I have a couple bullet points around this subject. First to echo what most people are saying I can tell you the city of MPLS is not going to allow you to add a unit. They do not get anything out of adding a unit. Property value does not go up enough to make them gain tax wise. Which is a complete hypocrisy because all city officials complain about not having enough affordable housing.
In MPLS you would have to go through a ton of hoops and hurdles to get rezoned for a triplex.
On another note people are saying it is not worth $400 a month to get that project done. It may be unlikely to get it finished for under $40,000 but if you could it is a good investment. Especially in a market like ours where a lot of MLS properties do not make sense.
If you can get that $400 and be in for less than $40,000 to do the project that is better than a 10% return on your cash. Hard to find in any other market. If you were in a great rental area that will always have demand for that I would do that investment. In fact we just did it on a single family home that is zoned multifamily. We are able to add $720 sq ft for $80,000 and we will be able to get $850/mo in rent. Taxes will not go up much, city assessments will not change, heat and electric paid by tenant. We add an additional garbage can, additional water usage but these numbers work.