It may be very difficult and expensive to create a separate zone for the second floor. A better solution would be to install a 1 ton mini split. It would also provide heating in the winter.

Thank you! I have looked into mini splits, but I couldn't tell if that would be more expensive than converting a current HVAC into zoned. Especially when we'd need 3 interior units for the 3 upstairs rooms.

You should be look into both options. Depending on the configuration, the mini split by actually cost more money, but it should work better than trying to add a zone to an existing system that was never designed to be have zoning.

To be done properly zoning needs additional room among other things proper installation. Often times HVAC systems are crammed into a closet, a crawlspace, an attic --- and in many cases all these spaces are barely large enough to put the HVAC system in, just by itself without zoning.

No one wants to look at the HVAC --- lets hide it by cramming it somewhere. They want the box to perform 'black magic' when it comes to comfort.

To make matters worse when your main area shuts off and the upper area is calling 550 sq. ft. worth --- you're going to try to cram probably 2-3 maybe 4 tons of AC into a 550 sq. ft. area.

That isn't going to work like you think it will. Zoning is an art & a science. But people go with the cheapest bid and then complain why it doesn't work. Remember the definition of working isn't just working during the heat of summer... it's also when temperatures moderate. Heating in my opinion is less problematic, but can have issues as well.

If you view any of this as 'expensive' don't do it. You'll probably waste money on something that will damage your system or cause some other problem symptom.