Air source heat pumps: planning limits and noise assessment
Most domestic heat pumps go in under permitted development, subject to conditions including a noise standard. The limits have changed more than once, so check the current Order.
Most domestic air source heat pump installations in England go ahead without a planning application, under Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015. That is the general position, and it comes with conditions attached: the equipment has to meet a recognised planning standard for noise, there are limits on size and siting, and the rights are cut back around listed buildings and in some conservation area situations. Air source heat pump planning permission becomes necessary when any one of those conditions is not met.
The limits in Class G have been amended more than once in recent years, including the siting restrictions and the size cap. Rather than repeating figures that may already have moved, check the current text of the Order on legislation.gov.uk, or the position with your council, before you order equipment. That is not a hedge. It is the specific thing that catches householders out.
When a heat pump is permitted development
Class G sets out a structure that has stayed stable even as the numbers have changed. The conditions cover, in outline:
- Compliance with a recognised planning standard for the noise the unit makes, assessed at a neighbouring property.
- A limit on the volume of the outdoor compressor unit.
- Restrictions on siting relative to the boundary of the curtilage and, for flat and pitched roofs, relative to the roof edge.
- Restrictions on installation on a wall or roof fronting a highway in a conservation area or World Heritage Site.
- Exclusion within the curtilage of a listed building.
- A requirement that the equipment is used solely for heating purposes and is removed as soon as reasonably practicable when no longer needed.
Two of those deserve emphasis. The noise standard is a condition of the permitted development right, not a guideline, so an installation that fails it has no permitted development rights at all and is unauthorised from the day it is switched on. And the listed building exclusion covers the curtilage, which takes in outbuildings and, depending on the history, parts of the garden.
The position for flats and for blocks of flats differs from that for houses, so check the wording that applies to your building type rather than assuming the householder rules carry across.
The noise standard attached to the class
The recognised standard used for this purpose is the MCS planning standard, which works from the manufacturer's sound power data and a calculation to an assessment position at a neighbouring habitable room window. The calculation accounts for distance, any barrier between the unit and the assessment position, and reflections from nearby surfaces. The pass figure is 42 decibels at that assessment position.
That figure is more demanding than it sounds once reflections are in play. An enclosed side passage between two houses, with the unit facing a wall a metre away, adds reflected sound that an open garden position would not. The same unit can pass in one position and fail three metres away.
Ask the installer for the completed calculation sheet, not just a statement that the installation complies. It should name the unit, its sound power level, the distance to the assessment position, the reflection and barrier assumptions, and the resulting figure. Keep it.
Conservation areas, listed buildings and Article 4 areas
In a conservation area or World Heritage Site the restriction bites on visibility from the highway, so a unit at the rear is usually acceptable while one on a front elevation is not. Check the precise wording, because it turns on the elevation the equipment is installed on rather than on whether the unit can be seen.
Within the curtilage of a listed building, permitted development is not available and a planning application will be needed. Separately, and this catches people, listed building consent is likely to be required for fixing pipework and brackets through a listed elevation or for internal works affecting historic fabric, and that consent is required whether or not planning permission is. Where a listed building is involved, a Heritage Statement explaining the effect on significance and the alternatives considered will be expected with the application.
An Article 4 direction can remove permitted development rights in a defined area, most often in conservation areas and most often covering front elevations. Councils publish the directions in force on their websites, and checking takes a few minutes.
How installation noise is assessed
Two separate regimes, and the difference matters.
Under the permitted development route, the MCS planning standard calculation is what counts, carried out before installation. Where a full planning application is needed, the council will normally expect a noise assessment following BS 4142:2014+A1:2019, comparing the rating level of the unit against the measured background sound level at the nearest neighbour, with corrections for tonal or intermittent character. That is a different and generally more demanding exercise, and it usually needs a Noise Impact Assessment from an acoustician rather than a calculation sheet from an installer.
Heat pumps have two acoustic features worth knowing about. The fan produces a broadly steady sound that is easy to model. The defrost cycle, which runs periodically in cold damp weather, produces a different and more noticeable sound, and it is the one neighbours complain about because it happens intermittently, often at night, in still conditions when background levels are lowest.
