When building a house there are a couple framing tricks that can add a significant amount of square footage with a minimal of additional cost. This method can also be used when taking a single story house and adding a floor.
To do this take the design of your house and if it's a single story house on a basement or crawl space take the length of your floor joists and extend them by two feet so they extend over the edge of the top of the basement or crawl space wall. If your house is fifty feet long this gives you 2x50, two for the overhang and fifty being the length of the house, for a total of one hundred extra square feet. Compared to excavating this extra space with foundation walls etc.the minimumin costs of the longer boards and longer trusses is much more affordable.This is truly the cheapest one hundred square feet you could add to a house.
If our house is a two story house just make sure your joists are two feet longer and overhang the first floor. Two feet is used because lumber lengths are increased by two feet as they get larger and two feet is allowed to cantilever over by most codes with no special engineering provisions , this applies if the floor joists are at least ten inches wide.If eight inch floor joists are used ( a rarity these days) then the overhang can only be eighteen inches.
The only drawback I can see doing this is the difficulty in insulating and air sealing between the floor joists that overhang but compared to the cheap additional square footage I would still encourage it's use when possible.
Check your codes to be sure this is allowed, generally they're happy to comply because more square footage means higher taxes for the county.
The cantilever part of the floor must only have a wall on it that supports only a roof load and not the load of a second or third story.