But, I'd like to focus only on value-adding renovations (for rentals of course). 

Putting in a ramp, adding grab bars in bathroom, raising the sink height, etc.  

Other things such as adding bedrooms, bathrooms, square footage justifying permits.

@Luke Spencer I am not on the management side but I can tell you it took 2+ years to fill 300 units at one of our facilities and the other just hit 300 units in 12/13 months so it really depends on the demographics, and local housing market trend, the more people moving in and out the higher the demand for storage.I think if you're looking at a saturated market then the upfront cash to turn the warehouse into self storage is not a good idea but then again, maybe those operators are on back roads ( low visibility ) and don't push google ads well.

I have yet to have one late charge added to a tenant.   

I second the others here - look into adding a CPA to your team.

Adding a 1/2 bath, new roof, central air or other typical improvements will increase the value of the property. 

It all started with removing some interior walls and changing the floor plan, removing a chimney, adding a bedroom and bathroom, installing patio door to the backyard, and completely renovating every other interior element What was the outcome?

The lender did everything wrong in this situation and now are paying for it.Now they are trying to sell the loan at a slight discount (oh did I say its a 0% interest loan and they jacked up the UPB of the loan instead of adding interest) - again another rookie mistake. 

I'm in the Tampa Bay area and I will put an ad on Craigslist, Indeed or post in a group on Facebook for AirBNB/STRs.