Pretty sure I don't even have the option. The city wouldn't allow it in this kinda historic district (I know, y'all out east and especially in Europe laugh at the idea of Colorado being "historic," but we take what we can get, and they want to maintain the original feel a little bit). So make sure your city and HOA, etc, allow it before you even start dreaming. Also investigate if it will price you out of your own neighborhood. You never want to be the most expensive house on the block.
When I did my addition back in 2001 I went with a 960 foot 2 story...the complete upper level is now part of the master suite, While the lower area is two bedrooms. What was the old downstairs bedroom in now my walk through office.