I did that to a shed that was built too low?  Not sure how that happened, was built not too long ago! lol

Very smart to take old concrete out!  Floor heat good! 16 foot byfold or 14 foot hyd door to lose no head room on your big door!  Don't have the inside and out side concrete in the same pour, but do put a water line "right" under the door, keeps snow melted 6, 8 inches away from door in winter. 

Might need alot of vents on the ends of roof if you don't have a overhang vented on the sides of shed to make sure it doesn't rain in the attic.