What Is a Unilateral Contract? How It Works & Examples

A unilateral contract is a legally binding agreement in which one party makes a promise that the other party can accept only by actually performing a specific act. Only the person making the offer takes on an obligation. The person on the receiving end owes nothing unless and until they complete the requested act, at which point the promise becomes enforceable.¹

How It Works

The mechanics are simple. One party (the offeror) announces a promise tied to a specific action. The other party (the offeree) accepts by doing the act, not by saying “I agree” or signing anything. Until the act is completed, no contract exists and the offeree is free to walk away.¹

Say your neighbor tells you, “I’ll pay you $200 if you mow my lawn while I’m on vacation.” You owe your neighbor nothing. You can ignore the offer entirely. But if you mow the lawn, your neighbor owes you $200. The mowing is both your acceptance and the consideration that makes the deal enforceable.

This structure puts the risk on one side. The offeror is locked in once performance is complete. The offeree carries no exposure because they never promised to do anything. And the contract springs into existence at the moment performance finishes, not when the offer is announced and not when someone starts working on it.

How It Differs from a Bilateral Contract

Most contracts people run into are bilateral. In a bilateral contract, both parties exchange promises and both are bound the moment those promises are made. A buyer promises to pay, a seller promises to deliver, and either can sue the other for backing out. A standard employment agreement works the same way: you promise to work, your employer promises to pay, and both sides have obligations.²

A unilateral contract flips that. Only one party makes a promise, and acceptance happens through action. If you post a $500 reward for your lost dog, you aren’t entering into a mutual exchange with anyone in particular. You’re making an open-ended promise to the world, and it only binds you when someone actually returns the dog.¹

The practical consequence shows up when something goes wrong. In a bilateral contract, either party can sue the other. In a unilateral contract, only the offeree can sue, and only after completing the act. The offeror cannot sue the offeree for failing to perform, because the offeree never promised anything in the first place.

Everyday Examples

Reward Offers

Rewards are the textbook case. A person or company promises money for information, the return of lost property, or some other specific result. The reward becomes enforceable the moment someone delivers what was asked for. Police department rewards for information leading to an arrest work the same way: nobody has to come forward, but anyone who provides qualifying information has a right to the money.¹

Contests

When a company offers a prize to the first person who completes a challenge, that is a unilateral contract. Nobody is required to enter. But the sponsor must award the prize to whoever meets the stated conditions.¹

Insurance Policies

Most insurance policies work as unilateral contracts. The insurer makes a legally enforceable promise to pay covered claims. The policyholder does not promise to keep the policy going; they simply meet conditions like paying premiums and reporting incidents to keep coverage in force. If you stop paying, the policy lapses, but the insurer cannot sue you for quitting. If a covered event happens and you have met the conditions, the insurer must pay.

Employment Bonuses and Sales Incentives

Employers create unilateral contracts all the time without labeling them that way. An employer who announces “anyone who exceeds their sales quota by 20% this quarter gets a $5,000 bonus” has made a unilateral offer. No employee is required to hit that target, but any employee who does has earned the bonus. The same logic applies to commissions, referral bonuses, and performance-based incentive programs.

Open Listings in Real Estate

An open listing agreement is a classic example. A seller gives several brokers the right to find a buyer, but none of the brokers are obligated to lift a finger. Whichever broker produces a ready, willing, and able buyer first earns the commission. The others get nothing.

When an Advertisement Becomes a Binding Offer

Most ads are not offers. Legally, a typical advertisement is an “invitation to treat,” which just means an invitation to start negotiating. A store that advertises a television at a listed price is not bound to sell it to everyone who walks in.

