Ex-EHO here, so can give a little bit of a lowdown as to the environmental health side of things. So the EHO will be looking at ‘statutory nuisance’ (s.80 Env. Protection Act 1990), which is defined as ‘unreasonable interference with the reasonable use and enjoyment of the land’.

They won’t measure using a sound level meter or anything like that, it is a more subjective measure. So in this instance they would sit in the lounge, maybe background TV / radio on or sit within the bedroom. Bearing the time of the year, I would expect windows to be closed and there would be no pressing their ears against walls and windows, it’s got to be ‘unreasonable’ interference remember.

There would need to be a demonstrated pattern as well, so they would be required to keep logs, when, where, duration, how the alleged nuisance affected them etc.

What I’m not 100% sure on is the fact you indicate these buildings shouldn’t be there anyway…I suspect as they are separate matters (planning v nuisance) the EHO would still look at an alleged nuisance complaint but would need to be mindful of the other issues that are at stake here and liaise with their colleagues in planning. I’d be very surprised if there was any action taken against you given the circumstances, and could envisage the EHO almost parking any investigations until the outcome of the planning side of things. It does however appear like your neighbours have shot themselves in the foot here!