Grandfathered Denver Duplex in Single Family Zoning Question

[Ross Carpenter]

We are looking at a property in Denver zoned U-SU-C (allows for single family home only) but has been operated as a triplex for many decades, and shows up as "RESIDENTIAL-DUPLEX" in the property type on the assessor website. 

My plan would be to complete a $150-$200k remodel, but I wasn't sure if by pulling permits it might jeopardize the grandfathered status of the property. Has anyone had any experience with this? 

What about if we demolished the detached garage and constructed a new (larger) garage?

Thanks for any advice you can provide.