If by 'convert' you mean 'tear down and build an actual structure', then you're on the right first step. Even then, I'm guessing a 4" slab is nowhere near code for Seattle.

Without diving too much into details, I don't see this being a cheap or easy endeavor, especially when you say you don't have experience at this scale.

Because I don't know the area all that well and maybe I'm completely wrong, maybe just get a couple of quotes. That will at least give you a ballpark of what to expect