I think this is a calculation a lot of homeowners are making right now--especially if they have a low interest mortgage, equity, and their house will cashflow as a rental.The choice is a personal one of course, but I think your future self will thank you if you pull out some equity from your existing home with a HELOC for the downpayment on the next house and a cushion renting your current home, use a property manager as you have other things to do, and buy the next house and do the same thing again.

We approached the existing lender for a loan assumption.

Let's assume that the neighborhood is a C class and the median income of the city is greater than 3X of the market rent.Here are some of the considerations I've come up with so far:- Units are in need of renovation and capex is too high or not available- Unit quality is not the same as market - Seller is worried about losing tenants due to increase- Vacancy rates are high or filling units have been difficult- Rent increase would take multiple increases over multiple lease periods to get to market rate if seller is trying to retain the same tenant- Seller inherited property and just want to liquidate- Seller needs to liquidate quickly (financial burden, sickness, quick exit from land-lording)It seems like I might be missing a warning sign about a deal if they are selling with current rents that are under market; but again, this seems to be most of the properties I've underwritten.And in the same vein, what should I be worried about when purchasing a deal with under market rent with the intention of raising them after purchase. 

Have you considered tapping into your existing rental equity for the down payment?

The current tenants are month to month and not interested in buying, and the existing mortgage balance is assumable (3.65%) for qualified buyers.I know it's a great house -- and with the assumable mortgage -- a great buy for someone looking to save with current interest rates sitting as they are. 

There are a lot of affordable properties in Knoxville, but the growth isn't terribly vibrant and the deman isn't what you'd expect for a city of that size and that close to the Smokies - you're mostly looking at college rentals for the meat of the move season, and the SF market, while it exists, doesn't really compare to Chattanooga. 

I still have one house with some existing K&T.

It also includes managing your existing investment well to ensure you aren't bleeding money due to bad tenants, excess maintenance, etc. 

RE is always changing and if you have only one niche then well your limited in opportunities and subject to the conditions that exist at the time for that strategy.  

Doesn't mean that markers don't exist.