Design Defect vs. Construction Defect
The legal distinction between a problem caused by a flaw in how a building was designed, a design defect, and a problem caused by the building not being built the way it was correctly designed, a construction defect, a split that determines which party is actually liable when something goes wrong.
Why it matters
Whether a problem traces back to the design or the construction changes who's responsible: under the [[concept-spearin-doctrine|Spearin Doctrine]], a contractor that builds exactly to a flawed design generally isn't liable for the design's own flaw, while a contractor that fails to build to a correct design is liable for that failure regardless of how good the design was.
On a real project
A wall develops water intrusion after construction, and an investigation finds the waterproofing detail was built exactly as the architect specified it, which points to a design defect in the original drawings rather than a construction defect in how the contractor executed them.
Who this matters most to
A Construction Litigation Expert Witness is often brought in specifically to determine, through forensic investigation, whether a given failure traces back to design or construction, since that determination usually decides who's actually liable.
Where this goes wrong
A dispute over a failed building system assumes the contractor is at fault simply because the contractor built it, without a proper forensic investigation into whether the failure actually traces back to the design itself. The real cause, and the real liability, only becomes clear once an expert witness investigates the failure in enough technical depth to separate the two.