Not normally part of the plumbing. An addition if the house cannot be connected to municipal sewer.
1/8th of an inch per foot
plumbing is most useful for our home, because we need to make water supply in different part of a home like kitchen, toilet, bathroom etc. To supply the water at required places of a home, we have to install and connect pipeline system that whole process is called plumbing and this overall functionality is done by plumbing professional.
That is the correct spelling of "toilet" (a sanitary plumbing fixture, or grooming).
By plumbing codes, no.
Plumbing or Hardware store
You can in most countries except those that have weak plumbing
If your plumbing system is properly installed there should not be any problem. If it isn't installed properly, you would already have water from sinks and bathtubs backing up out of the downstairs toilet.
pit toilets are like portapotties that have a hole that has plumbing but isn't done very frequently a flush toilet has a bowl that flushes the waste when wanted using a plumbing system
the plumbing drain vent is not installed or poorly installed.