Good idea to build house with attached ADU for rental investment?

[Kc Brown]

For those looking for additional details. 

I live in the Seattle area. I am planning on purchasing a lot somewhere in the Eastside (Redmond, Issaquah, Sammamish). And within the next 2-3 years I am planning on building my forever home on the lot. Provided that the County/City/HOA would approve, would it be a good idea to build an attached ADU for renting out? If the main home is between 2500-3000 Sqft (3-4 Bedrooms), I am thinking the ADU would be between 1000-1500 Sqft (2-3 bedrooms with 1 or 2 garage). How much value would this entire property add when compared to a Single Family home with around 3500 Sqft?