The answer for most homes is that you do not need planning permission — air source heat pumps are permitted development. But the rules were rewritten on 29 May 2025, several widely republished figures are now wrong, and the conditions decide where the unit goes rather than whether you can have one. That makes them worth reading properly before someone picks a wall.
Permitted development, under Class G
An air source heat pump installed on a house falls under Class G of Schedule 2, Part 14 of the General Permitted Development Order. If the installation meets that class’s conditions, no planning application is required.
“Permitted development” is often heard as “unregulated”, and it is not. It means Parliament has granted the permission in advance, on conditions. Miss a condition and you have not got permission — you have an unauthorised development. So the conditions matter as much as the headline.
What changed in 2025, and what is now out of date
S.I. 2025/560, in force 29 May 2025, made two changes that between them account for most of the wrong information currently in circulation.
First, the noise standard. The planning standard for air source heat pumps is now MCS 020 a), and its permitted development noise limit is 37.0 dB(A) at the assessment position — one metre outside the window or door of a neighbouring habitable room.
If you read 42 dB(A) on a page, in a brochure or in an older guide, that figure is from MCS 020 Issue 1.3, which has been superseded. It is a meaningful difference: 37 dB(A) is a tighter limit, and it does more to determine where a unit can sit.
Second, the boundary rule is gone. The previous condition requiring the outdoor unit to be at least one metre from the property boundary was removed. A unit may now sit closer to a boundary than it could before — position is governed by the noise assessment instead of by a measured setback.
That second change is more permissive but less convenient, because you can no longer check compliance with a tape measure. A unit tucked against a boundary can be perfectly fine where the neighbouring habitable-room windows are distant or screened; a unit set well back can fail where a neighbour’s window sits right beside it. The assessment, not the distance, is the test.
The Class G limits as they stand
- How many. One unit on a house that is not detached, and on a block of flats. Two units on a detached house.
- How big. The outdoor compressor unit up to 1.5 cubic metres on a house, or 0.6 cubic metres on a block of flats.
- Where. Not on a pitched roof. On a flat roof, at least one metre from the external edge.
- Noise. 37.0 dB(A) at one metre outside a neighbouring habitable room’s window or door, per MCS 020 a).
Listed buildings and conservation areas
Two designations change the answer.
A listed building sits outside this permission. An air source heat pump on a listed building needs listed building consent, and that is a separate application judged on the effect on the building’s special interest — which in practice means the position, the fixings and the visibility matter more than the noise.
In a conservation area, the unit cannot be installed on a wall or roof fronting a highway, and cannot be nearer to a highway than any part of the house. That is a genuine constraint on terraced and front-facing properties, and it is the reason some installations end up in a side return or a rear garden rather than the obvious spot.
It is worth adding that conservation-area designations are not uniform and local directions can go further — our conservation area guide for Croydon works through a real example of how an Article 4 Direction changes the picture for one technology and not another, which is exactly the trap people fall into.
Air conditioning: the condition that catches people
Class G applies to air source heat pumps where the heat pump is not solely used for the purpose of cooling.
A reverse-cycle air conditioning system that heats as well as cools is an air source heat pump and is permitted development under the normal Class G limits. A cooling-only unit is not — it has no householder permitted development right at all.
That is a planning consequence of an equipment choice, decided at specification stage and almost never mentioned at quotation stage. If cooling is the priority but the unit can also heat, you keep the permitted development route; if you specify a cooling-only system, you may need to apply. The full picture is in our air conditioning installation cost guide.
What to do with this before you commit
Three practical steps. Ask where the outdoor unit is going and why that position — a good answer will mention the neighbouring windows, not just the pipe run. Ask for the noise assessment against 37.0 dB(A), not a reassurance. And if your property is listed or in a conservation area, establish that at survey stage rather than after a deposit, because it changes the design rather than merely the paperwork.
Beyond planning, the questions that actually decide a heat pump project are heat loss, radiators, cylinder space and the tariff you will run it on. Those are set out in air source heat pump installation, and the numbers — the typical installed cost, what the £7,500 grant leaves you paying, and why the tariff matters more than the hardware — are in our air source heat pump cost guide.
Sources: legislation.gov.uk, Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2 Part 14 Class G, as amended by S.I. 2025/560 (in force 29 May 2025); MCS 020 a) Issue 1.1. Verified 14 September 2026 and re-checked 6 October 2026. England only — Wales, Scotland and Northern Ireland have their own permitted development regimes. This is general information, not planning advice for a specific property; where there is any doubt, your local planning authority will confirm in writing.