If she finish up the garage (which will increase the property value thus higher property tax), let's say maybe the rent can be increased to, say, $5500/mo.

So I bought a property in 2017 and the previous owner held it for 20 years my property taxes increased about $1,400 a year for that property.

Couple that with an increase in rent that im sure you would implement a few times within the next decade, I think the longer you can hold onto it the better.

First, you can't take advantage of the economies of scale like you can in a bigger facility....meaning that as you expand, your revenue will increase faster than your expenses which will have a positive effect on profit. 

@Kevin Pistole rates have increased nearly 1 point in the past 30 days. 

Basically her water and electric bill increased by hundreds of dollars the month before we purchased because the AC wasn’t fixed and because she didn’t know about the pipe.

Say I find one for 250K tomorrow that we love that will only increase our monthly mortgage payment from 1180 to 1800, with no money down right now, im sunk in the water.

We increase rents by $173 per door on average.

But that is also a complete myth!

As we are seeing the rents in Boston creeping up, the submarkets around the city are becoming increasingly viable options for buy and hold investment properties.