Combined rents on the property net roughly $4200/mo after expenses (PM, taxes, ins., etc.) 

Or any combination of these.

If your credit is strong you may be able to access an equity line for up to 95% combined loan to value. 

I pay myself and my wife a combined 120k/yr for this service (5-7 homes per year).2) My previous 19 SF BRRRR's are cash flowing 74k per year (P.I.T.I.) and in my current model, this Cash Flow is increasing by about 25-30k per year. 

These loans are structured with a combination of a first mortgage from a conventional lender (typically covering 50% of the project costs), a second mortgage from a Certified Development Company (CDC) backed by the SBA (covering 40% of the costs), and a down payment from the borrower (typically 10%).Bridge Loans: Bridge loans are short-term financing solutions used to "bridge" the gap between the purchase of a new property and the sale of an existing property or the availability of permanent financing.

If you property is functionally obsolete like only has 1 bathroom, adding a bathroom would likely pencil out.  

as Update to this post from 6 years ago, there are heloc's on owner occupied 2-4 unit properties up to 89.90% CLTV or combined loan to values (meaning multiple loans but all together not exceeding 90% of the value).Terms typically areinterest only first 10 years and drawable from the lineyears 11-30 its Principal and interest payment Prime index/rate + margin (this portion is fixed but prime is not)underwriting is done assume prime + margin + 2% stress test for underwriting meaning if your prime + margin was 10% then your underwriting rate used to determine your maximum line you can qualify for would be 12% rate as an example based on principal and interest payment over 30 years or 360 monthsIncome is the main determinant of your qualification and fico min 680+ is just the min score to get through the "front door,"Maximum DTI or debt to income allowed is 45% DTIMaximum lines on this product go from 500-750k so you can get a decent line size that can actually buy other BRRR's or fix flips and deals.

I build it like any other home there's nothing special except for some of the extras we add in for functionality. we sell our inventory though. 

And you can sell several and combine them to purchase something bigger (a consolidation exchange).   

The owner really wanted to sell to a business that could use the function room within the building.