Existing Structure - ADU - Question

[Santiago Casar]

Good evening good people, 

Bought a property in the vista, CA area. There is a main house with a back structure, looks like at some point it was a garage. It was converted to a two-story living space, about 2000 (sqft). Does anybody have experience on how to go about legalizing an existing structure? I want to take the legal route, but I am afraid city might force me to bring structure down or convert back to a garage. 

Appreciate any expertise or even legal contact.

Cheers,

Santiago