Air source heat pump noise and the MCS 020 standard

An air source heat pump on a house is usually permitted development, and the condition that decides it is a sound calculation rather than a planning judgement. The installation has to comply with the MCS planning standard, MCS 020, which requires the calculated sound pressure level at the nearest neighbour's habitable room window to be no more than 42 dB(A). Fail that and the permitted development right falls away, which means a planning application, a noise report and several months you had not planned for.

The permitted development conditions for a heat pump

Air source heat pumps on dwellings sit in Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015. Part 14 covers microgeneration equipment generally, and Class G is the air source heat pump class.

Read the current text of Class G rather than a summary. The class has been amended more than once, and the limits on the volume of the outdoor unit, the number of pumps permitted, the distance from a boundary and the treatment of flats have all moved. What has remained constant is the structure: the right applies only where the equipment is used solely for heating purposes, where it is sited to minimise its effect on the external appearance of the building and on the amenity of the area, where it is removed as soon as reasonably practicable when no longer needed, and where it complies with the MCS planning standards or equivalent.

That last condition is the one that catches installations. It is not advisory. If the standard is not met, the works are not permitted development, and an unauthorised installation is a breach of planning control however well it performs.

How the MCS 020 sound calculation works

MCS 020 is a calculation, not a measurement. It takes the sound power level of the unit from the manufacturer's published data, applies a correction for where the unit is placed, applies distance attenuation to the assessment position, and compares the result with 42 dB(A).

The placement correction is where most installations are won or lost. A unit standing in open ground radiates sound in more directions and loses more of it with distance. The same unit set against a wall reflects sound outward, and set into a corner between two walls it reflects more again, adding several decibels before any distance is taken into account. Placing a pump neatly in the corner formed by a rear extension and the main house is the most common way to fail.

Distance then does the rest of the work, and it is unforgiving at short range. Doubling the distance from the unit to the assessment position gives a useful reduction; moving a pump two metres further down a garden can be worth more than every other measure combined.

Where the assessment position is taken

The assessment position is one metre external to the window or door of a habitable room of a neighbouring property. Not your own window. Not the boundary.

That definition has consequences people find counter-intuitive. A pump three metres from your own kitchen window but fifteen metres from the neighbour's may pass comfortably while causing you constant irritation, because the standard is about protecting others. Conversely, on a terraced street where the neighbouring bedroom window is four metres away across a party boundary, a mid-range unit can fail without any obvious warning.

Where there are several neighbouring properties, the calculation has to be done for the worst case. A corner plot with two neighbours has two assessment positions, and a flat above a neighbouring commercial unit may still have a habitable room window.

Siting choices that pass and fail

Work through the options in this order.

1. Move the unit away from boundaries and from neighbouring windows. Distance buys decibels at no cost.
2. Take it out of corners. A free-standing position on a plinth, or a position against a single wall, beats an internal corner every time.
3. Put the unit on the side of the house away from the neighbour's habitable rooms, even if the pipe run is longer.
4. Choose a quieter unit. Sound power levels for units of similar output vary by several decibels between models, and the data is published.
5. Consider an acoustic barrier or enclosure, but only with the manufacturer's agreement, because restricting airflow reduces efficiency and can void the warranty.

Two things that do not help: a decorative timber screen with no mass, and a rubber mat under the feet where the problem is airborne rather than structure-borne.

When a planning application is needed instead

If the calculation cannot be made to work, or the siting falls outside Class G for another reason, the route is a householder planning application. The council will then assess the noise on its merits rather than against a fixed threshold, which is both harder and more flexible.

Expect the environmental health officer to ask for an assessment against BS 4142, comparing the rating level of the pump with the measured background sound level at the neighbouring facade. That is a different and more demanding exercise than MCS 020, because it involves a survey rather than a desktop calculation, and the tonal character of a compressor attracts a penalty. Our note on choosing the background level explains why the measured figure matters so much, and a Noise Impact Assessment prepared by an acoustician is the usual supporting document.

Heat pumps also produce low-frequency content that ordinary A-weighted figures understate, which is the subject of our piece on low frequency noise complaints. If the neighbour relationship is already difficult, plan for that conversation rather than hoping the calculation settles it.

Conservation areas and listed buildings

Class G is restricted in designated land. In a conservation area or a World Heritage Site the right does not extend to installations on a wall or roof fronting a highway, and there are further limits on pumps sited between the principal elevation and the highway. Check the current wording, because these restrictions have been adjusted alongside the rest of the class.

A listed building is a separate matter altogether. Permitted development rights do not remove the need for listed building consent, and fixing a pump, its pipework and its condensate run to a listed elevation is an alteration affecting character. Apply for consent, and expect the conservation officer to want the unit at ground level, out of principal views, with the pipe penetrations taken through an existing opening or a later fabric.

Evidence to keep with the installation

Keep the MCS 020 calculation itself, with the input data used. Keep the manufacturer's sound power figure and the datasheet version it came from. Keep the installer's certificate and the commissioning record. Photograph the installed position with a tape measure in shot showing the distance to the boundary and to the nearest neighbouring window.

That file does two things. It answers an enforcement enquiry years later, when nobody can remember where the pump was measured from. And it answers a conveyancer on sale, who will ask whether the installation was permitted development and will want more than an assurance. Where a complaint becomes a statutory nuisance investigation, the planning position and the nuisance position are separate questions, as set out in our note on what a condition cannot fix.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.