I need a large tree for privacy and would like it to be fast growing here in Seattle. I would not like it to be much more than 15 feet wide at maturity but the height doesn’t matter, also evergreen. Would an Incense cedar grow fast?
In addition to Calocedrus decurrens (Incense Cedar), you might also consider Podocarpus or Cryptomeria.
You may want to consult the locally created web pages of Great Plant Picks, and see which evergreen trees they recommend for our area. This is their list of conifers, and here is the information on Incense Cedar.
Incidentally, Great Plant Picks says that Incense Cedar does make a good screening plant. They claim it will mature at 35-40 feet tall in garden conditions (as opposed to in the wild), and about 10 to 12 feet wide, so your original idea of planting this tree sounds like a good one.