PT (pressure treated lumber) is required when closer than 6" to soil. In my area of the woods PT lumber is usually hemlock and the treatments work best with softer woods. Doug Fir is generally too hard for many treating facilities.
Ask at the lumber yard for the structural rating of their PT lumber. Also, you can always "substitute up" - an old and proven practice... For example, if a high quality structural 2x8 is called out and you have lumber of questionable or unknown pedigree, just go up a notch to 2x10.