But I started off really conservative because I lost a bundle of money on my first go-round. 

Must have multi-family zoning and be 500+ feet away from another non-owner occupied STR or you must live in the property for 6 months out of the year. 

Your tenant is experiencing life on earth and it shouldn't be a shock that the corners of the world are not all rounded and soft and warm to the touch.

My objective is to buy about an acre of land, that is already leveled, access is being done, and utilities are going to be 5 feet from the property; and then build 3-4 shell retail spots on my plot, owner occupy one of them with a business and lease out the others.I am meeting with a developer this weekend, but have some wrong numbers regarding the average cost of building a shell space. 

We have been doing it based on square feet, but the ADU consumes way less water than what that would actually calculate to (the main house has a backyard, more people, etc).

Be at least 660 feet from any other short-term residential rental as well as 2500 feet from a Resort Hotel4.

I agree, it's important to really understand the basics and start small to get your feet wet.

A 6.6% cap rate building (your 200 month payback on cashflows alone implies a 6.25% Cap Rate, rounding up to make the math in the next sentence easier), that stays at a 6.6% cap rate, appreciating at 3.4% per year, generates a 10% return that is highly likely to continue to pace with inflation over the long-run. 

I have a friend who bought two very small houses - 500 square feet each - for $1 million just two years ago. 

That being said, I have seen a couple of examples near my area where the main home is large enough with almost 2000+ square feet and multiple bedrooms and bathrooms that her family can live comfortably in while the other unit was a bit smaller and help offset the expenses.