Short answer: a structure that sits on or straddles the property line and serves both properties, a shared driveway easement, a party wall, a private road, is usually a shared cost, split by written agreement or, absent one, by a statutory default. Retaining walls, drainage/runoff, and encroachments are different: they usually turn on who altered the land or whose fault it was, so the honest answer is "it depends." Six fact patterns, six different rules. Below is the deciding factor for each, plus what actually controls before any statute does: the recorded agreement.
Shared driveway (easement) repair
When two properties share a driveway under a recorded easement, the deciding factor is whether there's a written maintenance agreement. California Civil Code §845 is the model most states follow in some form:
California Civil Code §845: "If the easement is owned by more than one person, or is attached to parcels of land under different ownership, the cost of maintaining it in repair shall be shared by each owner of the easement or the owners of the parcels of land, as the case may be, pursuant to the terms of any agreement entered into by the parties for that purpose. In the absence of an agreement, the cost shall be shared proportionately to the use made of the easement by each owner."
No agreement does not mean no obligation, it means the fallback is proportional use, not an automatic 50/50 split. A neighbor who won't pay their share can be sued for their portion.
Party wall (attached homes, townhomes)
A "party wall" is the dividing wall shared by two attached homes. The deciding factor is ownership of the wall itself, and the common-law default leans shared:
Cornell Law School, Legal Information Institute: "a party wall is generally presumed to be common property owned by two tenants in common unless there is some proof suggesting otherwise"
That presumption gets overridden constantly, an HOA or condo association may own the wall as a common area, or the deed may carry a specific party-wall agreement that puts damage one owner caused on that owner alone. Check the deed and HOA governing documents before assuming the common-law default applies.
Boundary retaining wall (lateral support)
This is where "shared structure" stops meaning "shared cost." The deciding factor is who changed the natural grade, not which side of the line the wall sits on. The doctrine is lateral support:
Cornell Law School, Legal Information Institute: "The right of lateral support is a common-law doctrine embodying the principle that the owner of land has a right in having their soil in its natural condition remain in its natural position without being caused to fall away by excavations or improvements made on adjoining land."
Practically: whoever cut into a slope or filled/built up land is the one who owes the ongoing duty to keep that soil supported. An uphill owner isn't automatically on the hook, and neither is a downhill owner, get a survey and pull old grading permits to establish who actually altered the grade before assuming either way.
Surface-water drainage / runoff
This depends entirely on which doctrine your state follows, and the doctrines genuinely disagree with each other. California's Supreme Court rejected one of the three approaches outright:
Keys v. Romley, 64 Cal. 2d 396 (1966): "we need not consider the common enemy rule for it has never been followed in California, and, in fact, was summarily rejected nearly a century ago in Ogburn v. Connor (1873) supra, 46 Cal. 346, 352."
A shrinking minority of states still use that "common enemy" rule (each owner may fend off surface water however they like, no liability to neighbors). Others use a "civil-law rule" that requires respecting natural drainage patterns. A growing number, including California, weigh reasonableness instead:
Keys v. Romley, 64 Cal. 2d 396 (1966): "A few jurisdictions, finding it undesirable to apply either the civil law or common enemy doctrines in their rigid or extreme forms, have evolved a rule of reasonable use which attempts to determine the rights of the parties with respect to the disposition of surface waters by an assessment of all the relevant factors."
The deciding factor: identify your state's doctrine first. Under a reasonable-use or civil-law state, a neighbor's grading, gutters, or fill causing unnatural or unreasonable runoff onto your land is what creates liability, not the mere fact that water crossed the line.
Private road maintenance
Like the shared driveway, the deciding factor here is a recorded agreement first, statutory default second. Maine's residential private-road statute (23 M.R.S. §3121) is a clear example:
Maine Legislature, 23 M.R.S. §3121: "In the absence of any such agreement, restriction, covenant, declaration, road association or method elected under section 3101, each residential property owner, after reasonable due process and notice, shall share equally in the cost of reasonable and necessary repairs to and maintenance of the private road when the private road is the primary means of access to the benefited property."
Note the fallback is equal shares per benefited property (not per owner, and not by use), a different default than the driveway-easement rule above. Check for a road association or recorded agreement before assuming either default applies.
Encroachment (a structure that crosses the line)
A fence, shed, driveway, or roofline that crosses onto your land does not automatically come down at the encroaching neighbor's expense. The deciding factor is the specific remedy a court (or a negotiation) lands on:
Cornell Law School, Legal Information Institute: "An encroachment is an unauthorized intrusion onto a neighboring property through the creation or extension of a physical structure (including flora) above or below the surface of land. Common remedies in the event of an encroachment include neighborly negotiation, the sale of relevant land, the granting of an easement, or injunctive relief."
For a substantial structure, courts weigh the relative hardship of tearing it down against the harm of letting it stay. Timing matters most of all: a long-standing, open, continuous encroachment can ripen into a prescriptive easement or adverse-possession claim in many states, so get a survey and object in writing promptly rather than wait.
What to do before you argue with a neighbor
- Pull the recorded easement, deed, HOA governing documents, or road-maintenance agreement first, in every category above, a written agreement controls before any statutory or common-law default does.
- For retaining walls and drainage, get a survey and check old grading permits before assuming "uphill" or "downhill" decides who pays. It doesn't, the party who altered the grade does.
- Confirm your own state's drainage doctrine (common enemy, civil law, or reasonable use) before treating a runoff dispute as automatic fault.
Have a specific dispute and want it looked up against your city or state's actual rule? Try the city/state lookup or ask a specific question. For more neighbor and shared-cost scenarios, see the full issues list.
This is general information, not legal advice; these rules vary by state and turn on the facts.