Air source heat pumps and the conditions attached

An air source heat pump on a house is usually permitted development under Class G of Part 14 of the General Permitted Development Order 2015, and the right comes with conditions that do real work. The one that decides most installations is noise. Class G requires the installation to comply with the MCS Planning Standards, which set a calculated sound level limit of 42 dB(A) at the assessment position, taken one metre from the nearest habitable room window of a neighbouring property. On a terraced or semi-detached plot with a narrow side passage, that calculation is what determines where the unit can go, and sometimes whether the right is available at all.

Two limits in Class G have been amended in England in recent years, including the volume allowed for the outdoor unit and the minimum distance from the boundary. Check the current text of Class G on legislation.gov.uk before you rely on a figure you read elsewhere.

The shape of the permitted development right

Class G covers installation, alteration or replacement of an air source heat pump on or within the curtilage of a dwellinghouse, and in the terms used by the Order, on or within the curtilage of a block of flats.

The limitations it imposes fall into four groups. Size, through a maximum volume for the outdoor compressor unit including its housing. Siting, through restrictions on distance from the curtilage boundary and on installation on roofs. Quantity, through a limit on how many units the right allows within one curtilage. And exclusions, which take listed buildings, land within the curtilage of a listed building and scheduled monuments outside the right altogether.

The conditions attached are separate from the limitations and they continue to apply after installation:

• The installation must comply with the MCS Planning Standards, or equivalent standards.
• The heat pump must be used solely for heating purposes.
• It must be removed as soon as reasonably practicable when no longer needed.
• Siting must be arranged so far as practicable to minimise the effect on the external appearance of the building and on the amenity of the area.
• On article 2(3) land, which covers conservation areas, National Parks, National Landscapes and World Heritage Sites, there are further restrictions on installation on a wall or roof facing a highway.

That third condition surprises people. A heat pump left in place and disconnected after a later conversion to something else is a breach, not a leftover.

Siting and the noise calculation

The 42 dB(A) figure is calculated rather than measured, using the manufacturer's declared sound power level for the specific unit, the distance to the assessment position, and a limited set of allowances for barriers and reflective surfaces.

What moves the result:

1. The declared sound power of the unit. Quieter models exist and the difference between models of the same output can be several decibels.
2. Distance to the nearest neighbouring habitable room window, which is the whole reason a rear garden position beats a side passage.
3. Whether the unit sits in a corner formed by two walls, which adds reflections and makes the calculated level worse.
4. Whether an existing solid structure sits between the unit and the assessment position.
5. The number of units, since two units on the same elevation do not have the same effect as one.

Get the installer to produce the calculation sheet and keep it. It is the document that evidences compliance with the condition, and without it you have no answer to a complaint or an enforcement enquiry years later.

Where the calculation comes out marginally under the limit, treat that as a warning rather than a pass. Neighbour complaints about heat pumps are typically about a low-frequency hum or about night-time operation, and neither is captured by a single A-weighted figure.

Conservation areas, listed buildings and flats

A listed building takes you outside Class G entirely. That means planning permission for the installation, and listed building consent under section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 for any works affecting the building's character, which a wall-mounted unit, pipework penetrations and a condensate run usually will. A Heritage Statement is the document that makes that case.

Land within the curtilage of a listed building is caught too, which catches outbuildings, former coach houses and garden positions at houses where the listing extends further than owners expect.

In a conservation area or other article 2(3) land the right survives, with restrictions on walls and roofs facing a highway. A rear garden position is normally the workable answer. A front garden unit on a terrace in a conservation area is normally an application.

For a flat, the position depends on the wording of the right as it applies to blocks of flats, and it depends much more on the lease. The freeholder's consent to fixing plant to the structure and to running pipework through common parts is a separate matter from planning, and it is the one that stops most flat installations. The same pattern applies to flues, vents and stoves on a house elevation, where owners assume the planning answer settles the question.

More than one unit

The right limits how many units the curtilage may have, so a larger house needing two outdoor units, or a house that already has a unit serving a swimming pool or an annexe, can exceed the limit.

Two responses. Specify a single larger unit where the heat demand allows, which is often possible once the fabric is improved. Or apply for planning permission for the second unit, which for a rear garden position on an unconstrained plot is usually straightforward and takes eight weeks.

Do not install the second unit and hope. An unauthorised outdoor unit is visible from a neighbour's window and the complaint route is easy.

The regimes running alongside

Planning is one of four things happening, and the other three are not optional.

The supply question is the one most often discovered late. An older property with a modest main fuse may need an upgrade, and lead times for a supply upgrade are measured in weeks or months rather than days.

When an application is the safer route

Five situations where I would apply rather than rely on the right:

The building is listed, or lies within the curtilage of one. There is no choice here.

The only workable position is in a front garden or on a wall facing a highway in a conservation area.

The noise calculation lands within a decibel or two of the limit, or the nearest habitable room window belongs to a neighbour who has already objected to something.

The scheme needs more units than the right allows.

Or the property has already been extended close to a boundary and the remaining space makes the siting conditions arguable.

In each of those cases, a Planning Statement explaining the heating strategy, the siting options considered and the noise position does most of the work, with a Noise Impact Assessment where the acoustic case is contested. Councils usually expect a suitably qualified author for the assessment, and professional review is strongly advised.

Where you need certainty rather than a view, a Lawful Development Certificate for proposed development under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that an installation falls within Class G. The fee is lower than a planning application and the certificate binds the council, which matters at sale.

Keep three documents in a folder from the start: the MCS calculation sheet, the installer's certificate, and a dated photograph of the installed unit with a tape measure showing its distance from the boundary. That file answers every question anyone asks later.

PlanScope can produce an AI-assisted draft Planning Statement, Heritage Statement or Noise Impact Assessment from your postcode and a few answers. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which documents a heat pump installation on your property is likely to need. This is general guidance, not legal advice; check the current text of Class G and the position with your local planning authority or a qualified professional before relying on it.