The fact that it is a new duplex, and yet the suite is not authorized, suggests that there is something that the builder did not include to make it authorized.
So assuming you buy the unit, you can either:
1) pay for the work & fees to get it authorized, and never have to worry about the city, or
2) rent it out eventually, but run the risk of having the city knock on your door one day
There are
pros and cons to having an authorized suite - the question is how much hassle are you willing to put up with? You can do all the work now, or do nothing now, and maybe have to do work later? Personally, I hate being hassled so I made sure my suite was authorized.