ADU options in SF East Bay

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Hi all, new here. I'm thinking about an ADU for my owner-occupied SFH property near downtown Lafayette. It's a good location, easy 10 min walk to BART and downtown shops / restaurants, and 2 blocks from great elementary and middle schools.

Currently I have a 2400 sq. ft. main home on a ridiculously large 19,600 sq. ft. lot. Minimum lot size is 10K, so unfortunately I can't split it. ADU seems like the next best option, as a rental for the next 10-15 years, and then potentially housing for boomerang kids after that.

Lafayette zoning code seems relatively permissive on ADUs.  It looks like I can build up to 1200 sq. ft. or 50% of main home, whichever is less (same in my case).  There's a 12 ft. height limit if I put it near the property line, which poses some design limitations, but not insurmountable.  There are parking requirements, but I think I'm exempt because of location < 1/2 mile from BART.

Any thoughts on ideal size / configuration?  Bigger is better?

Good resources for estimating cost, financing, income, cash flow?

I've read differing opinions on whether precut / prefab is possible.  Would love to hear more about that.

If any others have been down this road, would love to hear other thoughts, pitfalls, etc.

I'm also aware of SB827 on the table, which would dramatically upzone everything within 1/2 mile of a BART station.  That's a big wildcard, so perhaps waiting a bit to see how that plays out would be best.

[Seth Borman]

You need to try to buy 400 SF from a neighbor, or ask the city if they will vacate right of way to you.