Heat pump noise and the assessment planning expects

Most domestic air source heat pumps in England go in under permitted development, and the permitted development right carries a noise condition rather than a noise assessment. The unit has to comply with the MCS planning standard, which works by calculation: take the manufacturer's sound power level, apply corrections for distance and for the surfaces around the unit, and show that the result does not exceed 42 dB(A) at the assessment position, one metre from a habitable room window of a neighbouring property. Heat pump noise in planning terms is that calculation, until the right does not apply and the whole thing becomes an application.

The permitted development right and its conditions

Air source heat pumps on domestic premises sit in Part 14 of Schedule 2 to the General Permitted Development Order 2015, which deals with renewable energy installations. The right is subject to conditions and limitations covering the size of the unit, its siting, and compliance with the noise standard.

The limits have been amended more than once, including changes to the volume of the unit and to the restriction on how close it may sit to a boundary. Read the current consolidated Order on legislation.gov.uk before relying on any figure you have seen elsewhere, including figures in older articles and installer literature.

Some restrictions have been stable. The right does not apply within the curtilage of a listed building. In a conservation area, a National Landscape, a National Park or a World Heritage Site, a pump on a wall or roof fronting a highway falls outside the right. The unit must be used solely for heating purposes and be removed as soon as reasonably practicable when no longer needed.

Where the property is a flat or a maisonette in a converted building, check carefully, because the right is drawn around dwellinghouses and blocks of flats in specific terms and conversions do not always fit.

The noise calculation that applies to the right

The MCS planning standard exists to give a repeatable answer without an acoustician. It is a worksheet, and the inputs are the ones that matter.

You need the manufacturer's declared sound power level for the unit, in dB(A), for the operating mode being assessed. You need the distance from the unit to the assessment position. You need to identify the reflecting surfaces, because a pump in the corner of two walls is treated differently from one standing in the open. Then a barrier correction where a solid obstruction breaks the line of sight.

The output is a sound pressure level at the assessment position, and it must not exceed 42 dB(A).

Two practical points. First, the sound power figure quoted in a brochure is often for a mild condition at part load, and the figure that matters is the one at the design operating condition. Ask for the declared data sheet, not the sales sheet. Second, the assessment position is the neighbour's habitable room window, so a pump on a narrow side return between two terraced houses will usually fail even where the same unit passes comfortably in a large rear garden.

Where a full application is needed instead

The right falls away in several situations, and then the council assesses the noise on its merits rather than against a single threshold.

• The property is listed, or the pump would sit on an elevation fronting a highway in a designated area.
• The unit exceeds the size limit in the current Order, or more units are proposed than the right allows.
• A condition or an Article 4 direction has removed permitted development rights, which is common in conservation areas.
• The building is a flat in a converted house, or the installation serves a commercial use.

On a full application, expect the officer to look for an assessment against BS 4142:2014+A1:2019 rather than the MCS worksheet, comparing a rating level against the representative background sound level. That method adds acoustic feature corrections for tonality, impulsivity and intermittency, and heat pump compressors and defrost cycles regularly attract them. A pump that passes a 42 dB(A) threshold can still produce an adverse comment under BS 4142 if the background is very low, which is exactly the situation in a quiet rural garden at 2am.

Where design matters as well, a pump in a conservation area raises the same questions as external wall insulation in a conservation area: visibility from the street, the character of the elevation, and whether a less prominent position exists.

Night-time operation and defrost cycles

Heat pumps run when heat is needed, which on a cold night is all night. That is the fundamental difference from an air conditioning unit and the reason neighbour complaints cluster in winter.

Defrost is the other issue. Periodically the unit reverses to clear ice from the evaporator, the fan speed changes, and the sound character changes with it. Residents describe it as a change in tone rather than an increase in level, and that is precisely what a feature correction is designed to capture.

Design for the night condition. Site the unit away from bedroom windows, yours and the neighbour's, avoid narrow passages where reflections add several decibels, and mount on anti-vibration feet on a solid base rather than bracketing it to a bedroom wall. Structure-borne transmission through a wall is the complaint that no acoustic housing will fix.

Siting, housings and barriers

Acoustic housings are sold heavily and used badly. Any enclosure restricts airflow, and restricted airflow reduces the efficiency the heat pump was installed to deliver, sometimes to the point where the system cannot meet the heat demand on the coldest days. Check the manufacturer's clearance requirements before buying anything.

A solid barrier is usually the better answer. It needs to be imperforate, heavy enough not to transmit sound through it, and tall enough to break the line of sight from the unit to the neighbour's window. A slatted screen that hides the pump from view does close to nothing acoustically.

Better than either is distance and orientation. Moving a unit four metres further from a boundary, and turning the discharge away from the neighbour, costs pipework rather than equipment. And it does nothing to the efficiency of the system, which an enclosure certainly will.

What to submit

For a permitted development installation, keep the MCS calculation sheet, the manufacturer's declared sound data and a plan showing distances on file. There is nothing to submit to the council, but if a neighbour complains the enforcement officer will ask for exactly that package, and producing it the same week usually ends the matter.

Where a certificate of lawfulness is wanted for peace of mind, an application under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that the works are lawful.

For a full application, submit a site plan with distances to every neighbouring habitable room window, elevations showing the unit and any screening, the declared sound power data, and a Noise Impact Assessment using BS 4142 with the background survey period stated. Where the pump is part of a wider low carbon strategy, the reasoning belongs in the Energy Statement alongside the decision to go all-electric.

Enter your postcode on PlanScope to see which reports your property is likely to need before you order the unit.

This is general guidance, not legal advice. A lawful development certificate is the only formal way to confirm that works are lawful, so 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.