Air source heat pumps under Part 14 and the noise condition

An air source heat pump on a house is permitted development under Class G of Schedule 2 Part 14 of the Town and Country Planning (General Permitted Development) (England) Order 2015, provided it meets the conditions attached to that class. The condition that fails most often is not a dimension. It is the noise calculation.

Class G requires the installation to comply with the microgeneration certification scheme planning standards, or an equivalent standard, and those standards set a limit on the sound the unit may produce at the assessment position, which is the nearest habitable room window of a neighbouring property. The limit is 42 decibels. If the calculation comes out above it, the installation is not permitted development and needs a planning application.

That single number decides whether a terraced house in a tight urban street can have a heat pump without applying.

The conditions attached to Class G

The class covers the installation, alteration or replacement of an air source heat pump on a dwellinghouse or a block of flats, or on a building within the curtilage. The conditions that apply consistently are these.

The equipment must be used solely for heating purposes. It must be sited, so far as practicable, to minimise its effect on the external appearance of the building and on the amenity of the area. It must be removed as soon as reasonably practicable when no longer needed. It must comply with the microgeneration certification scheme planning standards or equivalent. And it must not be installed on a pitched roof, or within the curtilage of a listed building, or on land within the boundaries of a scheduled monument.

There is also a volume limit on the outdoor compressor unit and rules about how close the unit may be to a boundary. Both have been amended, so take the current figures from the text of the Order rather than from an installer's leaflet or an older article. That is the single most common source of bad advice on this subject.

On article 2(3) land, meaning conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, the pump must not be installed on a wall or roof that fronts a highway, nor on a wall or roof of a building within the curtilage. Flats have their own restrictions, including on flat roof positions.

What the noise assessment actually does

The microgeneration planning standard calculation is not a survey. It is a desk calculation that takes the sound power level of the specific unit from the manufacturer's data, applies distance attenuation from the unit to the assessment position, and adds or subtracts corrections for the acoustic environment around the unit.

The corrections are where installations pass or fail. Placing the unit in a corner between two walls reflects sound and adds to the result. Placing it in the open with a clear line to the neighbour's window is better acoustically than tucking it into a recess, which is the opposite of what most people assume. A barrier between the unit and the assessment position can reduce the figure if it is properly positioned and solid.

The assessment position is the nearest habitable room window of a neighbouring property, and it is measured at one metre from that window. Your own windows are not the test. In a terrace where the neighbour's kitchen window is four metres from the only sensible position for the pump, the calculation may simply not work at any location on the plot.

Keep the completed calculation. It is the evidence that the installation was permitted development, and a buyer's solicitor will ask for it.

When the calculation fails

Three routes remain open.

The first is to change the unit. Manufacturers publish sound power data and the range across the market is wide, so a quieter model at the same output can move a marginal calculation onto the right side of the limit. This is worth doing before anything else.

The second is to change the position or add a barrier. Moving the unit further from the neighbouring window, taking it out of a reflective corner, or placing a solid barrier in the sound path can all help. Check that the revised position still complies with the boundary rules in the Order.

The third is to apply for planning permission. A householder application for a heat pump is straightforward and is decided in eight weeks. The council will assess noise on its merits rather than against the 42 decibel standard, and it can and often does grant permission for installations that would fail the permitted development test, with a condition on the model and its position. Where the case is contested a short Noise Impact Assessment, measuring the existing background level rather than relying on a generic calculation, is what settles it.

Listed buildings, flats and designated land

A listed building is outside Class G entirely, and works to a listed building need listed building consent regardless of the planning position. Fixing a pump to the rear wall of a listed cottage is an alteration affecting the building's character, and it needs consent even where the visual effect is slight. A Heritage Statement assessing significance and the effect of the installation is the supporting document.

In a conservation area the right survives, subject to the article 2(3) restrictions on walls and roofs fronting a highway. What can remove it entirely is an Article 4 direction, and a number of authorities with large conservation areas have made directions covering householder alterations. Check the council's list before you commit.

Flats need the freeholder's consent and usually a check against the lease as well as the planning position, and in a block the practical constraint is normally where the unit can be fixed without affecting the common parts.

Getting certainty before installation

A lawful development certificate is the only formal way to confirm that works are lawful. For a heat pump, an application under section 192 of the Town and Country Planning Act 1990 for a certificate of lawful proposed development confirms that the installation falls within Class G, and it is decided on the documents rather than on an officer's view of whether the pump is a good idea.

Submit the manufacturer's data sheet, the completed planning standard calculation, a site plan showing the unit position and the distance to the nearest habitable room window of every neighbouring property, and a note on the barrier or acoustic environment corrections applied. Councils determine these quickly when the paperwork is complete.

Whether it is worth the fee depends on the property. On a detached house with 20 metres to the nearest neighbour, no. On a terrace, a semi, or anything in a conservation area, the certificate costs less than removing an installed unit and is the document that stops a complaint from becoming an enforcement investigation.

One last practical point. The installer's compliance paperwork covers the microgeneration scheme requirements and the electrical work. It is not a planning assessment, and an installer's assurance that heat pumps are always permitted development is not evidence of anything.

If the pump is going in alongside other work, an extension or a loft conversion, deal with it in the same application and show it on the elevations. You can enter a postcode on PlanScope to see which supporting documents a householder application on your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.