@Trevor Ramos an extremely important thing to understand on the front end is the difference between a Mother-In-Law (MIL) and Accessory Dwelling Unit (ADU) in the eyes of the city of Seattle (as well as surrounding cities like Des Moines) .

@Alek LupaWhile there are always folks who will live in a basement, this type of accessory apartment is generally less desirable than an above ground unit.In addition, digging down a crawlspace and underpinning the foundation is an expensive undertaking (provided it is possible, sometimes houses have crawlspaces, rather than a basement for reasons other than economics).  

Account Closed I would read into the local rules on Accessory Dwelling Units it may not be exactly your goal but a lot of cities with housing shortages allow ADUs aka In-law units to help alleviate housing shortages.  

One thing to look into it to see if you can put an accessory structure.

Most the info came from the county accessory website that is already free.

We used the proceeds to  add an accessory dwelling unit. 

Heres the deal: Field Card info: "Unique two family with an accessory apartment.

Over the course of a two years or, you're sitting on a 3-5 home portfolioOf course, multi-family properties are at a premium, and if you find a deal you may not be in the most desirable neighborhood, but rents are high even for modest accommodation.One very nice development specific to San Diego right now is how relaxed the zoning laws have become with regard to Accessory Dwelling Units (ADU's).

Looks like ADUs might be worth considering.  Interesting article;http://www.buildinganadu.com/what-is-an-adu

Rather, there is what's called an "Accessory Apartment" permit.