Cost of custom engineering/design plans for an ADU

[Derek Daun]

I was wondering what the expected going costs would be for design and engineering plans for an accessory dwelling unit and garage. We are planning on putting an ADU in our backyard to use as a rental for visiting medical professionals (we're right next to UC Davis Hospital), and as the occasional place for family to stay. With the new statewide ADU laws, it's really a no brainer from a cost standpoint since we have a huge yard, alley access, need to build a garage anyway. It would pull in 1500-2000 a month in rent.

Proposal would be approximately 30x(20-25)sqft footprint. (600-750sqft) Garage/workshop on the bottom, living space upstairs. 

I received a quote from a design firm I've used in the past for $8500. That includes everything needed for permitting and plans ready to give to contractors. This was little higher than I was expecting for this firm, since they are just a designer/drafter and an engineer. (no architect). But then again, they are fairly busy these days. This is my first time doing a project this big. Is this a reasonable ball park?