I have an unfinished expandable aread over my garage. I am in the process of finishing it out. The upstairs has its own Heat and A/C system with more than enough power to heat/cool this new space. My question, can I simply tie into the ducts to run a new duct to this room, or do I need to also consider an air return? There are two large bedrooms, a hall and a full bath upstairs on this systerm already, and there are air returns in both bedroom. Thanks for the info!

How large is expanable area?
If you say your HVAC system has enough power for the expansion what are you basing your calculations on? Is this a wing and a prayer guesst-a-ment or do you know for sure?

For duct sizing see this post.

http://www.askmehelpdesk.com/heating-air-conditioning/cold-air-return-358801.html

better install some returns

The area is 20' x 15', and was an option as a finished room when the house was built. The builder told me the unit for the unpstairs was large enough to handle the area - so I guess your point is good - better find out for sure. I'll take a look at the link for duct sizing, and try to find a calculator to determine if my unit if large enough for the extra space. Thanks for the help for this do-it-yourself guy with little AC experience.

Good luck with your project and post back here with how it all turned out.