Heat pump noise: MCS 020(a), permitted development and where to put the unit

Noise is the most common worry about air source heat pumps, and in the UK it also decides whether you need planning permission. The fan and compressor sit outside and work hardest on cold nights. Choosing and siting the unit with the rules in mind avoids a planning application, a disgruntled neighbour or an expensive move later.

The rule in England: MCS 020(a)

Most air source heat pumps on houses in England are installed under permitted development, without a planning application. Since 29 May 2025, a condition of that is compliance with MCS 020(a), the MCS sound calculation standard, whether or not the installation itself is MCS-certified ^1,2.

The limit is 37.0 dB(A) at the assessment position: 1 m outside the centre point of any door or window to a habitable room of a neighbouring property, measured perpendicular to that door or window ³. The calculation uses the heat pump’s sound power level in its standard operating mode, not a quiet or night mode.

The same 2025 changes relaxed other conditions: the rule that the unit had to be at least 1 m from the boundary was removed, the maximum size rose from 0.6 m³ to 1.5 m³, detached houses may have two units, and air-to-air heat pumps can now use permitted development too ¹. Other conditions still apply, for example for listed buildings and protected areas, so your installer should check the full list ².

Wales and Scotland

Wales changed its permitted development rules for air source heat pumps on 1 June 2026, also referring to MCS 020(a) and allowing larger units and two units on detached houses ⁴. Scotland has its own rules, which still refer to the older version of MCS 020 ⁵. Outside England, ask your installer or local planning authority which version applies.

How the calculation works

A datasheet gives the sound power level (LWA), for example 55 dB(A): the total acoustic energy the unit emits. The 37 dB(A) limit is a sound pressure level at the neighbour’s window. MCS 020(a) links the two with a simple formula ³:

Lp = LWA + 10·log(Q ÷ 4πr²) − barrier correction

• r is the distance in metres from the heat pump to the assessment position.
• Q is directivity: Q2 for one reflecting surface within 1 m (usually just the ground), Q4 for two (ground plus a wall), Q8 for three (a corner) ³.
• Barrier correction: a solid brick or masonry barrier gives −10 dB if it blocks the view of the unit completely and −5 dB if partly; a thinner fence gives −5 dB or −2.5 dB; no reduction if you can see the unit ³.

What distance do you need?

Minimum distance to the assessment position to reach 37 dB(A), with no barrier:

Every extra 5 dB of sound power nearly doubles the distance you need. Against a wall, only about one in six (17%) of the air-to-water heat pumps in our database reach 37 dB(A) within 2 m. That is why a unit squeezed down a narrow side passage between two houses so often fails, and why a solid screen can make the difference.

How quiet are heat pumps?

MCS 020(a) is a calculation for air source heat pumps ³. Ground source and water-to-water units usually sit indoors, so the figures below cover the 996 air-to-water heat pumps in our database, with data mainly from the German BAFA list and the Heat Pump KEYMARK ^6,7:

Bigger units are louder on average: the median is 50 dB(A) for 4–8 kW, 55 dB(A) for 8–12 kW and 59 dB(A) for 12–20 kW. The heat pumps sold as part of a hybrid system have a median of 50 dB(A). Compare within your size band, for example the quietest 6–8 kW heat pumps or 8–10 kW. Check that the sound power figure your installer uses matches the product’s standard-mode value.

Siting tips

• Avoid corners and narrow passages: each extra reflecting surface adds about 3 dB.
• Point the fan away from neighbours’ windows and your own bedrooms.
• Use anti-vibration mounts or a solid base, not a lightweight wall or fence, to stop structure-borne noise.
• Plan for defrosting: the unit sounds different for a few minutes and drains water; give it somewhere to go.
• Don’t oversize: a larger unit than your home needs is louder for no benefit.

If the calculation fails, the options are a quieter or smaller unit, a different location, a compliant solid barrier, or a planning application. Grants have their own rules; see our heat pump grants guide. Every product page on this site shows the indicative distance needed for that model.