Do you really mean "effort" (and if so, whose?) or do you mean cost? The other reply is correct that making it twice as long would minimize the need to redesign, but without doing the math (I am a civil engineer, but I can't be bothered) I suspect making it twice as tall would use the least additional materials and therefore be cheapest. (That assumes taking advantage of the extra height for storage is the client's problem, not the engineer's. Having to put in a second story floor would change things.)