I have heard of people removing kitchens and bathrooms from one unit (making it into a "storage room") to remove one unit from the count.

Hey the toilet is clogged - the plunger is in the bathroom or call a plumber.

It gets triggered when you are adding square footage to a new space, e.g. renovating a basement or even just adding a bathroom to a basement can trigger it.

Its about 1500 sqft and we are thinking of converting the smallest bedroom into a bathroom and making the den into the master bedroom.

You completely redo the bathroom in your unit for $10k.

Redo kitchen cabinets, bathroom fixtures, and vinyl plank throughout would put me in that realm but take a lot of investment.The property has several tenants that have been there for years and need new leases and rents raised.

I built a bedroom and a 1/2 bathroom down there because the upstairs only had 2 bedrooms. 

I would really like to get a rough estimate of what the following could cost.HVAC install for all unitsDig down basement and build out to make 1st floor an owners duplex down (2 bed, 1-2 bathroom depending on space, rec room)Adding laundry (stackable) in each unitI have had discussions with a realtor and one contractor, however curious in getting more opinions. 

What if the inspection reveals a non serious mold in the bathroom?

The house still needs to have a second bathroom installed (it has the plumbing) upstairs, and is probably worth $275-$300K as is.