Just curious to know if plumbing for the house MUST always be routed under
the foundation and up through the floors that way.
Would it be allowed to have the inbound feed join the house on the outside
of the foundation, say a foot or two above ground?
Absolutely yes, or absolutely no, or absolutely maybe. That's as
close of an answer as anyone could come with the paucity of
information. If, at some point, you decide to build a real house, you
will, of course, have to deal with real people, and not the imaginary
people on Usenet. Introducing yourself to the local building
inspector/department would be a stellar place to start. They will be
able to answer questions about real requirements for real houses.
Are we talking supply plumbing or waste plumbing?
Supply lines on the outside of the foundation would be subject to freezing.
Even if you live in a warm climate where that isn't an issue, you would
likely need to worry about UV exposure which could make the pipe brittle
and eventually crack (assuming plastic pipe).
Properly sloped waste lines would be less susceptable to freezing, but it
could still happen if temps are cold enough.
You also need to worry about physical damage if they are located on the
outside of a building.
As for the code legality, I'm guessing that would depend on your location.
I can't imagine why anyone would WANT plumbing on the outside of a house
(it's visually unattractive), but there are several older houses in my area
(Washington State) where waste plumbing was retrofitted into old homes and
routed on the outside of the building.
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