House Hack- Adding a 2nd Bedroom (Advice Needed)

[Grant Viola]

Hey Chicago Investors,

So good news, I have closed on my first property here in Chicago. I couldn't have done this without the bigger pockets community, and supplemental materials provided to me by this site and its members. So thank you!
I have my inspection on Tuesday, and official closing day in November if all goes well. This is a spacious 1 bed 1 bath, and I am looking to turn this into a 2 bed 1 bath. The dining room is very large, so I am planning to add a bedroom and create the remaining space as a hallway for both bedrooms. I will live in one room, and rent out the other. 

If any of you know the legality around this, HOA regulations, any helpful documents, advice, pricing estimates, or contractors you recommend for a job like this, I would love any advice I can get. Since there are no windows in this room, I will be making sure there is a gap from the ceiling to wall. Should I make this a "legal" 2nd bedroom. Or just get it built and call it a den. What is the best route for this project?

Thank you in advance! 

More Info--

3 story building
Part of HOA
Dining room is 11x15.2
Hallway will be 3 feet
Cook County (Near Wrigley)
I am willing to help contractors in any way to reduce price.