Converting Garage to ADU

[Pablo Hernandez]

I recently purchased a property in Lakewood, CO that is zoned R2. The previous owner built out the garage into a 2nd unit but did not complete it. 

I'm trying to pull a building permit to complete the rest of the garage and turn it into a duplex, but was told by the city that the lot needs to be 10,000 SQFT (ours is 9,450) - they did say we could build an ADU, so we went that route.

However, after digging further, they stated that the structure (that has already been built) needs to be 15 feet from the back neighbors property line.. It's about 7 feet. So "there's no way to build this into an ADU"..

The local planner of the day we spoke to let us know that when this was built, it was technically up to code. But now that they've changed the laws, it's not? 

My question is this - because the structure was built to code (when the parameters were 6 feet from back neighbors property line), are we grandfathered in?

The previous owner had the structure built to code, the framing and even started drywall. I just need to complete plumbing and electrical, which will be expensive but worth the investment. 

Hoping someone has had a similar issue and can help here.

Cheers!