HVAC technician can't do ductwork because of zoning limitations?
There are three pieces to this problem here.


What will the permiting authority accept?


If the inspector says he will approve your renovation without upgrading the existing HVAC system or adding a supplemental unit, then make sure you get that in writing. In my county in Florida, anything renovation that adds roof or that changes the conditioned area by more than 100 square feet requires a new heat loss calculation.


What can you get an HVAC contractor to do?


If they won't do what you want to do, then keep shopping. Considering the size of your job, you'll probably get a better response from a smaller company or an individual HVAC contractor. They are more likely to not dismiss a small job.


Are you prepared to do something more if your current system doesn't work out?



  I'm not particularly worried about the system handling the additional load


This could really go either way. If the current space is well insulated and properly ventilated, then that is the most efficient way to keep the room below it insulated. Adding a room above decreases the effectiveness of your HVAC system (more cubic feet to condition) and probably also increases the heat transfer in to your envelope (since you likely can't insulate as well and you're adding more exterior wall square footage to your envelope).

Be prepared to have to do something if your system can't keep up after your renovation. Since you'll have the walls open, etc., make a plan now for how an upgrade or supplemental HVAC might be installed. For example, instead of tying in to an existing vent nearby, maybe run the new vent all the way back to the current air handler. Or pre-plan a route for refrigerant lines and placement for an additional air handler.