You can only play games for so long.Chances are your next door neighbor is an IRS agent and turns you in when your tenants blocks their drive or some other random event disturbing your IRS neighbor.ADU catches fire and your insurance won't cover you, because it was an illegal and unpermitted structure.The city inspector told the last guy not to build or rent it, and is now pissed and wants to make an example of you and just red tags the whole property and no one can occupy for the next 1-2-3 years while you wait for a hearing.Then when you get the hearing, "staff" recommends you tear the whole structure down, and if you still want an ADU, you can apply for a permit and rebuild it.Your tenant quits paying rent and won't move out and you can't evict them, because there is no valid lease and they're just your "roommate".Probably a dozen other bad things can happen to you when you do stuff like this. 

Wouldn’t call it a red flag. 

I would start there.The Rock may have something to say about that.... 

It’s all about finding a balance between purchase price, financing terms, and smart rental management so you’re not forced into the red.

Also, real builders/experienced developers will often have a true technician, i.e. a civil engineer, architect, real estate attorney et al at least glance at the property info to make sure there are no major red flags.

@Natalia Perlova if the switch is in a common area and you are having trouble with the tenant especially if you think they switched the burner off deliberately (it should be a bright red switch plate in our area labeled oil burner).

Even you got hung up on the HELOC red herring.

Watching some of these videos and finding lots of red flags like that which beginners wouldn’t catch but experienced folks will know is b.s.

If they will not provide pictures of before and after that is a huge RED FLAG.

This info is public on the sec site and what we have found to be the red flags that have been good indicators of potential future problems1.