In addition, somebody else is paying off your mortgage and the property can be a good tax write-off (consult with a professional).

Lol.....Maybe I should develop a business model that installs them...for a slight additional fee. 

I started a GC company because it brings in additional revenue.Another important point, are you looking for a mentor or mastermind to join?

A high school friend of mine has a duplex little outside of Sacramento area but isn't really cash flowing and ate up a lot of his capital to invest in additional properties. 

If you're confident in your comps, it may be worth discussing with the appraiser or lender to see if there's room to provide additional context or request a review. 

We are looking at investment properties in Brooklyn, and found one we liked and want to add an additional floor (two stories as of now, would go to three), but the zoning is M1-2, which is commercial.  90% of surrounding properties are multifamily residential as well.

Spending a couple of hundred dollars can reveal major issues.

You will get convinced you can do seller finance or sub to, but it's not practical unless you are going to spend all your time doing that.

In addition I also offered cash and a quick closing timeframe.

But it's what my thoughts are and determines how we will operate and spend our money.