AFAIK it depends on the unit.
The Mitsubishi Ecodan, specifically, is classed as “ultra-quiet”, which avoids some of the planning requirements. They state:
Low Sound = Heat Pump Placement Flexibility
One of the regulations under Permitted Development, is that the sound pressure level of an air source heat pump must not exceed 42dB(A) 1m from the neighbour’s nearest room (Assessment Position).
With class-leading sound power levels of just 58dB(A), the Ultra Quiet Ecodan air source heat pump can be located much closer to the assessment position and pass planning.
This ultra-quiet performance means you can now choose the most convenient location for your Ecodan, causing no disturbance to neighbours.
It’s the one I had installed in our terraced house, no planning permission and no issues. It’s also quite a smart unit and I would recommend it 
You can read more about it in their product spec sheet.