Air source heat pump noise in a domestic garden

An air source heat pump can usually go in your garden without planning permission, but only if it passes a noise calculation first. The permitted development right for domestic heat pumps carries a condition that the installation meets the microgeneration certification planning standards, and those standards set a sound limit of 42 dB(A) at the nearest neighbouring window. Heat pump noise and planning permission are therefore linked by arithmetic rather than by opinion: if the calculated level at your neighbour's window exceeds the figure, the right does not apply and you need an application.

The noise condition attached to the permitted development right

Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015 covers the installation of an air source heat pump on domestic premises. The right is conditional, and the conditions have been amended more than once, including changes that relaxed the old boundary distance requirement and the limit on the number of units. Read the current text on legislation.gov.uk rather than an article, including this one, because the wording moves.

The condition that has stayed constant in substance is compliance with the microgeneration certification planning standards, commonly referred to as MCS 020. That document sets out how to calculate the sound level at a defined assessment position and requires the result not to exceed 42 dB(A).

Other conditions matter too. The unit must be used for heating purposes, it must be removed as soon as reasonably practicable when no longer needed, and on designated land, meaning conservation areas, National Landscapes, National Parks and World Heritage Sites, the right is narrower. A listed building needs listed building consent for the installation whatever the planning position, and that is a separate application.

The detail of the condition itself, including how the paperwork is expected to be kept, is covered in the heat pump noise permitted development condition.

Where the assessment position sits

This is the part most people get wrong. The calculation is not done at your boundary, and it is not done at the unit.

The assessment position is one metre from a habitable room window or door of the nearest neighbouring property. Habitable means a room used for living or sleeping, not a bathroom, hall or utility room. So the distance that matters is from your heat pump to your neighbour's living room or bedroom window, measured in a straight line, and on a narrow terraced plot that distance is often shorter than owners expect.

Two consequences follow. A unit tucked against your house on the far side from the neighbour can pass comfortably, while the same unit five metres away in the middle of the garden can fail, because the middle of the garden is closer to the window that counts. And where two neighbouring properties both have windows facing your garden, the calculation has to be done for the worst one.

The method behind the number, including the corrections applied for reflective surfaces and barriers, is set out in air source heat pump noise and the MCS 020 standard.

Siting decisions that decide the result

Before anyone runs a calculation, four decisions have already fixed most of the answer.

The first is which unit. Manufacturers publish a declared sound power level, and the range between the quietest and noisiest units of similar output is wide enough to turn a fail into a pass. Ask for the figure in writing before you order, and ask whether the unit has a night or quiet mode and what output it delivers in that mode.

The second is distance from the neighbour's window, which is a straight trade against distance from your own plant room and the length of the pipe run.

The third is what is behind and beside the unit. A heat pump standing clear on the ground radiates sound in fewer directions than one pushed against a wall, and one wedged into a corner formed by two walls is worse again. The calculation accounts for this with a directivity factor that increases with each reflecting surface, so an alcove that looks tidy can add several decibels. Avoid corners, avoid recesses, and avoid siting the unit so it fires across a hard paved yard towards the neighbour.

The fourth is whether a barrier can help. A fence only earns a correction where it blocks the line of sight between the unit and the assessment position, and where it is solid, continuous and close enough to work. A standard lap panel fence with gaps between boards does very little, and the difference between a barrier that works and one that does not is explained in when an acoustic barrier actually reduces noise.

What happens when the calculation fails

You have three routes, and they are not equally attractive.

1. Change the unit or the siting and recalculate. This is almost always the least costly answer, and a competent installer will run the calculation for two or three options before committing.
2. Add mitigation that the standard recognises. That means a barrier meeting the criteria, not a decorative screen, and it means checking that the enclosure does not restrict the airflow the unit needs.
3. Apply for planning permission. The right is not the only route, and an application is often granted.

What does not work is proceeding anyway on the basis that the neighbour seems relaxed about it. If the calculation does not pass, the right does not apply, the installation is unauthorised development, and the position surfaces when either of you sells the house.

Applying for permission instead

A householder application for a heat pump that cannot meet the standard is a small application with a narrow issue. The fee is the standard householder rate, the determination period is usually eight weeks, and the case officer will normally consult environmental health.

Officers assess it on amenity rather than against the 42 dB(A) figure, and a proportionate noise case helps. For a single domestic unit that usually means the manufacturer's data, the calculated level at the nearest habitable window, the background sound level in the garden if it has been measured, and a description of the mitigation offered. Where the situation is more complicated, for example a unit serving several flats or a site where the background level is very low at night, a full Noise Impact Assessment using the industrial and commercial sound standard is the proportionate response.

Expect a condition. Common wording fixes the unit model, requires it to be maintained in accordance with the manufacturer's instructions, and occasionally restricts operation to a defined output at night.

Neighbour complaints and statutory nuisance

Planning is not the only control, and a permitted or permissioned installation can still cause a problem.

Where a heat pump is intrusive in practice, a neighbour can complain to the council's environmental health team, which has duties under the Environmental Protection Act 1990. If the noise amounts to a statutory nuisance, the council can serve an abatement notice, and the defence available to a householder is that the best practicable means were used to prevent or counteract it. Compliance with the planning position is useful evidence, and it is not a complete answer.

Most complaints are not about the steady running level. They are about tonal hum at low frequencies, which the 42 dB(A) figure does not capture well, about the defrost cycle, which is louder and intermittent, and about vibration transmitted through a wall or through pipework clipped to a party structure. Anti-vibration mounts, a concrete pad rather than wall brackets, flexible pipe connections and resilient clips address most of it, and they cost very little at installation.

Talk to the neighbour before the unit arrives. It takes ten minutes, and it changes how a marginal noise situation is experienced.

Specifying and commissioning the unit

Keep a file. If the installation relies on permitted development, the calculation showing compliance is the only evidence you will have that the right applied, and you will want it at sale.

The file should hold the manufacturer's declared sound power data for the model actually fitted, the calculation with the assessment position identified on a plan, photographs of the installed position showing clearances and any barrier, the commissioning record, and the certification paperwork. If the installer will not provide the calculation, that is a reason to use a different installer.

Where the position is close to the limit, or where you have a difficult neighbour relationship, a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 puts the question beyond argument before the unit goes in. It costs a fee and some weeks, and it is the only formal confirmation that the works are lawful.

Enter your postcode on PlanScope to see which supporting documents a heat pump or wider retrofit application is likely to need. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval.

This is general guidance, not legal advice. Check the current wording of the permitted development right and the certification standards, and confirm the position with your local planning authority or a qualified professional before relying on it.