What are good strategies or online calculators to help make rehab estimates? 

You mentioned it was 3% interest only however the math calculates to a payment of around $3950 a month if it is amortized at 3% and 30 years starting with a unpaid principal balance of $937,000.  

Hey @Eric Garces, the first steps I would recommend as a new investor is to get a handle on analyzing the income, expenses and ROI of these properties in Bridgeport.You can use the Bigger Pockets calculators to start.

The image attached is an example calculation I did and I had to push the appreciation to 25% to be able to pull a good amount for the next down payment.With these calculations, it made me realize that I might have to find alternative ways to go from house hack one to house hack two.

RUBS seems like another easy idea that we'd have to calculate monthly, costs me nothing, and does a fair division of the bill based on the number of people per unit.

Of course that total would have to be at or below my max asking price based on the BP calculators. 

 - no description on how the $400 cash flow was calculated but for 2 units this is what many PMs would charge to manage 2 units.  1% rule and 50% rule would be very aggressive at this price point.- no description of condition was provided- it has historical appreciation below the inflation rate.  

Do your own estimates and ARV calculation.

I checked the manufacturer's specifications and they don't have any sort of table or calculation to utilize.

Calculate the cash flow using the 50% rule and you will likely show negative cash flow for the ADU when financed at high LTV.