Since tax bills are for entire parcels (typically, though not always) then it is highly unlikely that they somehow segregated the taxes for that small portion and paid it.

This is because the main house often includes value drivers that ADUs do not, such as larger living spaces and more comprehensive amenities.

@Adam OrtizRecommend you first figure out the property Class you want to invest in, THEN figure out the corresponding location to invest in.Property Class will typically dictate the Class of tenant you get, which greatly IMPACTS rental income stability and property maintenance/damage by tenants.If you apply Class A assumptions to a Class B or C purchase, your expectations won’t be met and it may be a financial disaster.If you buy/renovate a property in Class D area to Class A standards, what quality of tenant will you get?

But I want to understand:How quickly do rental units typically get leased?

That's my typical price range for Milwaukee suburbs and both will break even at 20% down, slightly positive at 25% and I usually end up with 30% down to get to the 1.2 DSCR.

Add this to smaller network and typically less knowledge for new RE investors.  

One of the biggest challenges of investing in Rustbelt cities is knowing where to invest and properly understanding the cooresponding pros & cons.Here's copy & paste advice we share to help to address this challenge:-------------------------------------------------------------------------------------------Recommend you first figure out the property Class you want to invest in, THEN figure out the corresponding location to invest in.Property Class will typically dictate the Class of tenant you get, which greatly IMPACTS rental income stability and property maintenance/damage by tenants.If you apply Class A assumptions to a Class B or C purchase, your expectations won’t be met and it may be a financial disaster.If you buy/renovate a Class A property in Class D area, what quality of tenant will you get?

They typically won't agree to that. 

Here is a list of what adding ADUs in my CA market is typically a poor RE investment:1) The value added by the ADU addition is often significantly less than the cost of adding the ADU.

That's typically where I see people referring to entry fees.