ronk said:
I wonder if neighbours objected at the planning stage?
The adjoining property owners would have been given the opportunity of commenting at the planning application stage. In addition the owner of the new house would have to give notice under the Party Wall Act 1996 so a further opportunity to comment of the detailed construction plans. Quite frankly you wouldn't want the new building joined to your property so it's not unusual to have a minimum 100mm gap between the properties and there would be no damp issues to any of the properties. There should be no reason why any litter of vermin would be attracted to the gap.