Convert Single-family to add more units

[Kyryl Sokolov]

Hello, Bigger Pockets! First timer here. I have bought single family owner occupied unit in seattle NR3 zoning(residential single family up to 2 ADUs allowed). 

I have 2 story house with the basement. 3/1 with opportunity to add second bathroom on the second floor.  

I am exploring the idea of removing the stairs in the main house to the 2 floor and stairs to the basement to convert the 2nd story to a separate unit(add stairs outside) and convert the basement(cut egress door). 

It seems like a very sophisticated project, but doable. I have around 150-200k.

Should I convert 2nd story unit to ADU or convert the house to the multifamily? What is the difference? What are pros and cons?

With the basement its a bit trickier. Height of the ceiling without floor is 6’ 10. It has some cracks on the floor and slight slope. To make it legal adu I need at least 7 feet of ceilings. House built in 1925, are there any grandpa low to make it legal with the current height without digging-out basement? Should I take a rout of a non-confirmed unit? What are the legal problem I can face by renting non-confirmed unit? Is it worth to dig-out 0.5 - 1 feet of the basement to make it confirmed? 

Where should I start? Architect(seems very expensive) or architectural designer will be enough? Or any other expertise could be consulted to start this project ?

I am new to the real estate business and this country, thats why I really appreciate any guidence here.

Thank you in advance folks!