In the fall, it depends on the temperature and amount of snow cover for starting both types of projects. In the spring, foundations would start after frost is gone (mid to late April). For second story additions, we have found that snow can easily be swept off construction areas before creating collateral damage to the rest of the home. So mild winter is not a bad time to do a 2nd story addition. We typically close in the new exterior walls and roof before the existing wall is removed on an addition.