Short answer. Usually no. Under Article 1479, an accepted unilateral promise to sell binds the seller only if the promise is supported by a consideration distinct from the price. If you paid nothing separate for the option itself, the seller can generally still withdraw the offer before you exercise it.
What the law says
An accepted unilateral promise to buy or to sell a determinate thing for a price certain is binding upon the promisor if the promise is supported by a consideration distinct from the price.
Civil Code, Article 1479 — Mutual Promises to Buy and Sell, and Option Money. Read the full provision →
An option needs its own consideration
Article 1479 says an accepted unilateral promise to buy or to sell a determinate thing for a price certain is binding upon the promisor if the promise is supported by a consideration distinct from the price. An option is one-sided: the seller ties his own hands and gives you time to decide, while you remain free to buy or not. The law will hold the seller to that only if you gave something separate — often called option money — in exchange for the promise to keep the offer open. That payment is distinct from the price of the thing itself. Without it, the seller's promise, even if you accepted it, lacks the consideration the article requires.
Why a free option can be taken back
If you paid nothing for the option, the seller has received nothing in return for waiting. In law, a promise unsupported by consideration is not enforceable against the person who made it. So the seller may generally withdraw the offer any time before you actually exercise it — that is, before you communicate your acceptance and are ready to pay the price. Your earlier "acceptance" of the option does not lock him in, because it is the separate consideration, not your say-so, that makes the option binding. This is a frequent and painful surprise for buyers who believed a signed but unpaid option guaranteed them the property.
Option money is not earnest money
It helps to keep two payments apart. Option money is the price of the option — the sum you pay just to hold the seller to his offer for a period. Earnest money is part of the purchase price, given when a sale is already perfected, and it proves the sale exists. Article 1479 is about the first. If what you handed over was truly a separate payment for the exclusive right to decide, the option binds the seller. If you handed over nothing, or the money was really a partial payment on an already-agreed sale, a different analysis applies. Getting this label right often decides whether the seller can walk away.
How to make the option stick
The safe course is to put the option in writing and to pay a real, separate consideration for it, however modest, clearly identified as the price of the option rather than a deposit on the thing. State the period during which the seller must keep the offer open and the fixed price at which you may buy. Once valid consideration exists, the seller cannot lawfully sell to someone else or revoke during the agreed period, and if he does he answers in damages. Absent that consideration, the article gives you no hold on him — a lesson worth acting on before you rely on an unpaid option.