Also look up the architect Steve Baczek, Peter Yost, and the builder Matt Risinger on YouTube.

Plus the addition likely had to get the blessing of an architect and engineer. 

@Ricio MontesYou will need to consult an architect on plans and some do specialize in ADUs

Architects, engineering, contractors, lenders, AHJ info, etc.

There is a ton of time working with engineers, architects, general contractors, etc and not to mention permitting times etc. 

I’m an architect in King County and I’ve done short plats, including one for my own property recently.

You will need a land use attorney, civil engineer and architect at a minimum.

I could do a 90 day refi next month however due to the city being so extra and strict on their permitting process i had to hire a structural engineer/ architect for a front deck. 

Bonus points if the lender can do const finance and then final occupied financing.To get that, you'll need:1) Figure zoning out and how many you can or want to build2) Go to architect and builder and get a sway on layout and costs3) Most painful, go to the governing entity for permits and pay your soft costs. 

It varies by project size and by architect.