However, if you build the ADU and not condomise it, they appraisal will only view it as a SFR and give you the rent as SFR at 75% if lease agreements is being used.

Due to the negative equity in the deal, if the Buyer ever wanted to sell, for whatever reason, the Buyer would have to either "short sell" or wait until the house appraised for more than the proposed sales price of $446,018.40. 

And then it won't appraise at the sale price and seller will have to come up with a lot more cash or cancel the contract.

I just did a DSCR the other month on a duplex in Detroit I just finished up.It appraised for $193,000 and each unit is rented for $1,200/mo.The DSCR is at 8.00% and PITI is something like $1,130/mo.The deals are out there, you just need to know where to look.

The seller won't require an appraisal like a bank will, so you should order your own. 

Yes, there are DSCR lenders that will do 3 months seasoning to use the new appraised value of the property. 

The Chicago-area 1-4 units that I have worked on recently for clients have done very well on their appraised values.

Assumptions: 5 year term. 80% LTV, Financing ~600K with an appraised value of 800K. $2,000/mo net cashflow. 

I am looking at how commercial office space is appraised.

We do them all the time in the $75k-$95k all in range and they are consistently appraising for $90k-$120k.It's a great way to scale if you have the capital.