Housesitting in Mexico

No beach in Mexico is private. Under the Ley General de Bienes Nacionales, the beach and the federal maritime-terrestrial zone (the ZOFEMAT — the 20-metre strip inland from the high-tide line) are federal property and common-use land, administered by SEMARNAT. A beachfront title never includes the sand, and a federal-zone concession grants temporary use, never a right to shut the public out. A 2020 reform (DOF 21 Oct 2020, LGBN art. 8) bars anyone from inhibiting, restricting, obstructing or conditioning access to the beach and the federal zone. Blocking it carries fines of 3,000–12,000 times the UMA (art. 154), roughly MXN 352,000–1.4 million at the 2026 UMA of MXN 117.31, and repeat offenders lose their concession. (A 2025 bill making access “free, gratuitous and permanent” passed the Chamber of Deputies in October 2025 and awaits the Senate.) So a sitter may walk the shore below the house, whatever a neighbour or resort claims.

Adverse possession (prescripción positiva) is real in Mexico — five years' continuous possession in good faith, ten in bad faith — but its first and decisive requirement is possession “as an owner”: holding the property as if it were yours, without the owner's permission. Possession that begins with consent — a tenant, a borrower, a caretaker, a housesitter — is “derived possession” and can never ripen into ownership, however long it runs (Código Civil, e.g. CDMX arts. 826, 1151–1152). A signed sit is the opposite of an adverse claim: it is proof James was there by permission.

Mexico doesn't use a simple day-count for residency. Under article 9 of the Código Fiscal de la Federación, you're a tax resident when you establish your home in Mexico; if you keep homes in two countries, it turns on your “centre of vital interests” — more than half your annual income arising in Mexico, or Mexico being your main professional base. Spending 183 days doesn't, by itself, make a sitter a resident. (The “183 days” people half-remember is a different rule — the trigger for taxing a non-resident's income from work physically done in Mexico.) It's true that once you are a resident, you owe tax on worldwide income — but the trope skips how you actually become one.

“One drink” is an exaggeration — the Mexico City limit (Reglamento de Tránsito art. 50) is 0.8 g/L in blood, roughly three to four drinks, with stricter zero-tolerance for commercial and transport drivers. But the back half is true and distinctively local: blow over at the alcoholímetro checkpoint and you're sent to “El Torito” for an administrative arrest of 20–36 hours that is inconmutable — you cannot pay a fine instead, the detention must be served — plus impound and licence points. This is Mexico City specifically, not nationwide.

The system is real and official: SASMEX, the world's first public earthquake early-warning, broadcasts over street loudspeakers and to phones, and the national drill (the Simulacro Nacional, run by Protección Civil) is held each 19 September at 11:00 — the date both the 1985 and 2017 quakes struck. The legal duty, under the Ley General de Protección Civil, sits on buildings and establishments — workplaces, schools, hotels — which must hold an internal civil-protection programme and run drills. There's no personal criminal duty on a houseguest to evacuate; stepping into the street is near-universal practice and a building-level requirement, not something a sitter can be punished for.

Firearms and ammunition are a federal monopoly under the Ley Federal de Armas de Fuego y Explosivos, controlled by the army (SEDENA). Article 84 sets seven to thirty years in prison for bringing arms, parts, cartridges or ammunition reserved for the armed forces into the country without permission — which is exactly how the well-known cases of foreigners jailed over a single forgotten round arise. Even ammunition that isn't reserved for the military carries six to twelve years if brought in without a permit (art. 84 Bis, raised in the 2025 reform). The trap for visitors: calibres that are ordinary at home — 9mm, .357 Magnum, .38 Super, .223 — are reserved military calibres here. A Mexican may keep one registered, low-calibre home-defence weapon at their address, so James finding Sarah's pistol needn't mean a crime by her — but he must never touch, move or carry it, and must never bring his own ammunition into the country.

Money isn't the test. Across legal systems, what makes an arrangement binding is that two people genuinely agreed to it and meant it to count — not whether cash changed hands or anything was signed, and it can be spoken or shown by conduct rather than written down. (Some systems look for something given on each side — here, James's care for Sarah's keys and a roof; others ask only that the promise was seriously meant. A housesit clears either bar.) So 'no cash, nothing signed' is the wrong line everywhere. The subtler, equally universal catch: a purely social favour can fall short of an intended legal commitment, and even a binding deal is painful to prove once memories differ. Agreeing clear terms up front helps with both, which is the point of agreeing them.

In most systems an agreement can be oral or formed by conduct; a signature and a formal document aren't what make it binding. What's hard about an informal deal isn't its validity — it's proving what was actually agreed once memories differ. Writing the terms down doesn't create the obligation; it records it, so nobody has to argue about it later.