As someone who has done Additions, land development and worked as a contractor in the area for 20+ years there are a few things to add to David's comment.
Feasibility:
1) Is the building site one which would allow a DADU per the jurisdictions requirements? Does this fall within City of Seattle, Unincorporated King Co or perhaps a specific city in/around "Seattle"? What are the setback requirements? Water availability for a second structure? Sewer availability? Once you have this basic stuff figured out you can draft up a Site Plan.
2) Bring the Site Plan to the City you wanna build in and speak with a Plans Examiner and get more data on what might be required. They will give you a whole packet of info of all the things required before submitting for a permit. This may/will include Survey, Engineering, Architectural and may include other things depending on your site such as Drainage, Road/Traffic Impact and/or find out if any Road Improvements would be required as these can be $$ cost items, Wetlands, etc. Find companies to give you a price on these items.
Also it'd be smart to understand from the Sewer District, Water and Power district/company where and how you will add these 3 key infrastructure pieces to the DADU. Is power in the area overhead or underground? Where does sewer run, do you have to go into the public right of way, under public sidewalks, into the street? etc. I'd get quotes from companies on these things because they will likely require excavation and be bigger ticket items. Last year I completed an addition in Tacoma that required an excavator to bring in new sewer/water and the excavation was a separate company than the plumbing company as sometimes these two do not go hand in hand.
3) Ballpark as David mentioned could be upwards of 300k for a 1000sf DADU or higher depending on your choice of finish materials. I recently did an 850SF addition in SeaTac and it ended up costing about 280k, labor, materials, permits, and various things such as survey, engineer, architect.
4) Add together the ballpark cost from #2 and #3 and see if you can finance this. It sucks to get into a big project and you NOT being able to finish because the money dries up.
It can all be pretty daunting if you're green on the subject but this is where I'd start - in a nutshell. However if you have more money than you have the interest or experience in the process to bring your vision into reality you can always hire a GC who has the experience/track record of this sort of stuff. Look for someone who will essentially not only coordinate the project itself to get done but also do all the preliminary (personally or by hiring someone) to get past the gated pre-project work such as collecting up all the survey, plans, studies, bids, etc. in order to get the permit.
Good luck!