Posted by: @dgclimatecontrol

↑

pparently they have to comply with MCS which they don't and installed by an MCS installer which maybe 90% are not.

In England that is no longer the case although you have to read source documents carefully to figure this out.  I cant comment on Scotland, Wales and Northern Ireland as these jurisdictions may not yet have caught up.

The source legislation (which incorporates the May 2025 changes) is here:

https://www.legislation.gov.uk/uksi/2015/596/schedule/2/part/14/crossheading/class-g-installation-or-alteration-etc-of-air-source-heat-pumps-on-domestic-premises/paragraph/G

This (vitally for the question you have asked) refers to a new document (the suffix (a) is significant!)

https://mcscertified.com/wp-content/uploads/2025/03/MCS-020-a-Issue-1.0-Final.pdf

which you have to read to realise that installation by MCS to MCS standards (other than the noise standard) is no longer required for PD.

Posted by: @dgclimatecontrol

↑

Also, all A2W heat pumps can heat and cool so the previous information saying must only heat was misguided.

The previous information was indeed misguided (although not all heat pumps can both heat and cool, some have cooling disabled).  The requirement was that the unit (to qualify for PD) must not be used for cooling (not that it was not capable for cooling).  This was further relaxed in May.

My advice is always to use 'guidance' to point you in the right general direction, but to read source documents if you want the actual facts.  Guidance can often be misleading, apply only to a subset of cases, or, depending on the source, be just plain wrong!

Here is a write up I did of the changes.  Nobody so far has questioned my analysis which I have published elsewhere and I do have a background in reading planning legislation (which does not mean that I am infallible - feel free to interrogate my logic!)

Changes to legislation in May 2025

The legislation governing permitted development was most recently amended in England with effect May 2025

The ‘headline’ change, which has received most attention, is the removal of the former requirement that the outdoor unit must be at least 1m from the property boundary.  This is, without a doubt, a useful relaxation, offering a greater degree of flexibility in placement and, in some cases, making it easier to meet the noise requirements and/or hide/disguise the unit.

There are, in addition, several equally significant changes which are worth discussing.

(1) The former requirement that the installation conforms to ‘MCS Standards or Equivalent Standards’ has been amended to require simply that the installation conforms to ‘MCS Standards’.  At the same time the legal definition of ‘MCS Standards’ has changed; the relevant document is now MCS-020(a) not MCS-020.  At first sight this may seem insignificant, but it is actually rather important. 

This change is both a relaxation and a tightening of the rules  MCS020(a) differs from the previous version in that it is a noise specification only and no longer requires that the installation must be carried out by an MCS installer.  To most who claim the BUS grant (which requires MCS) this subtle change is of no consequence, but for those who would install without the BUS grant, including those who are considering a DiY install, this is an important relaxation clarifying, apparently unambiguously, that installations other than by MCS contractors can still benefit from PD rights.

The noise calculation specified in the new document is substantially, but not precisely, the same.  The maximum permitted noise pressure due to the sound from the heat pump remains 37dB(A) at the most affected assessment point.   However the method specified now uses exact calculations and formulae not the previous ‘rounded up’ calculations based on tables.  Additionally the final stages in the previous calculation, where a nominal background noise was factored in, are omitted.  These stages added no value and only served to confuse, so this is a step forward!

More significant is a new, more refined, definition of barriers and their effects.  In the previous version of MCS-020 a ‘solid barrier’ could be assumed to attenuate the noise level by 10dB if it ‘fully obscured’ the heat pump from the assessment position.  The term ‘solid barrier’ was not defined, and many assumed a reasonably gapless garden fence would qualify. 

The new document makes it clear that a normal garden fence will not be sufficient to give the maximum 10dB attenuation.  To achieve this, a barrier must now satisfy the requirement that it is ‘a solid brick/masonry wall or a solid fence that is at least 18mm thick. There should be no cracks or gaps in the barrier. The barrier should extend horizontally by 1m or more from either edge of the heat pump’.  

This is significantly more onerous and the precise text should be carefully read.  A ‘normal’ fence, provided that there are no gaps, is also catered for, but the attenuation (in dB) is halved.

Incidentally, the meaning of the phrase ‘The barrier should extend horizontally by 1m or more from either edge of the heat pump’ is unclear.  The author has asked MCS to explain it but, at the time of writing, they are unable to do so, even though it is their name on the document!

In addition to the above, MCS020(a) now provides a calculation method where two heat pumps are present, and another change to the legislation means that up to two heat pumps can now qualify as permitted development, but only on a detached house.

(2) The size limit for the outdoor unit, previously 0.6cu m, has been increased to 1.5cu m, except for flats where it remains 0.6cu m.  This is an important change for properties which have a relatively high heat demand, as it allows a far greater range of dual fan heat pumps to be installed under permitted development, and also accommodates models (such as the Samsung HTQ range) which were just over the previous limit.  The increased physical volume may also allow a greater level of noise insulation to be added to heat pumps, which can only be beneficial.

(3) The, often misinterpreted, requirement that the heat pump ‘must not be used for cooling’ (which did not, contrary to many sources, prevent the heat pump being capable of cooling) has been further relaxed, and now states that the air source heat pump must ‘not [be] solely used for the purpose of cooling’.  This is an important distinction, as the rules now expressly allow a heat pump installed under permitted development to be used both for heating and cooling (but not cooling alone).

It should be noted that the above is a summary of the changes in the requirements made in May 2025.  The legislation included several other, unchanged, requirements which must also be complied with..