Do we need a 400A service to power a home and an in-law (ADU)?
Maybe some local code ,but 400 A is excessive power unless you have something like a weld shop. I have 200 A in 2500' plus 1000' garage+room  plus a 96' out building.  Includes 5 ton AC. circuits for an electric stove and and an electric dryer have never been used , but are dedicated . Also a 20 A 220 in the garage , I told the electrician I might want to weld one day, I never have but I bet I could if I wanted to. Looked in the box for curiosity : 28 breakers used, 2 -50 A, 7 -30 A , 19 -20 A . How in the world would one need twice as much power ( 400 A) in a home ?  I assume the licensed electricians hooked it up correctly.