Noise and neighbours
England, Northern Ireland, Scotland and Wales all refer to the MCS Planning Standards or equivalent. These are specified in the Microgeneration Certification Scheme MCS 020 (PDF) 
.
MCS 020 is primarily about noise and neighbours. It is a calculation designed to determine whether noise levels of an outdoor unit would impact on neighbours. The basic requirement is that the sound level of the nearest habitable room of a neighbour is below 42dB.
How to do MCS 020 noise calculations
The calculation form (DOC) is available on the MCS website. It is easy to this or an equivalent yourself.
Heat Geek have done a YouTube guide , and accompanying form that lets you very easily assess if your proposed installation meets the 42dB sound level requirement.
If the final result is a FAIL
If the result does not meet the required noise level, a system reposition and/or a sound-block such as a fence should be considered and the calculation redone.
If the sound assessment is still a fail, then the installation cannot be done under permitted development. However, it may still go ahead if planning permission is granted by the Local Authority.
If the final result is a PASS
If the final result is a pass, the all the other considerations for permitted development must be met before going ahead.
You should keep the document as evidence the assessment was completed.