I have a “sunken” utility room whose floor is about 12 inches lower than the rest of the house, and want to raise it to the same level as the other floor. The utility room floor is a concrete slab that has a toilet (the drain pipe is embedded in concrete). My plan is to add a riser to the existing toilet drain, terminated by a flange at the new floor level. Can anyone confirm this can be done without having to modify the existing vent stack? In other words, it there a limit to how far a toilet drain can drop vertically before it hits the closet bend where the vent branches off the line?

Thank you!