Where to start with building an ADU

[Justin Knighten]

I'm looking into building a detached ADU on my property like the garage pictured (apartment for rental/AirBnB upstairs, garage for me on the main level). I've talked with the planning department and it looks like I'd be in the clear as far as zoning and such. Where do I go from here? I need to get a rough estimate on the cost in order to figure out if it's something we can afford/that makes sense for us. I don't want to waste a bunch of contractors time if I'm just a tire kicker. How should I go about getting a good ballpark estimate on a project like this?