A lot depends on that country (and in the USA, some states) privacy laws. In Thailand, you can do what you like. In Canada you must justify the need for any CCTV camera, although it is not widely enforced, except perhaps in Montreal
I have fitted CCTV cameras, mainly outdoors but one in the entrance foyer of a flat that is a AirBnB unit in Vredehoek in Cape Town. The AirBnB ad specifically states that the cameras were installed to monitor the outside entrance and also to verify that people entering are entitled, ie apart from the guests, who are always preceded by the housekeeper who introduces them by name in the foyer, whilst facing the camera so they are aware of its presence
It also helps to stop guests from stealing items like towels, bed linen (one guest stole a microwave)
The flat is occupied by over 80 sets of guests a year and not one has raised an objection. There is a monitor in the kitchen where you can see the 8 cameras at different points. The owner lives in Camps Bay and can if he wishes view the images on his phone or computer, and also playback past events.
In the 2 years the cameras were in, he has seen 3 guests cars broken into, one disappeared from the street on a Sunday morning and several burglars depositing stolen property behind a swing gate to the garage driveway. He has called the local armed response when this happens, as 3 of the cameras send an alert upon movement. Trouble is, they get arrested and are out the next day
