If I add to the square footage of my house can I pull air from my existing AC duct line to supply air to the new room(s)?

Sometimes. If you are building a small room and your air conditioning system had originally been designed at the high end of your building needs, than there is a chance that you won’t need to upsize your equipment or ducts. If you plan on doing this yourself we recommend that you still solicit a consultation on your projects viability to avoid longer term and bigger problems down the road.