Utilizing empty lot space in this way helps to combat the increasing housing prices and limited supply many markets have been experiencing.

Two days later our offer to purchase the property + adjacent empty lot was accepted.

Here's a quick breakdown:Unforeseen Property Issues: Hidden damage, bad tenants, major repairs – these can eat into profits.Vacancy Woes: Empty units mean lost income, impacting cash flow.Market Downturns: Property values can drop, affecting your investment's performance. 

Do not go home until your pocket is empty.

If you are just looking for principal protection assets you can do a lot better than empty houses, if that's what you are doing. 

The house sits empty for a month or so while I am looking for a new tenant.

Because the laws are so tenant protection oriented I would rather have an empty unit than take a risk on an uncertain situation. 

So I offered to pay for the flight, if they returned a clean and empty house.

I'll loose a lot less money and sleep keeping the property empty for another month than renting to tenants that destroy the property.And your statement regarding must rent to the 1st qualified applicant. 

New unit: 4-unit multi family in which 3 units (with existing tenants that will carry over from seller) will remain investment; I will move into (currently empty) 4th unit AND USE as primary residence with primary mortgage.