Air source heat pump siting rules for a house

An air source heat pump at a house is usually permitted development under Class G of Part 14 of the General Permitted Development Order 2015, provided the installation meets the conditions in that class, including compliance with the microgeneration planning standard for noise. Air source heat pump planning permission becomes necessary where the house is listed, where the unit would sit on a wall or roof fronting a highway in a conservation area, where an Article 4 direction applies, or where the installation cannot meet the noise test at the neighbour's window.

Class G and the conditions that apply

Class G covers the installation, alteration or replacement of an air source heat pump on a house or a block of flats, or within the curtilage of one.

The conditions attached to the class do most of the work. The installation must comply with the relevant microgeneration certification planning standards, or equivalent standards. It must be used solely for heating purposes. It must, so far as practicable, be sited to minimise its effect on the external appearance of the building and on the amenity of the area. And it must be removed as soon as reasonably practicable when it is no longer needed.

The class also carries siting limits on where the unit may go, on its size, and on how many may be installed. Those specific figures have been amended in recent years, so read the current consolidated text of Part 14 on legislation.gov.uk rather than relying on an installer's leaflet or a guide written a few years ago.

Pitched roofs are the one restriction that has been consistent: a unit mounted on a pitched roof slope falls outside the class. Flat roofs are treated differently, with a limit on how close to the roof edge the unit may sit.

The noise test and the 42 dB(A) limit

This is the condition that decides most installations, and the one installers are most likely to gloss over.

The microgeneration planning standard for heat pumps requires the calculated sound pressure level from the unit not to exceed 42 dB(A) at the assessment position, which is taken at a habitable room window or door of a neighbouring property. The calculation accounts for the manufacturer's declared sound power level, the distance to the assessment position, and the acoustic effect of nearby surfaces such as walls and corners.

Three things push the result upwards, and all three are siting decisions:

• Placing the unit in a corner between two walls, which reflects sound and adds to the calculated level.
• Placing it close to the party boundary of a terraced or semi-detached house, which shortens the distance to the neighbour's window.
• Placing it under or beside a neighbour's window rather than under your own.

Ask the installer for the completed calculation, not a statement that the unit complies. The sheet should name the model, its declared sound power level, the distance to each relevant assessment position and the reflection corrections applied. If the answer comes back above 42 dB(A), the fix is usually to move the unit, choose a quieter model or select a lower fan speed setting, in that order. An acoustic enclosure is a last resort, because restricting airflow reduces performance and can increase noise at the defrost cycle.

Note that the standard governs the permitted development route only. A unit that fails it is not unlawful. It simply needs planning permission, and the application is then judged on amenity in the ordinary way.

Boundary distance, volume and how the rules changed

Older guidance on Class G refers to a minimum distance of one metre from the property boundary, a compressor unit volume limit of 0.6 cubic metres, and a limit of one heat pump per property. Those figures described the class as originally made.

The class has since been amended, with changes affecting the boundary requirement, the volume limit and the number of units permitted. The direction of travel has been towards making domestic installation easier. Take the current numbers from the consolidated Order on the day you specify the installation, because both installers and local authority websites are still circulating the older figures.

What has not changed is that the conditions still apply in full. A larger unit sited closer to a boundary still has to meet the noise standard at the neighbour's window, and the requirement to minimise the effect on external appearance and amenity so far as practicable still applies. Relaxing a dimensional limit does not relax the conditions.

Conservation areas and front elevations

In a conservation area, Class G does not permit installation on a wall or a roof which fronts a highway, and it does not permit installation on a building or structure within the curtilage of the house that fronts a highway.

That is narrower than people assume. A unit at the side or rear of a house in a conservation area can still be permitted development. A unit on the front elevation, or in a front garden alongside a building fronting the road, is not.

An Article 4 direction can remove Class G rights altogether in a designated area, and directions covering householder alterations in conservation areas are common. Check the council's Article 4 mapping before ordering.

Where permission is needed, the application is a straightforward householder one. Support it with a plan showing the unit's position and its distance to neighbouring windows, an elevation showing the unit against the wall, the noise calculation, and in a conservation area a short heritage statement covering visibility from the street and the effect on the character of the area. Screening in a painted timber enclosure with adequate free area for airflow answers most conservation objections.

Illustrative example

An illustrative case: a mid-terrace house in a conservation area, rear yard 4.5 m deep, neighbouring kitchen windows on both sides. The obvious position, in the corner beside the back door, sits 1.8 m from the neighbour's window and in a two-wall corner, and the calculation returns a level above the standard. Moving the unit to the far end of the yard against the rear boundary wall, with a single reflecting surface and 4 m to the nearest neighbouring window, brings it within the limit. The pipe run is 6 m longer and the installation costs more. It is permitted development, which the corner position was not.

Listed buildings and internal pipework

A listed building needs listed building consent for works affecting its character, and that duty is independent of any permitted development right. Class G also excludes installation within the curtilage of a listed building in the circumstances the Order sets out, so check both.

The external unit is only half the question. Heat pump installations in historic buildings involve internal interventions that conservation officers care about: core-drilling through solid walls for flow and return pipework, chasing for cabling, a hot water cylinder needing a plant space that did not previously exist, and often larger emitters or underfloor heating that affect historic floors.

The applications that succeed keep penetrations few, place them in later fabric or existing openings, run pipework in existing chases and voids, and record what is disturbed. Where a cylinder is needed, a former bathroom, a later extension or a modern partitioned space is a far easier location than a principal room.

Building fabric improvements usually have to come first for the system to work at low flow temperatures, and on a listed building those improvements have their own consent implications. Deal with the fabric strategy and the heat pump in the same application rather than in sequence, so the officer sees the whole picture.

When to submit a short noise note

Submit one whenever any of these apply, whether or not the installation is permitted development.

Where the assessment position is within about three metres of the proposed unit. Where the unit sits in a corner or an enclosed light well. Where the property is a flat or a terraced house with habitable windows on both sides. Where a neighbour has already raised the subject. And where you are applying for permission because the installation cannot meet the microgeneration standard, in which case the note has to do the persuading.

A useful note is two pages: the model and its declared sound power level, a plan showing the unit and every relevant window with distances, the calculation with its corrections, the resulting level at each position, and the mitigation proposed. Where the level is close to the limit, add the manufacturer's night setback specification.

Building Regulations apply regardless of the planning position, covering the electrical work, the condensate discharge, the structural fixing where the unit is wall-mounted, and the efficiency requirements for the system as a whole. Where you intend to claim grant support, the installer will normally need to be certified under the relevant microgeneration scheme, and the certificate is worth filing with the deeds.

The measurement discipline is the same as for any permitted development question. Where you measure from decides the answer, exactly as it does with the rear extension depth limits.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful.

Enter your postcode on PlanScope to check whether your house sits in a conservation area or an Article 4 area before you order a unit.

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