The famous exception comes from an 1893 English case that still shapes contract law. In Carlill v. Carbolic Smoke Ball Co., the company advertised that it would pay £100 to anyone who caught influenza after using its product as directed. When Mrs. Carlill used the product, caught the flu, and demanded payment, the company called the ad promotional puffery. The court disagreed. Because the company had deposited £1,000 in a bank to back the promise, it had shown clear intent to be bound. The court held that the advertisement was a unilateral offer and that Mrs. Carlill accepted it by performance.³

An advertisement crosses into a binding unilateral offer when it contains specific terms, names a clear action required for acceptance, and shows genuine intent to be bound. Vague marketing slogans do not qualify. A specific promise tied to a specific act does.

You Have to Know About the Offer

Here is a wrinkle that catches people off guard. To accept a unilateral offer, you generally need to know the offer exists before you perform the act. If someone posts a $1,000 reward for returning a lost dog and you happen to find and return the dog without ever seeing the notice, most courts will not let you collect. Your act was not motivated by the offer, so it does not count as acceptance.

The rule flows from a basic principle: acceptance requires some awareness of what you are accepting. A kind act done in ignorance of an offer is just a kind act.

The Act Must Be Something You Weren’t Already Obligated to Do

Every enforceable contract needs consideration, meaning each side has to give up something of value. In a unilateral contract, the offeree’s performance is the consideration. In Carlill, even the inconvenience of using the product as directed was enough, because the company benefited from the public actually trying it.³

But if you were already legally required to do the act, your performance does not count as fresh consideration. This is the pre-existing duty rule. A police officer cannot claim a reward for arresting a fugitive, because arresting fugitives is already part of the job. A contractor already under contract to paint your house cannot treat a separate “bonus for finishing on time” offer as a new unilateral contract if timely completion was already required. The act has to be something beyond what the person already owed.

Can the Offer Be Revoked Once You’ve Started?

Picture this. You offer someone $1,000 to paint your entire fence. They spend three days in the sun getting 90% of the job done, and then you revoke the offer. Under older common law, you could do exactly that. An offeror could pull a unilateral offer at any time before the offeree finished performing. The person who did most of the work got nothing.

Modern contract law fixes this through Section 45 of the Restatement (Second) of Contracts. Once the offeree begins the actual performance requested, an option contract is automatically created. That option contract makes the offer irrevocable, giving the offeree a chance to finish. The offeror still does not have to pay until performance is complete, but the offeror can no longer yank the offer away mid-job.⁴

Preparation Versus Performance

Section 45 protects people who have started performing, but not people who have merely prepared. This distinction trips people up more than any other part of unilateral contract law. Buying paint and brushes for the fence job is preparation. Actually putting paint on the fence is performance. Only the latter triggers the protection.⁴

Courts look at whether the conduct is clearly tied to the offer, whether it is definite and substantial, and whether it benefits the offeror rather than just the offeree. Driving to the store to buy supplies benefits you; putting those supplies to work on the offeror’s fence benefits the offeror. That shift is roughly where preparation ends and performance begins.⁴

If the offer gets revoked while you are still in the preparation phase, Section 45 will not help. A separate rule under Section 87(2) can sometimes rescue you if you reasonably relied on the offer and suffered real losses as a result. That will not guarantee the full contract price, but it may let you recover what you spent in reliance.⁵

Suing to Enforce It

If you completed the requested act and the offeror refuses to pay, you can sue for breach of contract. You have to prove three things: that a valid offer existed, that you completed the performance as specified, and that the offeror failed to honor the promise. Where there is a dispute over whether the conditions were met, the party claiming they were satisfied carries the burden.

Statutes of limitations vary by state, generally running from two to six years depending on whether the contract was written or oral. Because many unilateral contracts are informal, they often fall into the oral category, which carries the shorter window. Waiting too long to act can cost you the right to recover at all.

1
Legal Information Institute. Unilateral Contract
2
Open Casebook. Unilateral vs. Bilateral Contracts – Manufactured Difficulties Introduction
3
Justia Law. Carlill v. Carbolic Smoke Ball Co.
4
H2O. Restatement (Second) of Contracts 45 – Option Contract Created by Part Performance or Tender
5
H2O. Restatement (2d) 25, 45 and 87 – Option Contracts