Boundary & Easement  ·  October 22, 2026

Can a Neighbor Drain Water Onto Your Property in California? The Reasonableness Rule, Deadlines and Hillside Los Angeles

The first big storm of the season arrives, and the water that used to run down the hillside now comes over the wall in a sheet, straight from the neighbor's new patio. Or the neighbor below has built a wall that sends runoff back into your yard. Drainage disputes between California neighbors are common, especially on hillside lots in Los Angeles, and the law that decides them turns on specific facts.

California does not follow the rule that a landowner may simply fend off water however it likes, and it does not let an upper owner send water downhill however it likes either. Since 1966, the test has been reasonableness, on both sides of the property line. This article explains that rule, how it changes when a city's storm drain or a natural creek is involved, what Los Angeles grading rules add, and the deadlines that apply.

Can my uphill neighbor legally drain water onto my property in California?

Yes and no. Under Keys v. Romley (1966) 64 Cal.2d 396, a California landowner may let surface water continue to flow along its natural course, but must take reasonable care not to injure neighbors by changing that flow. If both neighbors act reasonably, the uphill owner who altered the natural drainage bears the loss.

What the California Supreme Court held

In Keys v. Romley, a tenant built an ice rink, paved around it, installed downspouts and regraded the lot. Rainwater then ran onto the neighboring store property in Walnut Creek. The trial court ruled for the store owners under the traditional civil law rule, awarding damages and an injunction, without finding whether the tenant had acted reasonably. The California Supreme Court reversed and sent the case back for that finding.

The court rejected the "common enemy" doctrine, which would let each owner fend off surface water by any means, and observed that the rule "has never been followed in California." It began instead from the civil law rule, under which an upper owner may let water drain as it naturally flows but is liable for discharging it in an unnatural manner, and then modified that rule with a duty of reasonable care. In the court's words, "It is therefore incumbent upon every person to take reasonable care in using his property to avoid injury" to adjacent property through the flow of surface waters. "No party, whether an upper or a lower landowner, may act arbitrarily and unreasonably."

Reasonableness is decided on the facts of each case. The factors courts weigh include the amount of harm, its foreseeability, the purpose with which the owner acted, and whether the utility of the owner's use outweighs the gravity of the harm.

If both owners acted reasonably, the court held, "the injury must necessarily be borne by the upper landowner who changes a natural system of drainage." That rule applies only when the upper owner changed the natural drainage. Water that crosses the line because the land slopes that way, without any alteration, is not, by itself, a basis for liability.

What counts as "surface water"

Keys defines surface water as water "diffused over the surface of land, or contained in depressions therein," such as rainfall runoff. It is distinguished from water in a natural watercourse, a defined channel with a bed and banks, and from flood water that overflows a river or stream. The distinction matters because, as discussed below, the loss is allocated differently for a natural watercourse.

Common examples of changed drainage on California lots

Each of these can change how water leaves a lot. None is unlawful on its own; what matters under Keys is whether it was done reasonably and what it did to the neighbor.

• New hardscape, pool decks and driveways that replace soil that once absorbed rain.
• Downspouts or area drains piped to the property line instead of to the street.
• Regrading or fill that redirects a sheet flow toward one point.
• Retaining walls with weep holes or drains that discharge onto the lower lot.
• A downhill owner's wall, fill or berm that blocks water that used to cross the property and sends it back upslope or sideways.

Can I build a wall or regrade to block my neighbor's runoff?

Not freely. Because California rejected the common enemy rule, a downhill owner who unreasonably blocks the natural flow of surface water can be liable too. Keys requires the lower owner to take reasonable precautions, and in the City of Los Angeles, local grading rules separately restrict changes that impede drainage across a property line.

The lower owner's duty

Keys requires reasonable conduct from both owners. A downhill owner threatened with runoff is expected to take reasonable protective measures, and a downhill owner who builds a wall, fill or berm that dams natural flow and floods the uphill lot, or a third neighbor, can face the same kind of claim. In Locklin, the court repeated that the rule of reasonableness "requires reasonable conduct on the part of downstream owners as well." Before building anything, get advice from a civil engineer.

