Basement vs ADU Value-Add in South Bay Area

[Mike Yee]

Hi all, 

I've been thinking about how to improve my house values while generate rental income in SF Bay Area. Assume lot size 8400 sq ft on corner. I've been thinking of two residential additional projects for long time: Basement Vs ADU. Local famous architects prefer to do basement and replace house good brick foundation as first project and then ADU. Here is a few questions I have.

1. Living square footage

Assume equal square footage and finishes and rental income, which would you prefer habitable walk-out basement (without kitchen) over detached ADU on top of detached garage or attached ADU?

2. Cost variance

It appears cost to construct habitable finished basement for house with existing 6 feet crawl space is higher than building ADU equal square footage and finishes and rental income

3. Home resell values

Which gives more home values, habitable walk-out basement, detached ADU or attached ADU?

Other questions I like to ask:

4. Would you prefer one story detached ADU 1200 sq ft or two story detached ADU 1200 sq ft? Assume one story costs 50% more than two story ADU.

5. For house with ADU, would you prefer to convert 2 car garage (440 sq ft) into 1 car garage (200 sq ft) and covert the other half garage as ADU partial living space (240), convert 2 car garage to ADU (440 sq ft) to save building cost, or keep 2 car garage and build ADU on top of car garage?

Construction cost a fortune in the Bay Area, not to mention design/engineering fee and permit process. Or should I consider invest money somewhere else like buy a house in more affordable area?

Thank you very much!