You can deduct expenses whether the unit is legal or not, providing that the unit is completely self-contained and has a dedicated entrance (not shared with your entrance).
I would suggest that you only add the suite if it is legal, and make sure you get all of the proper permits. Also use a contractor that has done several legal suites before.
You can contact the Planning and Development department of your municipality. Larger centres sometimes have a Secondary Suites branch in the P&D dept. You can also try your city's website.