Short answer. Repairs and improvements are treated differently. Repairs for preservation may be made at the will of a single co-owner, subject to notifying the others where that is practicable. Work that merely improves or embellishes the property is not his to decide alone; it requires a majority.
What the law says
Repairs for preservation may be made at the will of one of the co-owners, but he must, if practicable, first notify his co-owners of the necessity for such repairs.
Civil Code, Article 489 — Repairs and Improvements. Read the full provision →
Preservation is a one-person decision
The first half of the article is unusually permissive: repairs for preservation may be made at the will of one of the co-owners. A single co-owner may act without waiting for anyone to agree, and the reason is practical. A leaking roof or a failing beam will not pause while the family debates it, and property held in common would otherwise decay whenever the owners could not be assembled. The other side of that permission is that the expense, being for the benefit of the thing itself, is chargeable to all of them in their shares.
The notice condition, and what it is worth
The permission comes with a qualification: he must, if practicable, first notify his co-owners of the necessity for such repairs. The words if practicable excuse notice in a genuine emergency, but they are not a general excuse for silence, and a co-owner who never mentioned the work will find the reimbursement claim harder than it needed to be. A short written message describing what is failing and what it will cost, sent before the work starts, is usually the difference between a claim that is simply paid and one that is argued about.
Improvement and embellishment need a majority
The second half draws the line: expenses to improve or embellish the thing shall be decided upon by a majority. Making the property better is not the same as keeping it standing, and the Code does not let one co-owner commit the others to spending on it. The majority is reckoned by the interests in the property, not by counting heads, so co-owners with larger shares carry more weight. A co-owner who renovates on his own initiative may find the improvement is his own cost, however much the property gained by it.
The distinction is where the money is
Almost every dispute here is a fight about which category the work fell into, and it is decided on evidence of condition rather than on labels in a receipt. Keep photographs of the defect before the work, any inspection or contractor's report saying what would happen if it were left, the invoices, and the message sent to the others. Repairing a rotted roof structure is preservation; replacing a sound roof with a better one is not, and a single project often contains both. Separate the two in the paperwork while the work is being done, not afterwards.