Built a home in Phx, AZ. a few years ago. Had the HVAC contractor put a larger heat pump in to accomodate an addition that would be built at a later time. The addition is 300 sq. ft., above a garage, and is the only room above the single level home. My dilemma is, do I consider tying into the existing air handler utilizing an adjustable duct damper , consider going with a minisplit or possibly putting in a new 1 ton heat pump? Suggestions?