Short answer. A legal easement of lateral and subjacent support, plus a right to be told. Article 686 protects not only buildings standing when the excavation is made but also constructions that may be erected, and Article 687 requires the excavating owner to notify all owners of adjacent lands.
What the law says
The legal easement of lateral and subjacent support is not only for buildings standing at the time the excavations are made but also for constructions that may be erected.
Civil Code, Article 686 — Lateral And Subjacent Support. Read the full provision →
Support is owed to the land, and to what is not built yet
Lateral support is the support your soil receives from the soil beside it; subjacent support comes from below. The easement is legal, meaning it exists by force of law and nobody had to grant it. The sentence quoted extends it past the obvious case: the neighbour must not undermine you even as to constructions that may be erected. So an excavation beside a vacant lot is not harmless because there is nothing standing there yet, and a landowner does not lose the protection by having deferred building. The duty runs to the land's capacity to carry a structure.
The notice requirement is separate and it is not a formality
Article 687 obliges the proprietor intending to excavate to notify all owners of adjacent lands. Not the ones he expects to object, and not only the immediate neighbour on the side he is digging. The reason is practical: notice is what lets an adjoining owner inspect, record the condition of his property, get advice and ask for protective measures before the hole is open. An excavation begun without notice deprives everyone of that, and the failure is itself a breach quite apart from whatever damage the digging goes on to cause.
What you can realistically ask for
Not that the neighbour leave his land undeveloped — he may excavate, subject to the duty of support and to whatever the regulations require of the work. What you can press for is the engineering that makes it safe: shoring and sheet piling, staged excavation, dewatering managed so it does not draw down the soil under your footings, monitoring of movement, and a survey of your structure's condition before work starts. Those are ordinary requirements of a competent job, and asking for them in writing beforehand is far more useful than proving afterwards that they were missing.
Record your building before the digging starts
The claim, if it comes, is about what changed. So the document worth having is a dated condition survey of your house — photographs of every wall and floor, existing cracks marked and measured, levels taken — made before the excavation opens, ideally by someone qualified to describe it. Keep the notice you received, or note its absence. If movement appears, record it immediately and repeat the measurements over time, because settlement that is documented as progressing is a very different piece of evidence from a crack photographed once.
Related provisions
- Civil Code, Article 686 — Lateral And Subjacent Support
- Civil Code, Article 687 — Notice To Adjacent Owners