There are a few things to keep in mind from mechanics as well. 

Mechanic, Magician & Mindreader.

If they will allow the change, you'll need to submit proper drawings and plans and obtain permits for plumbing, electrical, mechanical (heating), and any other permits as required by your jurisdiction (often required to be done by licensed/insured contractors).Keep in mind you will need to change out your electric service panel to a 3-service entrance--1 meter for each apartment, 1 meter for 'common area'--exterior lights, etc. 

Look at the mechanical systems.

Learn the mechanics, pitfalls, upside.

Personally, it sounds like a lot more risk involving contractors, possible mechanic's liens, city permitting issues, insurance, etc.

I have a few off the top of my headNon-conforming zoned properties, way too hard to get around these (sometimes)High crime rates, D class neighborhoods - beware of bad neighbors tooLong list of building inspector code violations being transferred at closingToo many liens on the property (mechanics, tax, credit - you find these in the title search)Clouds on title (relatives in other states that have different plans for the property)Too much "spaghetti" on the table (hair on the deal, I am Italian and like food references)Happy to share my "Due Diligence Checklist" from our BiggerPockets Blog.

Who in Missouri knows the laws for Mechanics liens being filed on a property?

After all I don't need to know how my car works to be an excellent driver, I just need to know enough to optimize its performance (that and a great mechanic). 

Most of the cost in a living structure is mechanicals, not open space. 5.