Zoning, subcontractors, mother nature, P/I during construction and rent up, your loan doesn't get "fixed" until after construction, material inflation, etc, etc, etc.You have taken most of the risk off the table with family financing and family construction. 

Current Zoning if any2.

>Understand the cost/value of completing an ADU projectIn my market ADU additions in single family zoned areas are getting valuations far below the hands off cost of the ADU addition.  

We never advise breaking local regulations but we have managed some that were not zoned for STR, and did not have a problem until recently. 

But I like it.Have you spoke to Zoning yet?

Red Canoe Credit Union in Washington and Oregon, capped @ just under $50K

Can anyone recommend an architecture firm familiar with Detroit's review process, particularly one that can handle a zoning review?

Hello Ilya,Where are you looking at purchasing your 1st property.When converting a property from a single family to a multi family, you will need to check on the zoning requirements, depending on your location, not every location has zoning.As far as lending goes, it is far easier to finance a single family up to a quadplex, than financing 5 plus units, the DSCR Market for 5+ units is extremely low right now, and there are few if any lenders in that space.For financing 1-4 units, there are Millions of dollars chasing those loans, so you can easily obtain long term financing, at rates between 5-7%.Please reach out if you have a property identified, and I can help you with financing options.

I.e. what if one doesn't get along with the other-Any benefits to trying to officially convert it to a duplex with the city if it is zoned to allow MF?

@Shahaf MatityahuYou apparently checked with City Planning which zoning is part of, and they advised you it's for only 2 apartments.