Best ROI when adding sq ft and how to value

[Paul C.]

We own a 4/2.5 home that has extremely tall ceilings in a couple of the rooms where it is possible to add square footage, ~260sq ft. Some of the homes in the neighborhood were constructed with additional room 20 years ago and a few have done it more recently. We are thinking about possibly selling/renting(if it makes sense) our current home soon and moving out to the country where we build a home so I would only want to proceed if it is a good return on my investment(I personally like the natural light that comes in now, but like $$$ more). I plan on getting quotes for a couple of different options including a bonus room, bathroom & Bedroom, etc. What do you think would give me the best ROI possible? I'm thinking the bed & bath option. What is the math behind how I value the new sq ft? Would it be at the same $/sqft as the rest of my current house?