Air source heat pump siting, noise and the boundary rule

An air source heat pump at a house is permitted development under Class G of Schedule 2 Part 14 of the General Permitted Development Order, provided a set of conditions is met, and the condition that decides most installations is the noise one. The unit has to be installed in accordance with the relevant microgeneration certification planning standards, which require a calculation showing the sound level at the assessment position does not exceed 42 decibels. That assessment position is 1m from the nearest window of a habitable room in a neighbouring property.

Get that calculation wrong, or install without doing it, and the installation is not permitted development at all, whatever the boundary distances say.

The permitted development conditions

Class G covers the installation, alteration or replacement of an air source heat pump on a dwellinghouse or a block of flats, or within the curtilage of a dwellinghouse. The conditions that matter in practice are these.

The pump must be used solely for heating purposes. The installation must comply with the microgeneration certification scheme planning standards, or equivalent standards, which is where the noise requirement sits. There is a volume limit on the outdoor compressor unit. The unit must not be installed on a pitched roof, and where it goes on a flat roof it must not be within 1m of the external edge. On designated land there are restrictions on walls and roof slopes that front a highway. And the equipment must be removed as soon as reasonably practicable when no longer needed for microgeneration.

Part 14 has been amended several times since 2015, including changes in England during 2025 that relaxed the siting restrictions and raised the volume limit. So the single most useful thing you can do before quoting a dimension to anybody is open the current consolidated text of the Order and read Class G as it stands. The structure of the class is stable. The numbers inside it are not.

The boundary distance and what replaced it

For most of the life of Class G the condition required the outdoor unit to be at least 1m from the boundary of the curtilage of the dwellinghouse. That restriction ruled out a great many terraced and semi-detached installations, because the only sensible position for the unit was tight against a side or rear boundary.

That boundary restriction was removed in England by amendment in 2025, and the volume limit for the outdoor unit was increased at the same time. The policy reasoning was straightforward: the noise standard already controls the effect on neighbours, and a crude distance rule was blocking installations that the noise calculation would have passed.

What that leaves is a system where the acoustic test does the work. A unit 300mm from a boundary that passes the calculation at the neighbour's window is permitted development. A unit 3m from the boundary that fails the calculation is not. The distance from the boundary is now a design input rather than a rule.

Check the position for your nation. The devolved administrations have their own permitted development regimes, and Scotland, Wales and Northern Ireland each set their own conditions for heat pumps, so an English change does not carry across.

The noise test the conditions require

This is the part householders and some installers skip, and it is the part an enforcement officer will ask about.

The calculation works from the sound power level of the specific unit, declared by the manufacturer, and adjusts it for the distance to the assessment position and for the acoustic environment of the position in which it is installed. The adjustments are the interesting part. A unit in a corner between two walls reflects sound and scores worse than the same unit in open air. A unit under a soffit is worse again. A unit with a barrier in the sound path scores better.

The output is compared against 42 decibels at the assessment position, which is 1m from the nearest habitable room window of a neighbouring property. Habitable means a room used for living or sleeping, so a bathroom window or a landing window is not the assessment point.

Three errors that recur:

1. Using the manufacturer's headline sound pressure figure quoted at a convenient distance rather than the declared sound power level the calculation requires.
2. Taking the assessment position as the boundary rather than the neighbour's window, which can be far closer or much further away.
3. Ignoring the reflective surfaces created by the chosen position, particularly external corners, narrow passages between houses and recessed porches.

Keep the completed calculation. It is the evidence that the installation was permitted development, and the person who will want it is a buyer's solicitor in eight years.

Conservation areas, listed buildings and flats

The designations change the answer completely.

Within the curtilage of a listed building, Class G does not apply, so an air source heat pump needs planning permission and will usually need listed building consent as well, since fixing a unit to the building and running pipework through the wall affects its character. A heritage statement addressing the position, the visual effect and the intervention in historic fabric is what carries that application.

On designated land, which includes conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, the class restricts installation on walls or roof slopes that front a highway, and the detail of those restrictions has changed with the amendments. A unit in a rear garden, out of public view, is the uncontroversial position in a conservation area and the one most officers will accept without argument.

Flats are treated differently from houses. The class does extend to blocks of flats in respect of installations on the building, but a unit on a balcony or an external wall of a flat raises management, lease and party structure questions that planning does not touch. Check the lease before the Order.

And an Article 4 direction can remove Class G rights, as it can remove any permitted development right. Check the direction's schedule rather than assuming it covers only extensions.

Siting against the neighbour's window

The acoustic calculation deals with the single nearest habitable room window. Good siting deals with the house as a whole.

Positions that work: a rear corner of the garden away from both your own and the neighbour's bedrooms, with the fan discharge directed away from windows and across open ground. A side passage is tempting because it is out of sight, and it is usually the worst acoustic position available because the parallel walls reflect sound and the passage funnels it towards the back of the neighbouring house.

Think about low frequency content and about night-time operation. A unit that meets 42 decibels on the calculation can still be audible in a quiet suburban garden at 2am, and the complaints that reach environmental health are almost always about night operation rather than about daytime levels. A statutory nuisance investigation under the environmental protection legislation is a separate route from planning, and a unit that is lawful permitted development can still be the subject of an abatement notice.

So talk to the neighbour before installation. It costs an afternoon and it prevents the one outcome nobody wants, which is a lawful installation that has to be moved to keep the peace.

Where the scheme is larger than a single dwelling, or where the unit serves a block, a noise impact assessment following the recognised industrial and commercial sound methodology is the proportionate piece of work, and councils usually expect a suitably qualified author for it with professional review strongly advised.

Acoustic enclosures and how they look

Enclosures and barriers do reduce the calculated and measured level, and they come with two costs.

The first is performance. An air source heat pump needs a large volume of air across the coil and a clear discharge path. Enclose it badly and you restrict airflow, short-circuit warm discharge air back into the intake and lose efficiency, which shows up as a higher electricity bill and a system that struggles on the coldest days. Follow the manufacturer's clearance requirements rather than a joiner's instinct about what looks tidy.

The second is appearance. A timber-slatted box in a front garden in a conservation area reads as a visually prominent structure, and if the enclosure itself is more than 1m high next to a highway it has its own permitted development limits under the rules for means of enclosure.

The arrangements that tend to succeed are a solid barrier on the side facing the neighbour with open space on the other three, planting in front of a barrier rather than instead of it, and a plinth that lifts the unit clear of ground debris while keeping the overall height modest.

What to record before installation

Five things, kept in a folder with the deeds:

• The manufacturer's declared sound power level for the exact model installed, not a similar one in the range.
• The completed noise calculation, naming the assessment position used and the acoustic adjustments applied.
• A plan showing the unit position with distances to the boundary and to the nearest habitable room windows on all neighbouring properties.
• Photographs of the installed position and of any barrier or enclosure.
• Confirmation that the installation complies with the applicable microgeneration planning standards, from the installer.

Where any condition of Class G cannot be met, apply for planning permission rather than installing and hoping. A short planning statement addressing noise, visual effect and the carbon benefit is usually enough for a householder application, and the conditions a council attaches to such a permission are worth understanding in advance, as set out in air source heat pumps and the conditions attached.

This is general guidance, not legal advice. Check the current text of Part 14 of the GPDO 2015 and the position with your local planning authority before relying on it. A Lawful Development Certificate is the only formal way to confirm that an installation is lawful without a planning permission.

To see which documents a heat pump or wider retrofit project is likely to need, enter your postcode on PlanScope and work through the guided questions. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.