Plant noise assessment for a rooftop air source heat pump

Most domestic air source heat pumps go in without a planning application, and the reason is a permitted development class that comes with a noise condition attached. Heat pump noise planning problems start when that condition is treated as a formality. The unit has to meet a sound level limit at the neighbour's window, calculated before installation, and if it does not, the permitted development route closes and you need planning permission.

The limit itself is 42 decibels, A-weighted, measured or calculated at the assessment position. That position is normally the nearest habitable room window of a neighbouring property, taken one metre from the facade.

When a heat pump needs planning permission

Class G of Part 14 of the General Permitted Development Order covers air source heat pumps on domestic premises in England. It permits installation subject to a set of limitations and conditions covering where the unit can go, how big it can be, what it is used for and the noise it makes.

Those limitations have been amended more than once, including relaxations to the boundary distance rule and to the number of units allowed. Read the current consolidated text on legislation.gov.uk before you rely on any summary, including this one, because guidance written a year ago may describe rules that have since changed.

The situations where you will almost certainly need an application:

• The property is listed. Listed building consent is needed for a unit fixed to the building, and often for one in the curtilage of a listed building.
• The unit would sit on a wall or roof fronting a highway in a conservation area or World Heritage Site.
• Permitted development rights have been removed by an Article 4 direction or by a condition on the original planning permission, which is common on newer estates.
• The property is a flat and the installation affects the external fabric of the building.
• The calculated sound level at the assessment position exceeds the limit.

If you are relying on permitted development and the answer matters, a lawful development certificate is the only formal way to confirm the works are lawful. It costs half the fee of a householder application and it removes the argument entirely if you sell the house later.

The noise condition attached to permitted development

The condition requires the installation to comply with the relevant planning standards for air source heat pumps, which is where the 42 dB(A) figure comes from. Compliance is demonstrated by calculation rather than by measurement, using the manufacturer's declared sound power level and the geometry between the unit and the assessment position.

What the calculation accounts for:

1. The sound power level of the specific unit at the specific operating mode, taken from the manufacturer's data sheet rather than from a marketing brochure.
2. The distance from the unit to the assessment position.
3. Whether the unit sits against a wall, in a corner formed by two walls, or free-standing, because reflecting surfaces raise the level.
4. Any barrier between the unit and the window that fully blocks the line of sight.

The corner effect is the one that catches installers out. Tucking a unit neatly into the angle between the back wall and a projecting utility room can add several decibels compared with the same unit on an open wall, and it is often the difference between passing and failing.

Siting to reduce the level at the boundary

Distance is the control that costs you least. Sound from a point source falls off with distance, and doubling the separation between the unit and a neighbour's window produces a worthwhile reduction for no cost beyond a longer pipe run.

Order of preference in a typical suburban garden:

• The furthest available elevation from any neighbouring habitable window, even if that means a longer refrigerant or water run.
• An open wall rather than an internal corner.
• Behind an existing solid structure, such as a garage or an outbuilding, so the line of sight to the neighbour's window is broken.
• Away from the shared boundary with the closest neighbour, rather than from the nearest boundary in the abstract.

What does not help: pointing the fan away from the neighbour. The fan discharge has some directionality, but the compressor and casing radiate in all directions, and rotating a unit rarely changes the calculation by much.

One more practical point. Heat pumps run hardest on the coldest nights, which are also the quietest nights, when background sound drops and a hum that is inaudible in August becomes obvious in January. Design for the winter condition.

Acoustic enclosures and their trade-offs

Acoustic enclosures, louvred screens and absorptive fences all reduce the level at a receptor, and all of them restrict airflow.

A heat pump needs a large volume of air through the evaporator. Restrict it and the unit works harder, the coefficient of performance falls, and the running cost rises. Worse, some units respond to restricted airflow by running the fan faster, which puts some of the noise straight back.

The workable compromises:

• A solid barrier close to the unit on the receptor side only, leaving intake and discharge paths clear, with the barrier taller than the unit and long enough to break the line of sight.
• Manufacturer-approved enclosures designed for that model, with the clearances the manufacturer specifies.
• Anti-vibration mounts and a solid, level base, because structure-borne vibration through a wall bracket produces complaints that no amount of screening fixes.

Roof-mounted units on flats deserve the same caution. A unit bolted to a flat roof deck sits directly above someone's ceiling, and the vibration path through the structure matters more than the airborne level. Use isolated mounts on a purpose-made frame, and get the level checked at the windows of the flats below as well as at the neighbouring block.

Avoid wall-mounting on a habitable room wall, whether that room is yours or a neighbour's. A bracket bolted into the cavity leaf of a bedroom wall transmits low frequency vibration directly into the room, and the resulting complaint is about something the sound level calculation never captures.

Neighbour objections and what answers them

Where an application is needed, the objections follow a pattern: constant hum at night, a tonal quality, vibration through the party wall, and the fear that the unit will be replaced with a louder one later.

The evidence that answers them is specific rather than reassuring. A calculation sheet showing the sound power level of the actual unit, the distance, the correction for reflecting surfaces and the resulting level at the named window. A drawing showing where the unit sits and where the assessment position is. Confirmation of anti-vibration mounting. And a willingness to accept a condition limiting the sound level rather than limiting the model.

Where the scheme is larger than a single domestic unit, or the local authority has concerns about existing background sound, a full assessment under the standard method for industrial and commercial sound may be required instead. That method compares the rating level of the plant against the measured background, and a Noise Impact Assessment prepared that way is a different and more involved piece of work. The point at which acoustic screening stops helping is covered in what an acoustic barrier actually achieves.

Remember also that meeting a planning condition does not close off a statutory nuisance complaint later, a distinction explained in statutory nuisance after planning permission is granted.

Evidence to submit

For a permitted development installation, keep the following on file even though nobody asks for it at the time:

1. The manufacturer's data sheet showing sound power level.
2. The completed noise calculation with the assessment position identified on a plan.
3. A dated photograph of the installed unit and its mounting.
4. The installer's certificate.

For a planning application, submit the calculation, a site plan at 1:200 showing the unit, the boundary and the nearest neighbouring windows, an elevation showing the mounting height and any screening, and a short statement dealing with vibration isolation and expected operating hours.

The honest uncertainty in all of this is the neighbour's expectation. A unit that passes the calculation comfortably can still annoy someone whose bedroom window is four metres away and who has never heard mechanical plant in their garden before. Talking to them before installation is worth more than any document.

Enter your postcode on PlanScope to check which reports a project at your property is likely to need.

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.