An air source heat pump can be permitted development if its equipment, location and noise assessment meet the relevant conditions. From 29 May 2025, the rules removed the general 1m boundary setback and changed equipment allowances; proximity to a boundary is therefore not, by itself, the old automatic barrier. [1]
Equipment size and number
The current domestic rules distinguish houses from blocks of flats:
- A house's outdoor compressor unit, including its housing, must not exceed 1.5m³.
- For a block of flats, the corresponding limit is 0.6m³.
- A detached house can have up to two air source heat pumps under this route.
- A non-detached house or a block of flats can have only one. The allowance for a block is not one unit per flat. [2]
The permission concerns microgeneration air source heat pumps. It is unavailable where the relevant premises already have a wind turbine, and the pump must not be used solely for cooling. A reversible system is not disqualified simply because it can cool as well as heat. [3]
Position still matters
A unit cannot use this permission on a pitched roof. On a flat roof it must remain at least 1m from the external edge. Listed-building premises and scheduled monuments are excluded. Conservation areas and World Heritage Sites have additional highway-facing and frontage restrictions; elsewhere, a highway-facing wall installation cannot be above ground-floor-storey level. Check Article 4 directions and planning conditions as well. [2]
The equipment must be sited, as far as practicable, to reduce its impact on the building's appearance and the area's amenity. It must also be removed when no longer needed. A convenient service location does not automatically satisfy these conditions. [3]
The noise assessment is not optional
Compliance with the named MCS 020a planning standard is part of the permitted-development test. The transitional route for equipment meeting the former equivalent-standard provision ended at the close of 28 May 2026. From 29 May 2026, that transitional alternative is no longer available. [1]
Ask the installer to identify the exact unit, its proposed position and the applicable MCS assessment before you approve the layout. Keep that evidence with the equipment specification; a generic statement that the product is quiet does not demonstrate that the proposed installation meets the planning standard. [2]
Planning status and statutory noise nuisance are separate matters. Councils can investigate noise affecting neighbours, so lawful permitted development should not be treated as immunity from a later noise complaint. [4]
Building regulations and applications
Heat-pump installation also needs to comply with building regulations. Ask how the relevant work will be notified and certified, including whether the installer can use an appropriate competent person scheme. Planning eligibility does not replace those arrangements. [5]
Where the permitted-development tests fail, a planning application may still be considered. In the August 2026 NPPF, policy CC2 gives substantial weight to the benefits of improving the energy efficiency of existing buildings and installing low-carbon heating. That policy supports assessment of a proposal; it does not override the legal conditions of Class G. [6]
Refrigerant policy
Defra's 15 May 2026 update said it would not legislate in 2026 to change the HFC phasedown steps from 1 January 2027. Further reform remains under consideration, rather than an enacted blanket ban on domestic heat pumps. [7]