Just wondering if this is a good sign for buying...Small town, tons of growth in the county....someone just built a 42 unit rent based on income smack dab in the middle of said small town. 

When set up side by side they look just like the onsite built metal storage buildings we are all used to seeing but they are not a permenant structure.

She owes very little on her mortgage as she's lived there for 30 years.Our house value according to Zillow (since we haven't had appraisals done yet): $620k built 2000, does not need any major work.

Also, some more information that would be helpful is what part of town are you looking at?

Each city gives you different headaches, forcing you to shed money like crazy.Plan about $200k for site built, expect rent of around $2500/month, plus appreciation of about $400k, just estimate.For Hayward, doing a main house expansion (900 to 2000 sf) and an ADU (1000 sf) at the same time.

Looking at a two building deal in a rural college town in Tennessee.

Sometimes depending on the city or town there may be more activity when people are out of work.

Even worse, if your "about 4.5%" interest rate proves to be not conservative enough.Extrapolating your idea, if you did this every time you moved, you'd always still be looking at negative cash flow while they were being built, and likely while you were living there.I worry that somewhere along the way, you'll be taken advantage of by your building contractor/s, who might start seeing you as an endless source of guaranteed dollars.People tend to not be as careful with their money as they should be, if too readily available. 

Not sure what your state laws indicate, but here in North Carolina our State Bar Association approved forms have it built in to the lease agreement if the tenant files for Bankruptcy, the Landlord has the option to immediately terminate tenancy. 

Anything built within the last 25 years is likely wrapped up in an HOA, so getting your hands on the CC&Rs should be your top priority.