Seattle addition structure on the same lot

[Jody Ho]

Hey fellow investor,

I am looking at a property with LR-1 zoning (6000 sqft lot), and the existing structure (~3500 sqft) is built on one side of the lot with a big back yard.

I am wondering whether I can add another structure, says another 4-plex, behind it. Where should I start the feasibility study? Seattle Department of Construction & Inspection? or need to hire someone to evaluate it? 

Thank you in advance.