Unpermitted Unit in Triplex- Tempe, AZ

[Bain Craddock]

BP, 

I bought a "triplex" 2 years ago in Tempe, AZ, which has 2 main units and 1 studio in the back. A few days ago I received an email from the city saying there was a complaint about the studio which they now realize is unpermitted. The studio was a garage but had been converted to a living unit many years ago. I bought the property from a flipper, but it was originally built in 1960. The studio is currently being rented, along with one of the main units. 

The city gave me 3 options: 1) return the unit to a garage, 2) get an architect or draftsman to draw up the building plans and then pull permits, or 3) get a structural engineer to "stamp" it and say that it's up to code. A possible 4th option came up when I was reading some past BP entries and noticed that some older multifamily properties are grandfathered into being approved.

Does anyone have any advice on which route would be best to solve this issue? Thanks!