Jerry-built

Etymology

Of uncertain origin, appearing in mid-19th-century England, with folk etymologies tracing it to a bad builder named "Jerry," to the biblical walls of Jericho, or to nautical "jury" (temporary). It is not connected to the later wartime slang "Jerry" for Germans, which came decades afterward.

adjective · /JER-ee-bilt/
Built cheaply and flimsily, with poor materials and little care.
What it names
Shoddy construction as a specific fault — badly made to last — as opposed to hasty improvisation for a stopgap.
Example
The jerry-built extension started sagging within a year, its foundations barely deserving the name.
Related idea
Jerry-built means badly and flimsily made to begin with; to jury-rig is to improvise a working fix from whatever is at hand — one is a permanent thing built shoddily, the other a temporary thing built resourcefully, and the two are often confused.