Labor, supply chain (we are on an island) and red tape: from offers and closing, to funding to permits.  

While this is municipality dependent, if the contractor is not licensed you likely cannot obtain the necessary permits and approvals.

Health and safety inspections don't seem to care much about zoning or permitting issues.

This doesn't come cheap--these folks were making high six to seven figures depending on volume.We focused on lending to professional real estate investors--so our customers were frequent repeat borrowers. 

Duplex have different zoning that may permit additional units.

Rent to price ratios, # of new building permits vs new residents.

This is the only app I can find customized for tracking your hours and that of your cleaners, handymen etc...

@Maxwell Silva - In marketing / advertising, products and services are not expected to be sold the first time a potential customer is exposed to them.

You want both customers.  

Do you see Nike cold calling customers or driving for dollars?