The tenant would have their own private entrance (rear of the house), parking (1 spot in driveway + road parking also available for guests and whatnot), their own laundry, as well as their own outdoor patio area in the backyard.

My question is should I build a wall separating both units completely so that they do not have any shared area nor run into eachother unless outdoors?

We were attracted to the area because there is tons of new development, restaurant's, outdoor amenities (trails, parks, etc).

Services such as pool maintenance and cleaning can be written-off your taxes in the year they're performed.That sounds incorrect - Just because something adds value to a house doesn't mean it needs to be depreciated over 27.5 years.If the item is not attached to the structure of the house, it doesn't need to be depreciated over 27.5 years.An indoor pool may be required to be depreciated over 27.5 years.however, an outdoor pool does not need to be depreciated over 27.5 years. 

Golf, ski, woods, lake, and outdoor activities draw people to northern MI.  

Our thoughts are are a city/town that's a close destination to the outdoors, but also not too far from some major cities.Basically, the location isn't strictly determined yet. 

Hi, we own a home with a two bedroom one bath apartment down stairs with a separate entrance and it’s own large outdoor yard.  

The house is roughly 4000 ft.² it has an updated outdoor kitchen patio deck and a stone fence wall in the front.

What matters is how the indoors compares to the outdoors in the amount and type of spores detected.

If it were mine, I would do all of the following before shelling out money for a secondary system.Check ther attic insulation.Consider installing UV heat repelant tint (it has a service life) on all sun side windows.Have a thermo scan done of the exterior and interior to see where heat problems occur.Have the AC system check by a tech for proper amount of coolant, and to see if its too big or too small.Vent the cooking hood outdoors (if it's legal to do so).Install ceiling fans in the hallway upstairs and in the bedrooms.Have tech make sure the draw air system is sized (and positioned) correctly for each floor.Make sure the thermostat is not a (NASA Project of complication designed to micro manage the temp), and that it is working correctly.Some Techs are much better than others....Good Luck!