how profitable are Dadu's in Seattle

[Mark Weins]

Say i buy land in seattle with a house on it but the lot is big enough to build a DADU, if it cost 100k to build the DADU and I can sell it for 500k, and build it in a year, I could make 400k in profit every year by doing one deal. Does anyone have experience building DADU's in seattle who can answer this question? What is the profitability like when you build a DADU