Redoing an old bathroom with cast pipe. As is fairly common in old houses the floor joist was cut when the toilet was installed. If the soil pipe had dropped another 6" to clear the joist would it create venting problems? Is there a standard for the total amount of drop before the vent? I intend to replace it with 4" ABS.
This is making some assumptions, I am a existing structures inspector for a US city and I am going to assume you are in the US or North America.
There is not a restriction on drainage line drop from the toilet flange for the ICC IRC 2012, with some caveats.
Most siphon toilets in the US have traps of 2 to 2.5 inches. The maximum distance from the toilet's weir to the vent permitted is 8 to 10 feet running length respectively. IRC Table P3105.1
In the diagram below, the length of i plus j cannot exceed 10 feet if the toilet has a 2.5 inch trap.
If the distance of i is 8 feet and you decide to drop j to 3 feet, to the letter of the code, this is not allowed. That would exceed the 10 maximum between the weir and the vent stack.
The maximum drop in the drain fixture (the length of pipe between the trap to the next junction) cannot exceed the diameter of the pipe. However, the vent can be below the weir of the toilet, other fixtures cannot. IRC P3105.2
The drainage line for the toilet still have to obey the first clause of P3105.2, so this will affect your maximum horizontal distance, this is dependant on the slope you give the line, if give the minimum required (1/8 inch) then this is beyond the 10 feet and probably will not affect you.
If you slope your line to 1/2 inch per foot. Then the horizontal pipe (i) would have to be less than 6 feet.
All of this is to prevent water and waste from 'plugging' the entire width of the pipe, moving down the pipe creating a vacuum that siphons water out of traps.