Los Angeles grading and drainage rules

Building codes do not decide a lawsuit between neighbors, but a violation can be evidence of unreasonable conduct. In the City of Los Angeles:

• Los Angeles Municipal Code Section 91.7013.9 requires roof water, where the natural slope exceeds 3 percent or specified fill is present, to be collected by gutters and carried "to a street or watercourse, or approved drainage facility."
• Section 91.7013.10 requires building pads with cut or fill to drain toward an approved drainage device or facility or a public street, generally at a slope of at least 2 percent.
• Los Angeles Department of Building and Safety Information Bulletin P/BC 2026-057, "Drainage Across Lot/Property Line," states that an upstream owner may not increase discharge onto neighbors or change how runoff naturally occurs, that a downstream owner may not regrade or build in a way that impedes flow across the property line, and that altering a crossing drainage pattern with pipes or similar devices requires a drainage easement from the downstream owner and a recorded covenant and agreement.

These are City of Los Angeles rules. Unincorporated Los Angeles County and other cities, such as Glendale, Burbank and Pasadena, have their own codes. Check permits and inspection history for the neighbor's work with the building department; unpermitted grading or drainage work often surfaces there.

Self-help has limits

Civil Code Section 3502 allows a person injured by a private nuisance to abate it if it can be done without a breach of the peace and without unnecessary injury. Entering a neighbor's property to remove a pipe or cut a drain is risky and can lead to a counterclaim. A written demand, an engineer's report and, if needed, a court order are the safer path.

Retaining walls on hillsides

When a retaining wall between neighbors fails in a storm, a common question is whether one side is automatically responsible. In Quinn v. Coulton (July 20, 2026, A172217), a published decision in which uphill owners sued a downhill neighbor after a wall failed in a heavy San Francisco storm, the First District Court of Appeal held that "California law does not recognize a strict liability claim for loss of lateral support absent some excavation activity." Without excavation, the claim must generally be pursued on a theory such as negligence or nuisance. Civil Code Section 832 separately governs excavation next to a neighbor's land, including notice and, for an excavation deeper than the walls or foundations of an adjoining building and close enough to endanger it, at least 30 days for the neighbor to protect the structure. Wall and boundary questions often overlap with encroachment claims.

What if the water comes from a California city street, storm drain or creek?

Different rules apply. A natural watercourse is governed by Locklin v. City of Lafayette (1994) 7 Cal.4th 327, under which, if everyone acted reasonably, the downstream owner bears the loss. Claims against a California public entity also carry claim-filing requirements and, for inverse condemnation, proof that the public improvement was unreasonable and a substantial cause of the damage.

Natural watercourses

In Locklin, homeowners along Reliez Creek in Contra Costa County sued after upstream development increased flows and eroded their banks. The California Supreme Court held that "The test is whether, under all the circumstances, the upper landowner's conduct was reasonable," and that the rule applies to "both private and public landowners." But for a natural watercourse, if both sides acted reasonably, the loss falls on the downstream owner, who is expected to take reasonable protective measures. That is the opposite of the Keys result for surface water.

Inverse condemnation is not strict liability for flooding

Article I, Section 19 of the California Constitution requires just compensation when private property is "taken or damaged" for public use. Flood and drainage cases have narrowed that remedy:

• In Belair v. Riverside County Flood Control Dist. (1988) 47 Cal.3d 550, the court held that "the fact that a dam bursts or a levee fails is not sufficient, standing alone, to impose liability"; the owner must show the public entity acted unreasonably.
• In City of Oroville v. Superior Court (2019) 7 Cal.5th 1091, the court held that "A causal connection between the public improvement and the property damage alone is insufficient to sustain a finding of inverse condemnation liability." The damage must be substantially caused by an inherent risk of the improvement's deliberate design, construction or maintenance. The owners in that case had not installed a legally required backwater valve, which the court found significant.
• In Biron v. City of Redding (2014) 225 Cal.App.4th 1264, a storm drain designed for a 10-year storm was overwhelmed by a storm closer to a 100-year event; the Court of Appeal held the city, which had acted reasonably, was not liable.

