Similarly in real estate, if I want to build a ADU in the back of my house and it costs $100,000 but rents for gross $24k a year, I'll recoup the cost of the build cost in 4 years.

The gross income, if any, is generally taxable on Sch 1 of your return, and the mortgage interest and property taxes are potentially deductible on Sch A. 

A commercial policy is going to cover the dwelling, lawsuits against you (unless it is gross negligence on your part),and lost revenue in case of peril.In 2017, a cabin I owned burned to the ground in the Gatlinburg fires. 

Your going to supply 12/24 months of either business or personal bank statements and they will use the gross deposits/months.  

I see a lot of gross income numbers. 

With larger multifamily properties, most property managers receive a 3% management fee from the effective gross income (or EGI, which is income minus expenses, but before you pay the debt payments).

But, debt service isn't the only expense, you also have prop mgmt, tax, insurance, vacancies, and repairs.If your debt service accounts for around 40% or less of your gross income, then you should be good no matter how big you scale.I.E. 1K/month in rent, 400/month in mortgage payment. 

In Portland, OR and in the METRO Area prices:                       2013                 2019                Gross Inc       Avg Ann Inc$/Unit            $91,389           $182,537                 99.7%           12.2%$/SqFt           $108.57            $208.79                  92.3%            11.5%

But as @jscott said if your gross margin is too small you are in danger of losing money.

I suspect gross is between $40k and $50k per month, I am waiting for confirmation of this as well as what the expenses are.