Adding a bedroom and bathroom, how much value increase?

[Loren Thomas]

We currently own a 3bd 2.5ba house. We have a vaulted entry that I am going to close in and and add a bedroom and bathroom making it 4bd 3.5ba. Most of the houses in our neighborhood are 3-4 bedrooms. Is there a percentage difference in Snohomish county or even a agent looming that familiar with Snohomish County? I just looking for a ballpark motivation figure. The nearest comp sold up the street for 339k, but its not as good of lot and a split level, not an ideal comp. There haven't been a ton of recent sales near us.. (says zillow).

I know the best thing to do would be call an agent and have them tell me, but I don't want to waste someone's time like that cause we aren't selling til the end of the year, and my wife may have her own license by then.

I was playing around on zillow and trulia to see values, but they seem less than accurate. Especially since there a 30k difference in our houses value between the two.