Other claims against public entities

A city may also be liable under Government Code Section 835 for a dangerous condition of public property, which requires proof that the property was in a dangerous condition at the time of the injury, that the condition proximately caused the injury, that it created a reasonably foreseeable risk of the kind of injury that occurred, and that the city either created it through an employee's negligent or wrongful act or omission or had actual or constructive notice of it in time to protect against it.

In Los Angeles County, a storm drain may belong to the city, the County, the Los Angeles County Flood Control District or another agency. A government claim must go to the public entity responsible for the facility, so identify the owner before the claim deadline runs.

How long do I have to sue over drainage damage in California?

Against a private neighbor, generally three years under Code of Civil Procedure Section 338(b), with the start date depending on whether the condition is permanent or continuing. Against a California city or county, most damage claims first require a government claim, within one year after accrual for real property and six months for personal property.

Permanent versus continuing

A permanent nuisance or trespass supports one action for all past, present and future damages, and the three-year period runs from its creation. A continuing one supports successive actions, but each recovers only damages suffered within the limitations period. Under Mangini v. Aerojet-General Corp. (1996) 12 Cal.4th 1087, the test is whether the condition can be remedied at a reasonable cost by reasonable means, so an engineer's cost-to-cure estimate is often the key evidence in a drainage case. Our article on how long you have to sue over an encroachment explains the permanent-versus-continuing test in detail, including Baker v. Burbank-Glendale-Pasadena Airport Authority (1985) 39 Cal.3d 862, and our statutes of limitations guide covers other real property deadlines. If the drainage problem is a defect in a newer home's own construction rather than a neighbor's work, the claim against the builder falls under California's Right to Repair Act, which has its own notice procedure and shorter deadlines for some items; see our guide to SB 800 builder deadlines.

Deadlines at a glance

If one storm damaged both the house and its contents, the contents claim carries the six-month deadline. File any government claim within six months of the damage unless a lawyer has confirmed the longer period applies to everything you are claiming. Under Government Code Section 912.4, a claim the board does not act on within 45 days after it is presented (unless extended by written agreement) is deemed rejected; the deadline in Section 945.6 then depends on whether written notice of rejection was given: six months after that notice, or generally two years from accrual if none was given.

What you can recover

Remedies against a private nuisance include a civil action and abatement (Civ. Code Sec. 3501), and a court may order an injunction requiring the neighbor to stop or redirect the flow. Damages for injury to real property are generally the lesser of the reduction in value or the reasonable cost of repair, with an exception allowing reasonable repair costs above the loss in value where the owner has a genuine personal reason to repair (CACI No. 3903F). Insurance is a separate question: many homeowners policies exclude flood and surface water. In Linsao v. First American Property & Casualty Ins. Co. (Aug. 27, 2026, B340746), a published decision, the Court of Appeal upheld a denial of coverage for a Sherman Oaks hillside home damaged when storm runoff was channeled by an unfinished retaining wall on an upslope lot, holding that every possible cause of the loss fell within a policy exclusion, including the exclusion for inadequate construction. Read your policy and give prompt notice.

Evidence that matters in drainage cases

• Dated photos and video taken during rain, showing where water enters.
• Before-and-after records of the neighbor's paving, grading, downspouts or walls.
• Building permit and code enforcement history for both lots.
• A survey and topographic map.
• A report from a licensed civil or geotechnical engineer on flow paths and causation, with a cost-to-cure estimate.
• Proof of your own reasonable precautions, since California law expects them of the lower owner.

Drainage problems are often tied to easement and boundary rights, including whether a long-used drain line created a prescriptive right. Our boundary and easement disputes practice handles these cases throughout California, and our article on adverse possession and prescriptive easement claims covers related questions. If water is damaging your property now, contact us with photos and dates.

Common questions

This article is general information about California law as of October 22, 2026 and is not legal advice. It does not create an attorney-client relationship. Anyone facing a specific dispute should consult a lawyer about their own circumstances.

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