Average price for ADU/small Foundations

[Benjamin Fishler]

Hello BP users,

I acquired a property a while back and just got permits approved to add an ADU ("Companion Unit") in the back. The ADU requires a 600 square foot foundation. It's currently a completely open backyard with just minimal sloping (<1% grade) dirt in the area that needs the foundation and a 5' wide easy access opening to the backyard, and the structural drawings call out a very standard slab foundation. So nothing fancy that should jack up the price beyond the norm.

The problem is I've been quoted prices that vary as much as 225% among contractors, and everywhere in between -- and all of them seem extremely high based off of estimated *high* end price per square foot for slab foundations that HomeAdvisor, Fixr, AngiesList, and ProBuilder.com list what you should be paying for the San Diego area. Those websites estimate it at a high end of $8.50/sq ft for 2019 prices.

I've reached out to both contractors and subs [using California's contractor license lookup took], but I'm reaching out as an owner/builder and not a contractor or architect, so I'm thinking there may be large price inflation. So what is everyone else paying per square foot for a slab foundation?

For reference, we're talking a 24.5' x 24.5' slab 4" thick on grade w/ #3 at 18" on-center rebar with 15"W x 12"D footers, 2" sand over 10mil vapor barrier over 2" sand.

If anyone knows someone who is reasonable in cost/licensed (and bonus if they can do the rough plumbing design underneath the foundation), please let me know!