The cost will depend on a multitude of things. Initially will you be adding to the foundations "footprint" or will you go vertical on the existing structure? Both options have their pluses and minuses. Are you talking about adding a bedroom or a bathroom? Bathrooms are going to have more costs as you will now need a plumber, and bathroom fixtures and tile will cost more than a basic bedroom. Your choice of finish out will greatly affect your cost.
You should contact at lease three contractors and give them the exact same specs and get bids on the work to really see what this will cost. Remember there will be permits to purchase from the City of Austin. Some contractors will factor this into their bid. Also there may be "change-orders" or over-runs which will typically bring the price up as much as 15-20% over budget. I wish you the best of luck on your remodel.
I highly recommend Chuck Ponce. Please see his contact information below.
Kristee Leonard, Broker/Owner, GRI, SFR, BA
The Leaders Realty, LLC
PS I am a general contractor much more than you can imagine is involved for a room addition.
Lynn911 Dallas Realtor & Consultant, Loan Officer, Credit Repair Advisor
The Michael Group - Dallas Business Journal Top Ranked Realtors