There is a way to provide access to a finished attic or basement without tearing up and changing the first floor. This would be to build a small addition on the side or back of the house just large enough for a stairway.

You could also add exterior access if you wish. Enclosing the stairway allows it to be used at any time and in any weather in privacy. The only changes required to the existing structure would be to cut a door from the appropriate rooms into the stairwell.