I'd guess that probably 80% of the homes standing today were
built from plans with three sheets: Elevation,
basement/foundation and floor plan. Most lumber yards had
draftsmen who would draw up house plans for free if you
bought your lumber from them. Until lately, only big cities
had inspectors. My own home town finally got an inspector
in the late 1960's, and he was (theoretically) responsible
for everything including plumbing connections to the water
and sewer, septic, electrical and finished.
Outside of big cities, which had subcontracting, most
small-town generals had crews that set the footers and
basement forms, framed and did everything. Through the
60's, my Dad had a 6-man crew that built darned good houses
and the only subs were for excavation, plumbing, HVAC and
drywall. I did most of his electrical, we all pitched in on
the drywall and flatwork, painting, roofing etc. Besides
being a darned good education for a teenager, it also
produced a very responsibly constructed house.
If a house had a septic system, the old boy with the backhoe
was the one who laid it out from the seat of his backhoe.
IMHO, most of those old systems are probably as good as the
engineered ones, but if something DID go bad, it really went