I have a garrison colonial (standard 2nd story 12" overhang of 1st
floor) and would like to make the 1st floor even with the 2nd floor by
adding a wall. But not increasing the living space of the 1st floor. I
will however be putting a bay window in the wall.

My question is regarding a foundation. Can I dig out a 4ft deep trench
12" wide beside the existing foundation and fill it with concrete? Or
do I just need to dig a couple of 4' deep holes and add some posts to
support the new wall and window?
Thanks
Jeremy