Wash. Sup. Ct. Civil. Commit. R. CCR 3.3
Within two (2) judicial days after the person detained is advised one the record of the right to a jury trial, the person detained may demand a trial by jury in the hearing on the petition for 90-day or 180-day detention by serving upon the prosecuting attorney a demand therefor in writing and filing the demand therefor with the clerk. No jury fee shall be required. If no party, within the time above specified, serves and files a demand for jury trial, the matter shall be heard without a jury. If no party, within the time above specified, serves or files a demand that the matter be tried by a jury of 12, it shall be tried by a jury of 6 members, with concurrence of 5 being required to reach a verdict.
Wash. Sup. Ct. Civil. Commit. R. CCR 3.3