The previous owner added 2 additional un-permitted units in the basement making a total of seven units.

I'm fairly certain it was just built this way, so I'm going to search for old permits, blueprints, and anything I can find to learn more.  

We’ve just completed site plans and permitting for a brand new Self-Storage development here in Austin, TX.

The Town 'dump&pump' inspector showed up, apparently triggered by the sale of any homes with septics, then the fun & permits started.When they had the 20 year old unit inspected & pumped by the local septic guy everything passed.

For leases entered into on or after July 1 2020, the owner would only be permitted to occupy the unit, if the renter agrees in writing to the lease termination or the lease includes a provision providing for lease termination based on owner or owner-relative occupancy;Withdrawal of the rental property from the rental market;Intent to demolish or substantially remodel the unit;Owner is complying with a local ordinance, court order, or other government entity resulting in the need to vacate the property.'

You may have a variance that allows multifamily, you should be able to see it on Atlas if so, under Licenses and Inspections > Zoning Permit Documents.

How long on a permit?

Another problem would be paper work, permits contracts etc. 

I started doing the math and it seemed with how regulated short term rentals are becoming in Big Bear, mixed with the fees, permits, etc to even get started, it seemed as though we’d be lucky to even break even.

Some places require you to file for an STR permit and pay an annual fee.