Anyone can ask themselves how they would feel if the boss hung a camera at the workplace that records 24/7. It could be that the employee occasionally takes care of personal matters or puts their feet up...
When the craftsmen are on site, there is no risk of theft, so you would have to turn off the cameras or the craftsmen cover them. Unless you have justified suspicion that the craftsmen are stealing. Fear of shoddy work, without initial suspicion, is probably not a sufficient reason for camera surveillance. With a full order book, I as a craftsman would simply refuse to work for you if there are cameras hanging around.
Especially since you are not even the client of the craftsmen, that is your general contractor.
PS:
Legally also tricky: on large construction sites, the craftsmen have a container; in single-family houses, the break is usually taken directly at the workplace, i.e., inside the house. You must not film that under any circumstances, suspicion or not. Lunch break is not working time, it is private.
