I know the boiler flue can't but are the shower and cooker extract fans allowed to be vented out of the outside wall of an extension which faces onto the neighbours back garden???
There is nothing to stop you venting a flue or fan to the neighbours garden.
However, this may cause a Statutory Nuisance due to the flue gas plume or smells from the fans, and the neighbour could contact the local Environmental Health department.
In addition, it may cause a trespass under civil law, by allowing an escape from your property that is causing harm to the neighbour.
It just depends on where the things are venting to and the potential effect on the neighbours.
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