Oy veh caramba...what a nightmare , but if this is a good client they need you to finish it. To sum up, this is a SFR with a large detached 2 story biding consisting of a garage and living area on one level with a bath and a second level living area sf with a second bath. No kitchen or kitchens and from what you post county does not allow it. What distinguishes an ADU is it has a kitchen , particular an installed stove.
Can an owner buyer illegally use these spaces as rentals and put a fridge and hot plate in each level? Unless building inspector finds out, Yeah...but if that it is not the legal use (rental units with cooking facilities ) then we can't value it as such since HBU has to be legal use.
What is legal use of this building as is ? Guest or inlaw suites , bedrooms for teens/extended family, workshop use, home office, art studio etc. The building has contributory value with its large size but at what point does the value cap? See how depreciated cost approach figures as, find SFR with a large detached building and or guest house/ADU, find an area 4000 sf house perhaps as 4th comp etc. Some assignments are "ugly", (I've done my share)...but that's why the client needs you isn't it..