How hard to make a house have units?

[Byron Broadnax]

In my area, there are a lot of reasonably priced 5 Bedroom places (some in worse condition than others).  With that being the case, I want to kill 2 birds with one stone. Moving out and bring in another source of income.

But I'm wondering one thing. One of my goals is to grab one of those houses, rehab it, create a studio apartment for myself (I want the bare minimum in personal responsibilities) , and 2 other units to rent out. 

So the question is, how hard (and maybe more importantly, is it even reasonably possible) is it to make a single-family house into multi-unit house? I would figure a minimum amount of square feet would be needed, but I'm wondering if the effort is even reasonabily possible.