It depends on the jurisdiction, but the proverbial devil is in the details even where it may be allowed by the relevant governmental authorities.

Legality/illegality of "nanny cams" may provide some guidance. For example, in MA, my nanny-cam-using attorney acquaintances have the nannies sign consent letters allowing for possible in-house video surveillance. But that doesn't mean they can stick a hidden camera in the bathroom to monitor the nanny's activities where there is an expectation of privacy.