Hello, we have a. apartment complex in LA County and want to install security cameras in the common areas.

Install cameras around the parameter of the property today; post no trespass signs; change your for rent listing. 

The downstairs tenant has cameras in his unit and said he has some stuff on there. - the alleged loud tenant is month to month until 12/31.

First consider why you are putting in cameras?

.✅ Thirdly, in regard to objectionable tenancy and nuisance caused by disruptive tenants, you will need to document these violations with undeniable evidence.Because police reports are not usually able to be procured due to disturbances like drug trafficking or domestic violence, you’ll need to take matters into your own hands on producing that documentation.I recommend getting security cameras installed in all common areas and outside your properties.I’ve used SimpliSafe security cameras but you’ll need to install WiFi service at your properties so that you can maintain recordings in the event of violations and add to your lease that any tampering with said cameras constitutes a lease violation as well.This is still developing and once we get further clarity, we will host a round table with legal experts on how to best protect our properties and residents.

Instead of ring cameras at each door we would angle them down the hallways from the ceiling to get a shot of who is entering and exiting each unit.

I think it's illegal to have indoor cameras.

Yes, i have ring cameras etc.Excellent suggestions. 

Recommend you look at Lockly locks with built-in camera, so you can also see who looked at the property and tell if the door was properly closed. 

Camera can also be a violation.