Class G – Air source heat pumps & air conditioning on domestic premises

 

The permitted development right of Class G allows you to install, alter or replace an air source heat pump (ASHP) on a residential property without planning permission. This can be a house, bungalow or a block of flats. You are permitted to install it either on the property itself or within the curtilage. This must be a unit capable of heating, but it can now include a cooling element.

On the 29th May 2025, Class G was altered to relax the rules around Air Source Heat Pumps (ASHP). This applies to any units installed on or after this date.

You can now have up to two air source heat pumps (ASHP) on a detached house or bungalow, however you are still limited to one on a semi or terraced property or a block of flats.  A block of flats is any number provided that it is solely flats. It is two flats or more, so a converted house may well qualify.

Within a dwellinghouse, whether that is detached, semi or terraced you can now go up to 1.5 cubic metres in size. For flats you are still limited to 0.6 cubic metres. Any air source heat pump needs to be 1m away from the edge if on a flat roof, but there is now no limit as to how close to the boundary it can be.

Finally and maybe the biggest change is that you can now have a combined heating and cooling unit. Previously the unit had to be capable of heating only. This had ruled out any form of air conditioning units under permitted development. As long as the unit is not solely for cooling you are good to go.

Note that these units on commercial units, will require planning permission.

 

 

 

How do I apply for permission?

 

You do not need to request permission, Class G is your permission. However there are a few restrictions that you need to be aware of so that you do not fall foul of the conditions within the second part of Class G.

The air source heat pump must comply with the MCS Planning Standards – which, since the May 2025 amendment, means the standard known as MCS 020 a) (we link the version current at the time of writing, but it is always worth checking for a newer issue). Before 29th May 2025 an “equivalent standard” could be used; that option was removed, but an installation already planned under the old rules can continue on the previous basis until 28th May 2026. After that date, compliance with MCS 020 a) is the only route.

Importantly the unit does not need to be certified, but it must meet the standards. This is defined within Paragraph P of Part 14.

You can’t install an air source heat pump if you already have a wind turbine. It must also not be on a pitched roof or within 1 metre of the edge of a flat roof but it can be on a flat roof.

You are allowed to install it on the front wall of a property, which is not in a conservation area or on a World Heritage Site but it must be on the ground floor storey and not above.

The external device is, so far as practicable, must be sited so as to minimise its effect on the external appearance of the building and also to minimise its effect on the amenity of the area.

 

Not sure your unit qualifies?

 

Consider a Lawful Development Certificate. If you have any doubt about where the unit will sit, or whether it truly meets every Class G limit and condition, you do not have to take the risk on trust. This is especially worth thinking about for air-to-air units, where an officer unfamiliar with the certification position may wrongly assume planning permission is needed, but it applies equally to any air source heat pump whose siting is finely balanced against the boundary, a highway, or a neighbour’s window. A Lawful Development Certificate gives you the council’s formal confirmation that the installation is lawful, which is invaluable when you come to sell. Planning Geek can prepare and submit that application for you – reach out to us here.

 

Conservation areas / Listed Building / WHS etc.

 

You can’t install an air source heat pump on, or within the curtilage of, a listed building or scheduled monument without planning permission. Planning Geek can assist you with the application.

If you are in a conservation area or on a World Heritage Site you can’t install it on an elevation that fronts a highway.

It also can’t be closer to a highway, than the wall of the dwellinghouse or block of flats, if ground mounted. The full legislation can be seen below.

The external device is, so far as practicable, sited so as to minimise its effect on the external appearance of the building and also to minimise its effect on the amenity of the area.

 

Air-to-air heat pumps – the certification myth

 

This is the question we are asked most, and the one installers most often get half-right: can you install an air-to-air heat pump under Class G, given that air-to-air units cannot (yet) be MCS certified? The short answer is yes – and understanding why matters, because “your installer says no planning needed” is not the same as knowing the unit is lawful.

