Will my outdoor unit be quiet enough?
Estimate the noise an outdoor AC or heat pump unit makes at your neighbour’s window using the MCS 020 method, and see what changes the result.
On the product datasheet, often labelled sound power or LWA. It is not the sound pressure figure at 1 metre.
Hard surfaces within 1 metre of the unit
Count the ground, walls and fences that extend at least 1 m past the unit. Vegetation does not count.
Straight line from the centre of the unit to 1 m outside the window or door of a neighbouring habitable room.
Barrier in between
From the top of the unit, the window is
Fails the MCS 020 a) test
39.0dB(A)
at the neighbour's window, against a limit of 37.0 dB(A) (2.0 dB over)
With the same unit and surroundings, it would pass at about 6.3 m from the window. A solid barrier that hides the window, fewer hard surfaces nearby or a quieter model also help.
Lp = LW + 10 log10(Q / 4 pi r²) - barrier. Q = 4 for 2 reflecting surfaces, barrier attenuation 0 dB. Rounded to 0.1 dB. The MCS 020 a) calculation must pass at every neighbouring assessment position, so repeat it for each window or door that could be affected.
How this works
This implements the MCS 020 a) sound calculation for air source heat pumps installed under permitted development in England (MCS 020 a) Issue 1.0 (MCS, 20 March 2025), mandatory for permitted development from 20 September 2025). It predicts the sound pressure level at an assessment position, 1 metre outside the centre of a neighbouring habitable room's window or door, and compares it with the 37 dB(A) limit.
The formula is Lp = LW + 10 log10(Q / 4 pi r²) minus any barrier attenuation. Q is 2, 4 or 8 for one, two or three reflecting surfaces within 1 metre of the unit; a unit with more than three cannot meet the standard. A solid barrier at least 18 mm thick takes off 10 dB if it hides the window from the top of the unit and 5 dB for a partial view; a thinner solid barrier takes off 5 or 2.5 dB. The result is rounded to 0.1 dB.
Passing this test is one condition of permitted development, not all of them. This is indicative, not legal advice: confirm the final position with your local planning authority. A cooling-only air conditioner has no permitted-development route in England and needs planning permission, where the council sets its own noise conditions.
Sources
- MCS 020 a) Issue 1.0: air source heat pump sound calculation
- MCS: updated version of MCS 020 published
Questions people ask
What is the noise limit for a heat pump under permitted development?
37 dB(A) at every neighbouring habitable-room window or door, calculated with MCS 020 a). It has applied to permitted-development installs in England since 20 September 2025.
Where do I find the sound power level?
On the outdoor unit’s datasheet, as the A-weighted sound power level (LWA). Do not use a sound pressure figure quoted at a distance.
Does a fence reduce the result?
Only a solid, gap-free barrier counts. At least 18 mm thick, it takes off 10 dB if it hides the window from the top of the unit, or 5 dB for a partial view.