It will be finished within 45 days. 

If it was me...I'd check with zoning to find out how many dwelling units I could build, and I'd find what the requirements are for those units (type of house, height and setback restrictions, etc.).Then I'd find out what local builders think the cost to construct would be - just get a cost estimate per square foot for 2 or 3 different levels of finish.

Congrats on finishing your 6-unit project!

We have a builder and the plans are very close to getting finished.

In working on construction loan financing, the lender informed me I'll need to unencumber that portion of the property (makes sense) before closing on the construction loan.I understand there to be two methods generally speaking: refinancing my mortgage and removing that portion of the property in the process OR attaining a partial release from the lender to remove that land.I *strongly* want to avoid refinancing as I have an amazing rate, but partial releases seem complex and also require a survey.

I fully renovated the upper unit with well-above-average finishes, and it's been averaging ~$2,850/month in gross rental income over the past 12 months as an STR on Airbnb. 

I would also look at suing them for the cost / expense to finish the job. 3.

GC never pulled any city permits yet charged me (didn't find this out until the job was finished). 

If you have something like a finished basement or garage I could see adding one there but no need to take up space and make it look junky with one in each room.  

@Tiamo Wright is getting g this over the finish line.