways. On average they cleared the old 42 by 1.08 dB and the new 37 by 7.3 dB, and every one that passed the old sum also passed the new one. So the lower number is not simply a tougher rule. It is a different sum.
MCS dropped the old method's assumed 40 dB(A) background noise step and set a corresponding limit of 37 dB(A)
reason for the change, from MCS's consultation response
The source page carries no date. Read 30 September 2026.
Source: MCS, consultation response on MCS 020 · checked 2026-09-30 · JSON
This is the calculation in the standard, step for step: the heat pump's sound power, the hard surfaces around it, the distance to your neighbour's nearest door or window to a habitable room, and any solid barrier in between. It shows the level against both limits. It runs in your browser and sends nothing anywhere.
About 38.0 dB(A) at that position.
Indicative. Your MCS installer does the official assessment. They check every assessment position around the house, not just one, and it is their sum that counts for permitted development.
The result is our arithmetic, run on the formula MCS 020 a) prints: Lp = Lw + 10 log(Q ÷ (4 × π × r²)) − AB, rounded to one decimal place as the standard says. Checked before this page is built against the three worked examples in the standard's Appendix A, which it reproduces. Filled in above with one check case: a 60 dB(A) unit on open ground (Q2), 5 m from the window, no barrier, comes to 38.0 dB(A). That is over England's 37 dB(A) and under the 42 dB(A) still used elsewhere.
Lp = Lw + 10 log(Q ÷ (4 × π × r²)) − AB
formula, sound pressure level in dB(A) at one assessment position
Page dated 14 April 2025.
Source: MCS, MCS 020 a) Issue 1.1 · checked 2026-09-30 · JSON
What the inputs mean, in the standard's own terms:
The sum takes the heat pump's A-weighted sound power level from the maker's data (never the low noise mode figure, and never a sound pressure figure), adds a correction for how many hard surfaces sit within 1 metre of it, takes off the spreading over distance, and takes off a correction for a solid barrier in the way. Distance is measured to the nearest 0.1 metre from the middle of the unit. More than three reflecting surfaces, as in a small lightwell, does not meet the standard at all.
Lp = Lw + 10 log(Q ÷ (4 × π × r²)) − AB
formula, sound pressure level in dB(A) at one assessment position
Page dated 14 April 2025.
Source: MCS, MCS 020 a) Issue 1.1 · checked 2026-09-30 · JSON
| Barrier | No view | Partial view | Full view |
|---|---|---|---|
| Type 1 | 10 dB | 5 dB | 0 dB |
| Type 2 | 5 dB | 2.5 dB | 0 dB |
| No barrier | 0 dB | 0 dB | 0 dB |
The first version of the standard printed this table with minus signs in front of each figure. The formula already subtracts the barrier, so a sum run off that table literally would add 10 dB instead of taking it away. MCS corrected it in Issue 1.1, the version the law names, and this page uses Issue 1.1.
The law that changed the rules in England came into force on 29 May 2025. It made MCS 020 a) the only standard that counts, removing the words "or equivalent standards". It allows up to two air source heat pumps on a detached house and one on any other house or a block of flats. Installations that met the old rules could still go ahead under them until the end of 28 May 2026. That window has now closed.
In force 29 May 2025: MCS 020 a) is the only standard that counts, and a detached house may have two heat pumps
statutory instrument, England
Covers 29 May 2025 onwards.
Source: legislation.gov.uk, SI 2025/560 · checked 2026-09-30 · JSON
37 dB(A), worked out with MCS 020 a) at every assessment position, which is a point 1 metre outside a door or window to a habitable room of a neighbouring property. It has applied since 29 May 2025.
Yes. Scotland, Wales and Northern Ireland still use the older MCS 020, and its limit is 42 dB(A). MCS says that stands until further notice.
No. MCS 020 a) says the sound power level in low noise mode shall not be used. It also has to be the sound power level, not a sound pressure figure, which is a different and usually lower number.
The standard's answer is that the heat pump is then not permitted development. It also says moving the unit may bring the level down. Outside permitted development, planning permission is the route, and your council's planning team is the place to ask.
Not in England. MCS says the change on 29 May 2025 let heat pumps be installed within 1 metre of the property boundary. The noise sum still has to pass.