Emma
Your project will fall under commercial building code for sure, so Mike is spot on with his comments. In building code world for the last 10 years or so whenever you have one unit on top of another you fall under the commercial code, even if it is a duplex.
You will need a fire sprinkler system. Almost definitely in the whole building. So you will want to plan on some renovations in your other units and a little bit of lost rent.
You will also need to provide fire walls/ceilings between all of the units. This is not a huge expense, but something to be aware of.
The final thing that has really come bite some project I have been working on in the but is the distance to the nearest fire hydrant. In really old neighborhoods they are sometimes not spaced as tightly as new code requires and it can be expensive to add them if the right conditions are not in place already.
One lesson that I have learned the hard way is keep you tenant until you have a building permit in hand. Anytime you have to deal with a city it can end up taking way longer than anticipated.
Good luck and let me know if you have more questions.