• Tenant — if the leak comes from something they installed or caused (appliance hookup, clogged drains, negligence).
• Unit owner / homeowner — if the leak originates inside their unit/house (internal plumbing, water heater inside the unit).
• Neighbor / adjacent unit owner — if the source is in the neighboring unit and migrated to you (e.g., upstairs toilet, bathtub).
• HOA / condominium association — if the leak starts in a common element (roof, exterior walls, shared plumbing stacks, building envelope, common-area pipes) or is the result of failed building systems the HOA must maintain.
• Landlord / property manager — for rental properties, landlords usually maintain structural systems and major plumbing unless the lease shifts certain responsibilities to tenants.
• Contractor — if a contractor’s work caused the leak (poor installation, damage).
• City/utility — rare, but possible when leaks are caused by municipal water mains, street work, or utility lines.

What decides it (key factors)

• Source of leak — tracing (or a professional moisture map/inspection) usually answers this.
• Ownership & maintenance obligations — lease language, deed, or HOA CC&Rs that state who maintains what. Example: some HOAs maintain roofing and exterior walls; owners maintain interior finishes and in-unit plumbing.
• Negligence / cause — if someone’s careless act caused the leak, they may be liable even if it’s not their pipe.
• Insurance coverage — who’s responsible for repairs vs. who pays damages (often handled through homeowner/renter/HOA insurance).
• Local law / building code — in some jurisdictions statutes or case law affect liability and required actions.

What to do right now (practical steps)

1. Stop ongoing damage if safe — shut off water if you can do so safely, move valuables, and place buckets/towels.
2. Document everything — photos, short videos (with dates), meter readings, visible stains, and affected materials.
3. Notify the likely responsible parties immediately — landlord/HOA/neighbor/manager and your insurance company. Put notices in writing (email/text + follow up).
4. Arrange a professional inspection — plumber and/or moisture-mapping/water-intrusion specialist to locate the source and provide a written report. (This is the single most useful item for determining responsibility.)
5. Mitigate — dry affected areas (fans/dehumidifiers) to limit mold/damage — keep receipts. Failure to mitigate can reduce insurance coverage.
6. Collect repair estimates & reports — plumber report, contractor estimate, moisture map, and any communications.
7. If dispute arises, escalate — HOA board, property manager, formal demand letter, then lawyer or small-claims suit as needed.

Evidence that usually determines responsibility

• A moisture map or plumber’s report showing source location.
• Time-stamped photos and communication logs.
• Lease/CC&R/maintenance schedules that assign responsibility.
• Work orders / contractor invoices showing recent work that could have caused the leak.

Quick templates — copy/paste and send

Landlord/HOA notification:

Subject: Urgent: Water leak reported at [Address/Unit #] — please inspect
Hi [Name], I’m reporting a water leak discovered on [date, time]. Affected area: [brief location]. I have taken photos and contained the immediate damage. Please arrange inspection and repairs ASAP and confirm next steps. I will share inspection reports when available. — [Your name, phone]

Neighbor notification:

Hi [Neighbor name], I believe water is coming from your unit into mine (water noticed on [date/time], location: [where]). Could you please check your plumbing and/or allow a plumber/inspector access so we can identify the source? Thanks — [Your name]

Insurance notification (initial):

Claim notice: I discovered water damage on [date]. Source unknown/possible source: [if known]. I have photos and have taken steps to mitigate. Please confirm coverage and next steps. Insured: [name], policy #: [if known].

When to involve lawyers or small-claims

• If you have clear evidence the other party caused the leak and they refuse to pay for repairs or damage.
• If the HOA/manager refuses to act on a common-area leak that is causing damage.
• For personal injury or large property-loss disputes, get an attorney.

Final note / offer

If you want, I can:

• Draft a tailored written notice for your landlord/HOA/neighbor using details you give me, or
• Draft a checklist for the inspector so they capture the evidence you’ll need to assign responsibility, or
• Walk through what typical HOA/condo rules say about responsibility (I can look up California/Los Angeles HOA statutes and sample CC&R clauses if you want me to research).

Which of those would help you most right now?