Land is supported both from underneath (subjacent support) and from the sides (lateral support). Changes on one party’s land may undermine the support for another party’s land or property. California law has a series of rules governing this.

First, in the situation in which the land surface is owned by one party, and the subsurface, directly beneath that land, is owned by another party, the surface owner has the absolute right to subjacent support, Marin Municipal Water District v. Northwestern Pacific Railroad Co. (1967) 253 Cal. App. 83, and the owner of the subsurface has an absolute duty to provide such support. Platts v. Sacramento Railway (1988) 205 Cal. App. 3rd 125. If this duty is violated, the injured party may sue for damages or injunctive relief to stop the acts, which are causing the harm.

Second, there are owners of land, which is next to each other or coterminous.. Their mutual rights are governed by Civil Code Section 832. The statute provides that owners of adjoining land generally are entitled to lateral and subjacent support from neighboring land. To make it possible to construct improvements on land, however, without having unlimited liability to the neighbor, Section 832 sets out the rules for excavations on one’s own land.

These rules apply to proper and usual excavations for the purpose of construction or improvement. First, the party doing the digging has to give reasonable notice to his or her neighbors about when the digging will occur and how deep it will go. Second, the party doing the excavation shall to use ordinary care and skill, and to take reasonable precautions to provide support to his or her neighbor’s land. As a rule, this support needs to be provided only for the land, not for the buildings. This distinction between land and buildings is centuries old in the common law.

The statute does, however, provide some protection for buildings. First, for excavations of less than nine feet deep, if they are so close to a neighbor’s building as to endanger it, the non-excavating neighbor is given thirty days to take reasonable measures to protect his or her building. Among other things, he or she is permitted to enter onto the land of the digging party, to the reasonable degree necessary to protect the building.

Second if the excavation is deeper than nine feet, the excavating party must act so as to protect both the land and the buildings of his or her neighbor. If the digging party needs to enter onto his or her neighbor’s land, to protect the buildings, and the neighbor denies such entry, then the digging party is not liable for the damage.