Short answer. An engineer or architect is liable if, within fifteen years from the building's completion, it collapses because of a defect in the plans and specifications they drew up, or defects in the ground. Article 2192 of the Civil Code, read with Article 1723, channels the injured third person's claim to them.
What the law says
If damage referred to in the two preceding articles should be the result of any defect in the construction mentioned in article 1723, the third person suffering damages may proceed only against the engineer or architect or contractor in accordance with said article, within the period therein fixed.
Civil Code, Article 2192 — When the Engineer or Architect Answers. Read the full provision →
Where the injured person must go
When a building collapses and hurts a passerby or neighbor, the law directs the victim's claim to the people truly responsible for the structure. Article 2192 of the Civil Code provides that if the damage should be the result of any defect in the construction mentioned in article 1723, the third person suffering damages may proceed only against the engineer or architect or contractor in accordance with said article, within the period therein fixed. The word "only" is important: rather than suing, say, the current owner, the injured third person is channeled to the professionals whose work caused the failure, on the terms Article 1723 lays down.
When the engineer or architect answers
Article 1723 sets out exactly when the designer is liable. It says The engineer or architect who drew up the plans and specifications for a building is liable for damages if within fifteen years from the completion of the structure, the same should collapse by reason of a defect in those plans and specifications, or due to the defects in the ground. Two things stand out. First, there is a fifteen-year window from completion; a collapse after that window generally does not trigger this liability. Second, the fault must lie in the design, the plans and specifications, or in the ground. A collapse caused by something else is a different question.
The contractor, and shared liability
The designer is not the only one exposed. Under the same article, The contractor is likewise responsible for the damages if the edifice falls, within the same period, on account of defects in the construction or the use of materials of inferior quality furnished by him, or due to any violation of the terms of the contract. So poor workmanship, substandard materials, or breaking the contract puts the builder on the hook. And when the roles overlap, liability combines: If the engineer or architect supervises the construction, he shall be solidarily liable with the contractor. A design professional who also oversees the build can be made to answer for the whole, together with the contractor.
Acceptance is no escape, but time limits apply
Two timing rules round out the picture. First, an owner's acceptance of the finished building does not let the professionals off: Acceptance of the building, after completion, does not imply waiver of any of the cause of action by reason of any defect mentioned in the preceding paragraph. Moving in is not a surrender of the right to sue for a hidden defect. Second, once a collapse happens, the victim must act promptly, because The action must be brought within ten years following the collapse of the building. Miss that deadline and the claim is lost, however clear the defect.