You can be creative with the building department regarding some permits.

As long as they don't know your real intent..

It's not uncommon to have water and electricity, or even a bathroom in a "detached workshop".

What is your plan for heat? What is the climate? You don't want plumbing on outside walls.

Will you have a dedicated circuit panel for the "detached workshop"?

Get electrical, and plumbing all roughed in. Get an inspection. Good with the City, and then finish your detached workshop into the apartment.

What you want to avoid is going through all this work, the city getting tipped off and having zero permits for anything. Then there's a good chance they'll make you tear the walls open to inspect after the fact.

Also, an oven is what makes a space into an apartment. Building and planning departments get REALLY hung up on an oven in more than one room. So you would not want any indication given by the electrical that an oven is going in. That wouldn't be wired in until after inspection.

I understand your position though, the less the city knows the better. I understand it quite well.

@James: " anything big I'm missing or not considering?"

Yeah, it's illegal. Excavating for the water and sewer will have to be inspected, also the plumbing venting, and the electrical. You will have no certificate of occupancy. You won't be able to sell without that cert.

Is you property zoned for a rental?

No, we're zoned R1. In fact in my original post I stated the conversion could not be done legally. I understand your point but was only asking for construction input not legality. Not that I need to justify it but... the same has been done throughout our neighborhood and most of Southern CA. In fact in many MLS listings of homes for sale here you will see "garage converted to unpermitted ADU". It's known and common, building dept has bigger fish to fry. I get that we can't include it in official square footage or bed/bath count but we will certainly be able to enjoy it and have inlaws stay with us more comfortably. It's a small risk we're willing to take.

Spending 75-100K on an illegal conversion that can cause fines and jail time, plus has to be torn down in the best of circumstances just seems to be a real waste of your money and time.

Do you think that a licensed plumber or electrician would risk the loss of their license working illegally? Not in this lifetime. You'll be left with unlicensed, uninsured hacks for some of the most life threatening work that you can have done in your home.

If you Air BnB rental burns down, do you think your homeowners insurance will cover you? Not bloody likely. Especially if there is loss of life. You will be personslly responsible for any judgements from any lawsuits.

Is creating an Illegal rental worth the risk of losing everything you own? Or will ever own?

I don't quite understand why you'd convert a garage instead of just adding an addition???

Ichabod, I think I can get the conversion done for ~$20k doing the grunt work myself and contracting out the skills. I already have water/sewer lines and the electric I need in the garage. I think in Los Angeles I'm looking at probably $150/sf for an addition making it quite a bit more expensive than converting my 350sf garage. Also, being a few miles from the coast we never use our garage for anything other than holding surfboards and bicycles... weather here is consistently amazing!

No one's going to jail... People do this all the time. The problem is the zoning stuck in a different era. One family, one house, one piece of property. Times are changing.

Dang, you already got water and sewer out in the garage? Sweet.

I do recommend, if you don't already, get a separate sub panel for the garage. And have all the electrical permitted.