Increased value of home?

I'm going to add an addition in my home (14x22) exact size as my living room making my old living room the dining and sitting room. this well known carpenter will do it for $18,000 (includes doing drywall, shingle, 2 windows, door, tile,Fireplace and adding pergola in front yard) I thought that was a very good price He told me this addition increases my home value by $40,000 Is this right