Trulia & Zillow start with data from public records. I'd start by going to King County's site and see how it shows up there. The challenge is if you correct those records and show your home is larger or has added rooms, you may find your taxes going up.
Professionals don't rely on the data from here or Zillow. Both sites recognize that local Brokers are better at determining values than they are.
You may find that adjusting the information on Zillow may impact it here, as both sites are owned by The Zillow Group. I'm not sure how much overlap they have, but it couldn't hurt.