Qualifying for a refinance

[Edward N Hunter]

Hi BiggerPockets long time Listener first time positing. A little background I bought a SF house in 2017 with owner financing and built a ADU in the back almost finished. The front house is renting for $3000 a month. Yearly taxes are $6600 insurance is $984 for the year. The mortgage balance is $450,000. I will have no other debts when I apply for the refinance currently I have less the $6000. I make $30,550 annually before taxes from work.

My question is How much do I have to rent the in-law unit in the back to quality for a refinance? 
Is there anything else I need to consider besides DTI and mortgage companies only counting 75% of the rent as income?

Thanks for your help in advance.