You won't figure this out from Costar or Zillow or most other data sources. 

If you buy mostly off-market you'll get zero commission, so a major factor for you will be if you plan to shop mostly via the MLS (which can be a good strategy especially for SFH where the data tends to be better quality). 

@Joe HuberCo-star data isn't perfect, and all datasets are subject to outlier effects, so I wouldn't put too much stock in the actual 15-25% number as much as the directionality.

They have gone up in value quickly in the past year. 

So while data may be true or applicable to them, it may not be to you.

I know that Privy is planning on adding an analytics tool for STR based on Airdna data, but we'll see how robust that is.

I've a firm believer that email is not a data repository nor a task manager.

And it will constantly be refilling too as the property goes up in value and your tenants continue to pay down the mortgage. 

If you find a smoking deal and the appraisal comes in way higher than the purchase price you may be able to get away with putting 15% down.If you're looking for high leverage it is best to buy a property with a bridge loan, add in value, and then preform a refi.

It's easy to search on Redfin/Zillow with the keyword "ADU" now.I found some recent transactions data (almost all from second quarter of 2021) in the bay area that might be of interest to you: https://www.redfin.com/CA/Camp...https://www.redfin.com/CA/San-...https://www.redfin.com/CA/Sant...https://www.redfin.com/CA/Sunn...https://www.redfin.com/CA/Sant...https://www.redfin.com/CA/San-...You can see that in all of these cases (whether with detached or attached ADUs), ADU added more value than the estimated construction cost, which shows that building an ADU can be a proven strategy for bay area home owners to not only improve cash flow, but also immediately add equity value.