The confusion comes from mixing up two different things. There is MCS product and installer certification (the MCS mark on a unit, and an MCS-registered installer), and there is the MCS Planning Standard, which is the document called MCS 020 a). Class G, at condition G.1, does not require the unit to be certified – it requires the heat pump to comply with the MCS Planning Standards. Those are two separate tests, and only the second one is a condition of permitted development.

Historically, MCS product and installer certification covered air-to-water heat pumps (wet systems feeding radiators or underfloor heating). Air-to-air units – the type that heat and cool through indoor fan units, essentially reversible air conditioning – sat outside the certification scheme, which is where the “air-to-air can’t be MCS certified” line comes from. As at July 2026 that is being actively changed: MCS is bringing air-to-air products and installers into the scheme, and air-to-air units have recently been brought into the Boiler Upgrade Scheme grant framework. So this is a moving picture, and it is always worth checking the current MCS position before relying on it.

But even while air-to-air could not be certified, it could still be permitted development, because certification is not what Class G asks for. What Class G asks for is compliance with MCS 020 a) – and MCS 020 a) is a noise calculation, not a product test. Anyone competent can carry out that calculation for an air-to-air unit. If the calculation is done and the result is within the noise limit, and the other Class G limits and conditions are met, an air-to-air heat pump is permitted development in exactly the same way as an air-to-water one.

The practical lesson: if an installer tells you an air-to-air unit needs no planning permission, they are probably right on the outcome – but ask them for the completed MCS 020 a) sound calculation. That calculation is the evidence that the installation actually complies, and it is what a council would want to see if the lawfulness were ever questioned. Without it, “no planning needed” is an assertion, not a demonstration. If you want that position confirmed in black and white, a Lawful Development Certificate records the council’s agreement that the installation is lawful. In fact with some local authorities or officers, thinking it must be certified, a certificate might be in order. Planning Geek can assist you with this. Reach out to us here. 

 

What MCS 020 actually requires – the noise calculation

 

Because “comply with MCS 020 a)” is the condition that unlocks permitted development, it is worth setting out what that document actually asks for. It is a sound calculation – the point of it is to show, on paper before installation, that the heat pump will not be too loud at the nearest neighbour’s window. This is the part installers rarely explain, so here is the substance.

The limit. The calculated sound pressure level must be 37.0 dB(A) or lower (as an LAeq over a 5-minute period) at every relevant assessment position. If it comes out above 37.0 dB(A) at any assessment position, the installation is not permitted development.

The assessment position. This is a point 1 metre outside the centre of any door or window to a habitable room of a neighbouring property – measured square to the window. A habitable room means a room used for sleeping or living, not a bathroom, hallway, utility or similar. Assessment positions have to be considered on all sides of the property, and at ground floor, first floor and above – because the closest window is not always the loudest, and a first-floor bedroom with clear line of sight can be worse than a nearer ground-floor window behind a wall.

What goes into the calculation. The installer works through a set procedure for each assessment position:

• the sound power level of the specific unit, taken from the manufacturer’s data (not the sound pressure level, and not a “low noise mode” figure)
• a directivity factor (Q) reflecting how many reflecting surfaces are near the unit – ground only, ground and one wall, or a corner between walls; a unit boxed in by more than three reflecting surfaces (for example in a small lightwell) cannot meet the standard at all
• the distance in a straight line from the unit to the assessment position
• a barrier correction for any solid wall or close-boarded fence between the unit and the window, depending on how thick and solid the barrier is and how much of the unit is hidden from view – a solid masonry wall with no line of sight gives the biggest reduction, an open or gappy fence gives none

Those figures are put into the standard’s formula to give the sound pressure level at that window, and the result is checked against the 37.0 dB(A) limit. Where two heat pumps are installed, the contribution of each is calculated and then combined. The installer is expected to keep a copy of the completed calculation and give one to the homeowner – and it is exactly that document you should ask for.

Why this matters for siting. The calculation is why position is everything. Moving a unit further from a neighbour’s window, putting a solid wall or a close-boarded fence in the line of sight, or keeping it out of a tight corner that bounces the sound around can be the difference between passing and failing. If a first design comes out over 37.0 dB(A), it is often fixable by relocating the unit rather than abandoning the project.

