Converting Single Family into Duplex/Triplex

[Artem Belayev]

Hello, my name is Artem, I had a question that i was hoping someone on here could answer?

I'm looking into purchasing a single family home, The platform is perfect to add a few kitchens and walls and turn it into a duplex or even a triplex. I was curious if anybody knows the legal side of doing this or has done it on here? The zoning on the property is residential but its big enough for more than a single family. Do i need any special permits to do it? is it an expensive process to convert the zoning? Location - Shoreline WA

Thank you!!!!