Actually, the water rate goes down when you have an ADU if you have DWP.

If you budget the negative cash flow over the course of the hold period and have reserves set aside to fill the gap, there is no problem (assuming your investment meets your target appreciation rates - I understand there is no guarantee there). 

There was one here local to me that was on the market for over 2 years before it sold at a significant discount.

Fit buy box of A or B rated school that will appreciate over time.

Are $300/squares for GAF HDZ good rates.

A quick check of new listings results in many listing that seem well below market rates.   

The first question is, can I use a second home mortgage (10% down and lower interest rate) for this. 

What would you assess your personal success rate is for 2024? 

The issue though will be a lender will not give you 100% acquisition if thats what you are trying to do by getting it at a discount. 

I have found people willing to owner finance their house to me at a lower interest rate.