Siting decisions that avoid an objection
Siting does more for neighbour relations than any specification choice.
| Situation | Better approach |
|---|---|
| Narrow side passage | Avoid; reflections raise the level substantially |
| Facing a neighbour's bedroom window | Reorient the fan discharge or move to another elevation |
| Hard against a wall | Leave the manufacturer's clearance, which also protects efficiency |
| On a timber deck or hollow structure | Mount on a solid base with anti-vibration feet |
| Fixed to the house wall | Prefer a ground-mounted position to avoid structure-borne noise |
| Shared boundary in a terrace | Keep the discharge direction away from the party boundary |
The single most effective move is to keep the unit off the building fabric. A wall-mounted unit transmits vibration into the structure, which travels through the party wall into a neighbour's house where no external measurement will pick it up. Ground mounting on a concrete pad with anti-vibration mounts avoids most of that.
Illustrative example. A semi-detached house with a unit proposed in the 1.2 metre side passage, facing the neighbour's kitchen window six metres away, fails the calculation on reflections alone. Moving it to the rear garden corner, ground-mounted, with the fan facing the owner's own house, passes comfortably and costs a little more in pipe run. The figures are illustrative.
Applying where permitted development does not apply
If the installation cannot meet the Class G conditions, a householder planning application is the route. It is not usually a difficult application, provided the noise question is answered properly.
Submit the unit specification with sound power data, a site plan showing the position and the distances to neighbouring windows, an elevation showing the appearance, and the acoustic evidence. Where the council's environmental health team is consulted, a BS 4142 assessment answering their likely questions in advance saves a round of correspondence. A short Planning Statement setting out why the position was chosen, and which alternatives were rejected and why, is worth the half hour it takes.
Expect a condition limiting the noise level at the boundary or at a named window, and possibly a condition on hours if the scheme is unusual. Read any condition against how the system actually runs, since a heat pump that cannot operate overnight is a heat pump that cannot heat the house.
Evidence to keep after installation
Permitted development rights depend on conditions being met, and the burden of showing that falls on the householder, usually years later when selling the property.
Keep the MCS certificate for the installation, the completed noise calculation sheet, the manufacturer's data sheet for the specific unit, dated photographs showing the position and its distance from the boundary, and the commissioning record. Store them with the property documents rather than in an email folder.
Where the position was arguable, or where a neighbour has raised a concern, applying for a Lawful Development Certificate is the only formal way to confirm that the installation is lawful. It costs a fee and some paperwork, and it converts an argument into a document. Solar installations follow a similar pattern under different classes, as our note on rooftop solar under Class J sets out for commercial buildings.
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. You can enter your postcode on PlanScope to see which reports a site is likely to need.
Frequently asked questions
Do I need planning permission for an air source heat pump?
Usually not. Most domestic installations in England fall under Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015, subject to conditions on noise, size and siting. Permission is needed where a condition cannot be met, within the curtilage of a listed building, and in some conservation area situations. The limits have been amended, so check the current Order.
What is the 42 decibel rule?
It is the pass figure in the MCS planning standard used to satisfy the noise condition in Class G. The calculation works from the unit's sound power data to an assessment position at a neighbouring habitable room window, accounting for distance, barriers and reflections. Failing it means the permitted development right does not apply and the installation is unauthorised.
Can I fit a heat pump to a listed building?
Not under permitted development, since the right is excluded within the curtilage of a listed building, so planning permission is required. Listed building consent is also likely to be needed for fixings, pipe penetrations and any internal works affecting historic fabric, and that is a separate consent. Expect to explain the effect on significance and the alternatives you considered.
Why do neighbours complain about defrost cycles?
Because they sound different from normal running and happen intermittently. In cold damp weather the unit reverses to clear ice from the coil, producing a change in tone and sometimes a brief rush of sound, often at night when background levels are lowest. Steady fan noise is easier to model and easier to ignore than an intermittent change.
Should the unit be mounted on the wall or on the ground?
On the ground where possible, on a solid base with anti-vibration mounts. A wall-mounted unit transmits vibration into the building structure, which can travel through a party wall into a neighbouring house and produce a complaint that no external noise measurement will explain. Ground mounting also makes the manufacturer's clearances easier to achieve.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.