My 1075 sq ft single-story house (built in 1950) is constructed of concrete block (both interior and exterior walls) and heated by a 2.5 ton HVAC system. The house stays relatively cool in the summer except on the hottest days. I'm located in Greenville, SC. Heating is more of an issue as I have no insulation in the block walls, and lots of single pane casement windows. The windows are being replaced in the next month.

My question regards the new addition that I am attaching to my existing house: what is the most cost effective heating/cooling solution? My addition is 680 sq ft (master bd/bth, laundry, and sun room) has ample windows (Uvalue .28-.3) and R19 insulation in the 2x6 walls.

Does the different type of construction (concrete block vs 2x6) between old house and new addition necessitate a zoned HVAC system? Would a ductless dual heat pump in the new addition suffice? I'm trying not to kill my already tight budget.

(BTW, my current 2.5 ton HVAC is 15-yrs old, but looks/acts brand new. Given my location, I don't lean on it too heavily. Load calculation for 1800 sq ft indicates 3-ton HVAC should be installed, mostly from AC point of view)