I can comment with some expertise on companion units in San Diego City limits. We are one of the VERY few who have successfully gotten a permit and occupancy for one. (My in-laws moved in this past fall.)

The process is very kafka-esque.

First – your lot has to be double the “nominal” size for your neighborhood. South University City has a 5000 minimum lot size and our lot size, being on a canyon, is just over 10,000. They will not consider a permit application for a CU unless you meet this requirement. It is their first “weed out” denial..

Second – you have to have parking for the CU – and your primary house has to provide the proper dimensions for “street wall”. In our case, we were a few inches short, but were able to negotiate rather than go to extreme measures to meet this requirement. The “street wall” requirement means that you have to have the width between the front side of your house and the side property line – and there has to be space in that width – in front of the primary house. If your primary house is on a narrow lot, or if you have the width on the wrong side of the house – this can present issues.
(We had the witdth on the non-curb-cut side – and if they hadn’t given us a compromise we would have had to remove our front landscaping to provide access to the non-curb side for the parking… and FWIW, my in-laws don’t drive so it would have been for NO purpose.)

– The CU has to go on the back 1/3 of the lot. In our case this meant doing some hillside work. (Big grading/retaining walls.) This added a LOT to our costs.

If you are looking for a place to purchase that will meet the requirements – look for larger lot (most of San Diego will qualify with a 10k sf lot.)
Look for how the side year and front of the house works for the street wall wierdness. Look for a level lot.