They have a vision board with ideal traits and attributes and build a business model with processes and systems around that.

Going from 1-2 may be a lot easier but when you start adding more than 2 and depending on how high the structure is you get into fire escapes, sprinkler systems etc...The more important question you need to ask is not is there a market for this but can I legally make a 2 unit within the local building codes and keep it financially on point so the project will still be profitable when completed

I have come across a lot of lots that just aren't aren't worth building on for many different types of reason (soils, subsurface water, etc..). 

James..Mechanicals, heating, cooling, septic or sewer, water source and piping, electrical and upgrades and roofing for a few.

The owner said he will warrant the electricity, plumbing, septic/water line, and air conditioning in the contract.

BAD DECISION - water runs downhill so my seal-coated and still leaking roof is destroying my ceiling and my 2 1/2 year old rehab. 

so long story short, I had an A/C leak... got the leak fixed but the water that cane out got under the floors.. the tenant put in the request to my PM and my PM’s flooring contractor estimated a total of $1900...

Part of that rehab is that the units are currently two bedrooms but HUGE so we are turning them into 3 beds AND putting hvac that runs into each unit and sub metering water so that the tenants will pay all utilities. 

He has a solid network and system there, since he has 100 units.

@Jon Underwood Fair point, I just saw this as a cautious way to test the waters.