 

Air Conditioning Units

 

Following the change in May 2025 you can now have combined heating and cooling units. You cannot have a purely air conditioning unit. It must be combined with an Air Source Heat Pump.

Previously any form of air conditioning unit was not permitted development. 

One important factor is that if the unit was installed before 29th May 2025 it will not be permitted development, even if it is a combined unit. This change is not retrospective. 

Planning Geek can also assist with a planning application for an Air Conditioning (AC)  unit if required. Complete our fee proposal form here.

 

Air Source Heat Pump Legislation

 

Class G – installation or alteration etc of air source heat pumps on domestic premises

Permitted Development

G. The installation, alteration or replacement of a microgeneration air source heat pump—

(a) on a dwellinghouse or a block of flats; or
(b) within the curtilage of a dwellinghouse or a block of flats, including on a building within that curtilage.

Development not permitted

G.1 Development is not permitted by Class G unless the air source heat pump complies with the MCS Planning Standards.

G.2 Development is not permitted by Class G if—

(a) in the case of the installation of an air source heat pump, the development would result in the presence of more —
(i) more than one air source heat pump on, or within the curtilage of—
(aa) a dwellinghouse which is not a detached dwellinghouse;
(bb) a block of flats;
(ii) more than two air source heat pumps on, or within the curtilage of, a detached dwellinghouse;

(b) in the case of the installation of an air source heat pump, a wind turbine is installed on the same building or within the curtilage of the dwellinghouse or block of flats

(c) in the case of the installation of an air source heat pump, a stand-alone wind turbine is installed within the curtilage of the dwellinghouse or block of flats;

(d) the volume of the air source heat pump’s outdoor compressor unit (including any housing) would—
(i) in the case of the installation of an air source heat pump on, or within the curtilage of, a dwellinghouse, exceed 1.5 cubic metres;
(ii) in the case of the installation of an air source heat pump on, or within the curtilage of, a block of flats, exceed 0.6 cubic metres;

(f) the air source heat pump would be installed on a pitched roof;

(g) the air source heat pump would be installed on a flat roof where it would be within 1 metre of the external edge of that roof;

(h) the air source heat pump would be installed on a site designated as a scheduled monument;

(i) the air source heat pump would be installed on a building or on land within the curtilage of the dwellinghouse or the block of flats if the dwellinghouse or the block of flats is a listed building;

(j) in the case of land within a conservation area or which is a World Heritage Site the air source heat pump—
(i) would be installed on a wall or a roof which fronts a highway; or
(ii) would be installed so that it is nearer to any highway which bounds the curtilage than the part of the dwellinghouse or block of flats which is nearest to that highway; or

(k) in the case of land, other than land within a conservation area or which is a World Heritage Site, the air source heat pump would be installed on a wall of a dwellinghouse or block of flats if—
(i) that wall fronts a highway; and
(ii) the air source heat pump would be installed on any part of that wall which is above the level of the ground floor storey.

Conditions

G.3 Development is permitted by Class G subject to the following conditions—

(aa) the air source heat pump is not solely used for the purpose of cooling;
(b) the air source heat pump is, so far as practicable, sited so as to minimise its effect on the external appearance of the building;
(c) the air source heat pump is, so far as practicable, sited so as to minimise its effect on the amenity of the area; and
(d) the air source heat pump is removed as soon as reasonably practicable when no longer needed.

 

[ Transitional provision (No longer applicable)

(1) Paragraph (2) applies where development (“previously permitted development”)—

(a) is permitted under Class G of Part 14 of Schedule 2 to the GPDO immediately before 29th May 2025, and
(b) is, by virtue of the amendment made by article 5(2), no longer permitted under that Class on and after 29th May 2025.

(2) Where this paragraph applies, a developer may, despite the amendments made by article 5(2), carry out previously permitted development until the end of 28th May 2026. ]

 

 

 Air Source Heat Pump Page Updated:  20th July 2026