Make that "square foot ranch," not a "square ranch!"

There's no way of telling without knowing the tonnage of the unit (even better to know how many CFM/ton the furnace/air handler is set to, but that's not something that you can check most likely).

I thought there was a multiple return air register requirement over a certain square footage, regardless of the size of the unit. 1800 sq ft sticks in my mind for some reason.

The house is still in the design stage, and I want to make sure the return air register locations are accounted for in the floor plan.

A Manual D calculation is done to size and design ductwork, not a rule of thumb. A 5 ton requires more return air than a 3 ton, regardless of square footage. A set return air amount will be needed depending on the system; however, it would be wise to have ample return spread throughout the home.

Thanks, I know there will have to be calcs done to size the system, ducts, and registers included. I am just trying to make sure the return air registers are accounted for in the design, and they are where we want them, not where someone else wants them.

If it is a cooling only set up with no heating unitconnected to it , some companys may go with 1 central return air filter grill in 2nd floor hall way ceiling with air handler in attic. this is ok for small size house. The more returns the better for larger houses. and if you plan on a furnace with this then yes depending on tons cooling the returns must be located per design of the duct layout.