Legalize ADU (King County, WA)

[Eugene K.]

Hello, first time homeowner here!

Previous owner added bedroom, bathroom and kitchenette to the basement and even rented it out but no permits were pulled.

In county data it's just finished basement and no data regarding room and bathroom there. 

I'd like to fix some issues up to code and obtain Certificate of Occupancy. But I'm afraid of too picky inspections and that I'll need to remove all walls, ceiling and floor (basement has kind of inside crawl space - elevated floors for plumbing I guess)
I'll do some apparent work anyways: add vents for bathroom/dryer/microwave, insulate and enclose the furnace (at least from thr bedroom), check for gfci/arc breakers, may be put resilient channels and more drywall to reduce noises from main floor.. also - add heating.. etc.

Does someone have experience permitting someone's work? If they would like to see electric and plumbing, can they force me to gut everything?
Easiest way - just do my changes and rent it. Tax will not go up, I may be liable for any harm/damage in the basement, I can rent it quickly.

If I go with official way - it'll take lot longer, pricier with big probability of pain in the ***. 

• My strategy is to hold and rent this property. 
  Basement has it's own entrance and no entrance from the main home. That makes is illegal ADU and I'm not allowed to have an oven there.
  I'd like to use rent as qualified income to reduce my DTI ratio (for new loans)
  Some work that I'd like to do will require permit (part of the wall is still unfinished and I might add few electric outlets)

Any advice is appreciated!