Rentals.
There are currently 6 fire lookouts in Washington that are available as pre-booked rentals through various agencies. Heybrook and Evergreen seem to be the most popular and therefore, the most difficult to reserve. Some of the seasons on these rentals are short, but Steliko, Hamma Hamma, and Clearwater are open year-round.

Heybrook — 1,824′
Glacier Peak/North Stevens Pass
Recently remodeled, Heybrook is available May 1 – October 31 on a 6-month rolling basis. Dates book up fast, good luck!

Evergreen Mountain — 5,587′
Glacier Peak/North Stevens Pass
Evergreen is available for rental between August 18 – September 30. Booking for 2020 begins February 18.

Steliko Point — 2,586′
Chelan Mountains
A new rental as of late 2019, reservations are available by calling the Entiat Ranger Station.

Hamma Hamma Guard Station— 560′
Olympic National Forest
The Hamma Hamma Guard Station was added to the officially standing fire lookout list in late 2019 and is available for rental year-round on a 6-month rolling basis.
First come, first served.
7 fire lookouts in Washington are currently available for first-come, first-served stays. Know that these are all incredibly popular locations and first come, first served also means different things to different people. Some believe that once a lookout is occupied, it is rude to invite yourself in. Others believe that the lookout is open to all. I’ve had great fortune with solitude at some of these lookouts and I’ve also shared some pretty incredible experiences with others.
I recommend checking your expectations at the trailhead, having an open mind, and never assuming you’ll have the lookout all to yourself. Please treat these structures and fellow guests with respect. Leave them better than you found them and consider a donation to the volunteer groups who help maintain these for everyone to enjoy. We have very few open fire lookouts in Washington State. Be the reason we have more, not less.
Note: the status of any of these lookouts could change at any time. Always be prepared with a backup plan if the lookout is locked, damaged, or not available.

Burley Mountain — 5,304′
Mount Adams Area
Burley does seem to be open for overnight stays and many have reported staying the night. There is also ample camping space on the summit.

Mount Pilchuck — 5,340′
Mount Pilchuck State Park
Note: Though many in the lookout community have said Pilchuck is not officially open for overnight stays, the Forest Service website does indicate it is. I was told to ask permission from the Ranger District first.

Winchester — 6,510′
Skagit Range
Note: After repeated damage from visitors, Winchester is now closed from November through May or later.

Hidden Lake — 6,890′
Central North Cascades
This one is incredibly popular. Know what you’re getting into if you make a winter ascent. Hidden Lake is surrounded by avalanche terrain and isn’t for the inexperienced.