Location?...nat gas available?...size of each zone, 2 story?...basement or crawl space?...

Why can't u run the ducts in the attic? Or install any equipment in the attic?

Single story, natural gas is available. The original part of the house consisting of 3 small bedrooms, attached garage, hallway and 2 small baths have a lower ceiling height than the addition. The original has 8ft ceilings (small crawl space) and the addition has 12ft ceilings with no crawl space. It seems like there isn't a way to run the ducting from one side of the house to the other without a soffit at the 8ft level. We really don't like the way that will look and are grateful for any ideas to solve this.

Your location and size of each zone sq footage, old and new...what type of system do you have now for old zone and where is equipment located?

Have you had any dealers out yet and have they made any recommendation?

IMO

It makes sense for the addition to have its own HVAC system. Trying to extend the current system is likely not to work well. I would avoid installing the equipment in the attic. You could build a utility closet and have the duct work run in floor. You want some high registers and returns for the 12 foot ceilings.