Oakland: Consequences/Legality of renting unpermitted unit?

I've noticed in general that a lot of SFH have unpermitted units (studio or 1 bed/bath). I also know that it is quite common in SF and Oakland to rent these out to folks who are ready to pay rent due to the lack of affordable housing in the area.

Is it possible to rent out an unpermitted unit? Is there a way you can be legally protected by disclosing it in the rental papers or another way? I'd like to one day own a SFH and build an ADU on there but some that have already built one look enticing, however not many are permitted. How do landlords who rent these unpermitted units manage their tenants and rent? Or would the general suggestion just be to work with the city to legalize it?