One crucial piece of information the Planning Dept provided me with is that if the buildings are located anywhere within the property lot's "setbacks" (specific square footage adjacent to the property line), the city would not allow the buildings to be permitted, and could even order them to be tore down.

If it's legal in your state use the Rubs system to bill the tenants for the gas they use based on the ratio of square footage and number of occupants.

I was able to achieve something similar on one building by refinishing an attic space and adding an additional master bedroom, bathroom and square footage. i.e. 

Renters pay for: location, square footage, layout, amenities in that order. 2.

Since then, I've been more inclined towards an extra bed over an extra bath (though of course this varies with the property, and you still need a functional layout with the square footage you have to work with). 

What I do like to see on the assessors site is any reference to the improvements on the land, square footage, etc.  

Will the appraiser include the square footage of the RV (300 square ft)?

Every year when I get the Property Tax figure I simply plug it in and it automatically generates the increase per month-per apartment based on square footage.

If my client is able to get the permit approved with certificate of occupancy, would the extra 400 sq ft (garage size) be applied to the total square footage of the house?

At least you are gaining sq. footage at $100+ value + a half-bath.