I would look for a map of old mines in the Renton district, or an abandoned storm drain in the civilized edge of Redmond. Taking over an abandoned coal or iron mine would probably be best. You want one that was an open pit, that now has trees around it. You put a roof over the pit, camo netting or that stuff they use to conceal cell phone towers or both over the roof, and then set up a sump pump. The mine is already designed to allow trucks to carry the ore out, so you have a spiral road leading downward, You just need to build platforms for parking, and maybe hollow out spots to put pre-fab construction for living quarters.
Toxic rain is a concern. Drainage is a big concern. You will want to make sure that you aren't building in a toxic site, check the background count. Make sure it is truly abandoned, having Mitsuhama Ore Extraction Solutions (MOES) show up at your secret hideout with a new mining crew would be awkward. Having the city maintenance crews come around to check drainage in your supposedly abandoned storm drain tunnel would be equally bad.
Hire the mob to do the work. You want the same people who build escape tunnels and safe houses for the Don. It costs a bit more, but the Mob can ensure silence. Or do the work yourself, but it will take a while, especially if you have no construction experience. Everything is much harder than you think it is. The workers make it look easy because they are pros and have been doing the work since they first apprenticed at 15-16 years old. Your runner is probably not a professional contractor.
Good luck, have fun!