The hardest part is doing the legal paperwork to separate the units, setting up ( condo) fees for each unit ( usually based on square footage), the condo board, common area, parking, maintenance, etc... off course there are physical requirements too.Converting a single family into a duplex is much more complex.

There are other reasonable methods of prorating, such as square footage.

Annual #'s to run - CDD fees attached to taxes... 3800-4,800k, 1,400 for "nocatee" splash park amenities on the low end -from what I understand its based on front footage of lot...

Garage conversions you'll see often times in smaller single family homes that need extra square footage.

Before becoming project manager with DHM I would always charge a fee based on the square footage of the rehabbed property to hold the client’s hands through the process.

For example, don't ask how many bedrooms, bathrooms, Square footage, taxes,  and etc.

The upper level tenants were asked to be charged a flat rate of $75 since they don't have control of the heat/ac which I agreed to.The lower unit is 65% of the home's sq footage(1,000 sq ft). 

all things being equal, a single story is better" I agree with that, but usually having 2 USUALLY means you get more square footage.

In order to compensate for the differences in square footage of your subject properties, you can divide the sales prices by the square footage of living space to come up with a cost per square foot.

Once you get access you can pull your own comps and do a little better job of filtering.You need to be looking at homes with similar bed, bath, square footage, same neighborhood, same construction, similar year built, etc.