Air source heat pumps and permitted development limits
A heat pump at a house is usually permitted development, but the 42 decibel noise condition and the designated land rules catch a lot of installations.
Most air source heat pumps at a house in England go in under permitted development, using Class G of Part 14 of Schedule 2 to the GPDO 2015. The right comes with conditions rather than a free hand: one on noise, one on the size of the outdoor unit, several on where it can sit, and a set of exclusions for listed buildings and designated land.
The class was amended in 2024, which removed the old rule keeping the unit a metre from the boundary and raised the size limit. Plenty of installers and advice pages still quote the previous figures, so check the current text of Part 14 on legislation.gov.uk before you order anything.
The right in Part 14, and what it covers
Class G permits the installation, alteration or replacement of an air source heat pump on a dwellinghouse or a block of flats, or within the curtilage of either. That includes the outdoor unit, its housing and the pipework.
It does not cover ground source systems, which have their own class, and it does not cover a heat pump serving a commercial building. It also does not remove any other consent you need. A leaseholder in a converted flat still needs the freeholder's permission under the lease, whatever the Order says.
Where the right applies, no planning application is needed. Where any condition is broken, the installation is development and needs planning permission, which for a householder is a householder application.
Siting, size and the conditions attached
| Condition | What it requires |
|---|---|
| Noise | The installation must comply with the MCS planning standards, or equivalent |
| Size of the outdoor unit | Volume capped, including any housing, at the figure in the current text of Class G |
| Pitched roof | Not permitted on a pitched roof |
| Flat roof | Permitted, but at least 1 metre from the external edge |
| Designated land | Not on a wall or roof fronting a highway on article 2(3) land |
| Listed buildings | Not within the curtilage of a listed building |
| Use | Solely for heating purposes |
| Removal | Removed as soon as reasonably practicable when no longer needed |
The 2024 amendment removed the 1 metre boundary separation and raised the volume ceiling from the old 0.6 cubic metre figure, which had ruled out most units capable of heating a whole house. Take the current number from the Order itself.
Article 2(3) land means conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads. The restriction there is specific: it bites on a wall or roof that fronts a highway, so a unit tucked into a rear garden in a conservation area is usually still permitted development. A unit on the front elevation of the same house is not.
The noise condition, in practice
This is the condition that most often fails, and it is the one homeowners never think about.
Class G requires the installation to comply with the MCS planning standards. That standard sets an assessment position 1 metre from the centre of the nearest habitable room window or door of a neighbouring property, and a limit of 42 decibels A-weighted at that point. The calculation takes the unit's declared sound power level and applies corrections for distance and for reflective surfaces nearby.
Three things push a compliant unit over the line:
- Siting in a corner where two walls reflect sound back towards the neighbour.
- A terrace or narrow side passage, where the assessment position is only a few metres from the unit.
- Choosing a larger model late in the design without redoing the calculation.
Ask the installer for the completed calculation sheet and keep it. If the unit cannot meet 42 decibels at the assessment position, the options are to move it, to specify a quieter model, to add an acoustic barrier that the calculation can credit, or to apply for planning permission and support the application with a noise assessment.
Where the right does not apply at all
Listed buildings are out. Class G excludes installation within the curtilage of a listed building, so the route there is a full householder application plus listed building consent, and the heritage case has to address the fixings, the visual effect and the reversibility of the work.
An Article 4 direction can remove Class G across a conservation area or a single street. Check the council's list of directions before assuming the right exists, because several authorities have made directions covering external alterations that catch heat pumps without naming them.
Scheduled monuments, and land in a Site of Special Scientific Interest, bring separate consents. And a house built under a recent planning permission may carry a condition removing Part 14 rights, which happens more often on new estates than people expect.
Get any of this wrong and the installation is unlawful development from the day it goes in. Enforcement time limits have changed, and the move to ten years matters here, because the window in which a council can act on an unlawful installation is longer than it used to be.
Flats and shared buildings
Class G applies to a block of flats as well as a house, which surprises people. The practical obstacles are different.
The right belongs to whoever is carrying out the development, but the lease usually does not allow a leaseholder to fix plant to the external wall or to occupy common parts. Expect to need the freeholder's consent, a licence to alter, and agreement on where the condensate goes.
Where several flats each want a unit, the size and noise conditions are assessed for the installation in question, and a bank of units on one elevation raises a cumulative noise question that the standard calculation for a single unit does not answer. On anything beyond one or two, get an acoustician to look at the combined position before the first one is fitted.
Illustrative example. A mid-terrace house in a conservation area wants a 9 kilowatt unit in the rear yard, 2.4 metres from the neighbour's kitchen window with a boundary wall on one side and the house on the other. The declared sound power gives a calculated level above 42 decibels at the assessment position because of the two reflecting surfaces. Moving the unit 1.5 metres along the yard, away from the corner, brings it under. The figures are illustrative.
Confirming it, and what to do if it fails
A Lawful Development Certificate for proposed use or development, under section 192 of the Town and Country Planning Act 1990, is the only formal way to confirm that the installation is lawful. It costs half a householder application fee and it produces a document a future buyer's solicitor will accept. For a heat pump in a conservation area, or anywhere the siting is tight, it is money well spent.
Where the certificate route is not available because a condition is broken, apply for householder planning permission. The supporting material is short: a site plan showing the unit and the nearest neighbouring windows, a product sheet with the declared sound power level, the noise calculation, and a brief Planning Statement explaining why the position chosen is the least harmful one available. Councils decide these in eight weeks and most are approved on their merits.
Rules in this area have moved three times in recent years, and further changes to permitted development for low carbon technology have been consulted on without being made. As at September 2026 the position is as set out above. The planning law timeline is the place to follow reform news, and the full list of report types will tell you what a larger renewable energy scheme needs.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
Do I need planning permission for an air source heat pump?
Usually not. Class G of Part 14 of Schedule 2 to the GPDO 2015 permits installation at a house or block of flats where the conditions are met, covering noise, unit size, roof position, designated land and listed buildings. Break any condition and the work becomes development needing a householder planning application.
What is the 42 decibel rule?
The MCS planning standards set an assessment position 1 metre from the centre of the nearest habitable room window or door of a neighbouring property, and require the calculated sound pressure level there to be no more than 42 decibels A-weighted. Reflective walls, narrow side passages and larger units are the three things that most often push a design over.
Can I put a heat pump on a listed building?
Not under permitted development. Class G excludes installation within the curtilage of a listed building, so you need a householder planning application and listed building consent. The heritage case should address the fixings into historic fabric, the visual effect on the elevation and how far the work could be reversed later.
Has the one metre boundary rule gone?
The requirement to keep the outdoor unit a metre from the curtilage boundary was removed in England by a 2024 amendment to the Order, which also raised the volume limit above the old 0.6 cubic metre figure. Many older guides still quote the previous rules, so take the current wording from Part 14 on legislation.gov.uk.
How do I prove the installation was lawful when I sell?
Apply for a Lawful Development Certificate for proposed use or development under section 192 of the Town and Country Planning Act 1990 before the work starts. It is the only formal confirmation that the installation is lawful, it costs half a householder fee, and it gives a buyer's solicitor a document to rely on.
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.