In these situations lots of builders take the approach you mention.
Cutting door into lower level floor.
Strip the frame down to the side 2 by 4 inch studs top header plate and floor or subfloor.
Sweep over the floor with a carpenter s level to reveal any depressions you may have missed.
Raise the lower end of the level until the bubble is centered then measure the distance from the bottom of the level to the floor to find the depth of the dip.
Shop made sanding block to chamfer the cut edge of the door make a sanding block by cutting a 2 by 3 4 inch strip of wood 1 2 inch shorter than the diameter of the peel and stick sanding paper.
Cutting a few inches from the top of the existing joists.
Lay a long level downhill on the floor with one end of the level on the level part of the floor and the other end at the lowest point.
Mark any concrete bumps that are raised above the rest of the floor with an x and any dips that are lower than the floor with an o.
Leveling a floor can be either a do it yourself project or a job for the pros.
On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
Determine the underlying problem.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
If you decide to cut off an exterior door make sure you have a plan for sealing the bottom of the door against the threshold when the door is closed.
Glenn mathewson a code official in westminster colo responds.
Remove any old door and molding such as a casing or bottom threshold inside the opening.
Use a piece of chalk to mark all obvious high and low spots on the floor.
Many new homes are built with lower profile joists to achieve this effect but that doesn t help much when it comes to remodels.
Hold a 16 inch long two by four block of wood vertical 4 to 5 feet up and centered in the doorway that will be cut out.
Before taking steps to level a floor you must find out why it s not level.