Q- I am planning on expanding my house by finishing my basement and perhaps adding a room onto the back. How much can I add with my current hvac system?
Bucky in Johns Creek
A - Not much.
An under-sized unit will perform inefficiently and will be costly.
Heating and air conditioning systems are sized by the livable square footage of a house. That probably doesn't include an unfinished basement, and it certainly doesn't include an addition.
Your favorite heating and air company should offer, for free, to come out and size your house and give you an exact answer.
One of the possible solutions for adding more ummph to your current system would be adding a smaller system just for the basement. This is something you will need to decide on before your finish the basement as you will want to run the ductwork before completing the walls.
Your extra room could then be nicely controlled by a mini-split (such as a Mr. Slim unit made by Mitsubishi).
It is also possible to include a system for your basement and your addition. Your HVAC company will easily be able to measure the size unit you need and give you estimates.