Hi there, costs may vary from area to area, are you thinking on DIY, buy the materials and have a handy man to do it for you at say $40 per hour, or call a small job builder/plasterer to do it for you? Because I am running a few renos at the same time, I use the handy man option, I have an account (15% discount) with the materials supplier, so I get the wood, plaster, glue, door, frame, etc, etc… and then my handy man does the job, including picking it up from the supplier. A job like yours would cost me around $500 all up, doing It my way. Not bad for the leverage of adding an extra room… works every time… Happy Investing