Disputes over water running off developed land are governed by one of three rules, and which one applies is state-specific.

Common enemy. A landowner may repel surface water without liability, historically absolute and now generally modified by a reasonableness limit.

Civil law. The lower parcel must accept the natural flow from the upper, and the upper may not increase or concentrate it.

Reasonable use. The modern majority. Liability turns on whether the landowner’s alteration of flow was reasonable, weighing the utility of the conduct against the harm.

What creates liability under any rule. Concentrating diffuse flow into a channel or pipe discharging at a point; increasing volume by paving and directing it to one location; and blocking a natural watercourse.

Drain districts and county drains in many states are statutory systems with their own procedures for petitions, assessments and maintenance, and private claims about a public drain may have to proceed through them.

Development approvals. Stormwater management requirements and detention standards are conditions of site plan approval, and compliance is evidence of reasonableness without being conclusive.

Practical resolution. An engineered study of pre- and post-development flow is what resolves these disputes, and it is far cheaper than the litigation.