I have ADU at one of my rental property. Originally it was a one room cottage. There are some unpermitted addition of bedroom, kitchen and closet. My tenant reported to county. County asked me to legalize it. The added rooms don't have proper foundation. I don't think i can legalize it. My only choice is to demolish it.

Note to self, evict tenant that squealed. make sure future LLs know about his/her "reporting."

I know of companies that will put a foundation under an existing structure. It won't be cheap but might be better than tearing down the entire thing.

We have "legalized" unpermitted ADUs by pulling (and paying for) all the permits that SHOULD have been pulled to start with. Then go through the whole process of paying plumbers, electricians, drywall and insulation guys to make tiny repairs and sign off on their work. Then the inspector signs off.

Start with an inquiry with the county on what they are going to require since it's already built, they may be very reasonable with what they require or they may be nit picky to the point it will be cheaper to tear it down.

The foundation can be repaired in place. Call a couple of foundation repair companies and see what they would charge for underpinning the foundation to bring it up to code. If you can find a decent company (foundation repair companies are worse than used car salesmen about trying to oversell) it shouldn't be more than a very few thousand to fix the foundation. It's pretty straight forward with a plumber and electrician to get them to check those off. In some cases you may have to cut some sheetrock to expose enough to have it checked off. Sheetrock is easy to fix. Of course you will have to get rid of the tenant to proceed with those fixes.

An answer to your question: I had a 5 unit who's neighbor re-worked his natural drainage to my property so water undermined my foundation. The City ordered me to tear down my building. I was going to loose $600,000. The City was okay with that. The neighbor could no care less and his insurance told me to drop dead!

So as a contractor, land surveyor, draftsman, designer and licensed to work on foundations, I submitted a plan to the city for repairs to my foundation. The City refused to review my drawings or accept money towards the permit. So my property has several dead addresses associated with it. Online I pulled a foundation permit using an alternative address.

I then jacked up the building, to stabilize and level it. I jack hammered out the old foundation. Installed new forms and rebar. Then ordered a pumper and cement truck (many) and pumped in the new foundation. I have a dozen jack hammers and around 50 jacks. I had my 12 man crew and did this work over a 3 day period.