You need a builder not a Realtor.
You question needs some answers.
What kind of Roof line are you dealing with for this addition?
What kind of construction (slab? Wood, Block, stucco?)
What kind of A/C? Are you going to need to upgrade your system to handle the 400 feet?
What are the codes and or deed restrictions you are dealing with?
What sort of floor covering? - wood, tile, carpet?
how many windows doors
do you have ample electric to handle?
A good idea for you might be for you to review what you want done, talk to several builders
(get a guy with a license) and choose the guy that makes the most sense to you.
Round number $20,000 to $30,000 - but could be very different than these numbers depending.