Your best tact is to talk to your local zoning administrator, health department official, and building department; or whatever they are called in your jurisdiction.

What are the dimensions of them both? Just curious since they seem so tiny!

I don’t actually have plans yet. Doing owner builder with the help of a few friends (electricians, plumbers, handymen). That’s a rough estimate of size, I’m hoping that I can get it 450-600 sqft. Might have to connect breezeway to garage or main house to avoid fire safety setback.

If you have a septic, thats not gonna happen. They are sized to the primary dwelling only, with no expansion. You would have to attach to the city sewer system to do an ADU. Which our neighbor just did as a DIY GC, and built an ADU. 45K for the sewer attachment of both structures, and another 225K for the 500sf structure.

The sticking point at the end came down to the driveway for the ADU. It was poorly placed (everything was poorly sited and built) and ended up costing him another 60K as he had to legally abandon the septic tank and fill it before he could route a driveway over it. The perils of self GCing.

This was in a low cost area of the country, thats ”lightly” regulated. But no areas of the country lets you add more residence than the septic is good for.

I added two drywells on my septic system in "highly regulated" city of Sacramento, without any problems, to add two bedrooms to the house. The issue with septic systems is potential capacity of the tank and space for the drywells or leach field. Also, the distance from any water wells on your or neighbor's property. Your first response from the architect was the best advice.

I think you need to get your ducks in a row before you make huge mistakes. I am not sure about the rules in California but in Florida you will not be able to build this unless you have licensed contractors for all the work and that incldes them getting all the permits to code . No DIY for this job IMO. I do not think there would be a way to tie into the septic you have

They don't call California the "Land of NO" for nothing................................

Thanks for all the help everyone. Definitely put my mind right.

I have a bathroom that is connected to the main house but with an outdoor access door. Could I make an ADU addition to the home and just use the existing restroom? ADU needs its own seperate bathroom, but would this work or would I need to close up the main house entrance to bathroom?

I'm not going to touch the zoning, etc., stuff ... get a local expert/authority for that.

Just looking at your question from a regular person point of view (not thinking about zoning, permits, etc.): the bathroom should not be accessible from the main house, since that would make it no longer a private bathroom, and the bathroom being attached to the main house but detached from the new space? The new space is basically getting an outhouse ... Using that bathroom, you're making a man cave/she shed/workshop/whatever more than a self-contained apartment type thing ... you know, somewhere to hang out than an actual dwelling.

Another thing that might be worth noting, I don't think septic capacity only looks at number of bathrooms ... or possibly at all. I think the addition of the kitchen and bedroom will actually be your big sticking points on that front ... or the whole detached part of your plan ...

I assume you need to talk to the city zoning department/planning board to get approval for the building itself either way ... they can tell you who to talk to about the septic matter, too too.

Thanks for the info! I posted this so I could get alternative views and ideas. I actually would prefer a structure permitted as a shed to avoid some of the ADU requirements and permitting.
I’ll explore having a shed with a kitchen and shower and composting toilet or incinerating toilet.
I understand as a shed I couldn’t rent it out but that’s fine I’m not looking to do that.
I’ll talk to the building department.

You may want to talk to a few local architects. They may have some good input and insight along the lines of something in your last comment.

I live in unincorporated LA county, so county rules apply. First, septic system is sized to the number of bedrooms, not bathrooms. We are not allowed to connect ADU to main septic system even though it is sized for more bedrooms than the existing house.