A lock-out agreement, also called an exclusivity agreement, is a seller's promise not to negotiate with, or accept an offer from, anyone else for a fixed period while a specific buyer carries out due diligence, arranges finance, or finalises terms. It does not commit the seller to sell, and it does not commit the buyer to buy. What it buys the buyer is time and certainty that the seller will not use that time to entertain a competing offer.
Why the agreement must have a fixed time period
English contract law is firm on this point: a lock-out agreement is only enforceable if it has a defined, fixed time period. An open-ended promise to negotiate exclusively, with no end date, is not enforceable, because it amounts to an unenforceable "agreement to negotiate in good faith." The leading authority is Walford v Miles [1992], a House of Lords decision that held an open-ended agreement to negotiate in good faith is void for uncertainty: a court has no workable way to decide when good-faith negotiation has ended or been breached, since there is no objective standard for when negotiations should conclude.
The House of Lords in Walford v Miles did, however, leave the door open to a properly time-limited lock-out. That door was confirmed in Pitt v PHH Asset Management Ltd [1994] 1 WLR 327 (CA), where the Court of Appeal upheld a lock-out agreement as a valid, enforceable collateral contract precisely because it had a fixed period, two weeks, and was supported by consideration. The distinction is the whole case: promise not to negotiate with anyone else, for a defined period, in exchange for something of value, is enforceable. Promise to negotiate in good faith indefinitely is not.
What happens if a seller breaches a lock-out
Even where a lock-out is valid and enforceable, the remedy available to a disappointed buyer is limited. Tye v House [1997] 2 EGLR 171 confirmed that the remedy for breach of a lock-out agreement is damages, typically to cover wasted due-diligence costs the buyer incurred in reliance on the exclusivity, not specific performance forcing the seller to sell. In practical terms, a seller who breaches a lock-out and sells to someone else can simply wait out any claim: the buyer cannot force the sale to go ahead, only recover the costs the breach caused them to waste. This significantly limits how much protection a lock-out genuinely provides, and buyers should treat it as a tool for creating space to negotiate, not a guarantee of an eventual purchase.
This principle has continued to be reaffirmed by the courts. In KSY Juice Blends UK Ltd v Citrosuco GmbH [2025] EWCA, the Court of Appeal again confirmed the same underlying distinction between an enforceable, time-limited exclusivity commitment and an unenforceable open-ended promise to negotiate in good faith, underscoring that this is settled, current law rather than a historical curiosity.
A lock-out agreement is not a substitute for the underlying sale contract, and it does not override the "subject to contract" status of any price or terms discussed alongside it. A seller can still be negotiating a subject-to-contract price with the same buyer during the lock-out period, non-binding right up to exchange, while the lock-out itself operates as a separate, narrower promise: not to open that same conversation with anyone else in the meantime. Keeping the two elements distinct in the drafting, the non-binding heads of terms on one hand and the binding, time-limited exclusivity promise on the other, avoids confusion about what is actually enforceable if a dispute arises.
How to draft a lock-out agreement properly
Three drafting points consistently separate an enforceable lock-out from one that will fail if tested. First, the period must be precise: a specific end date or a clearly calculable duration, not language like "a reasonable period" or "until terms are agreed." Second, there should be identifiable consideration for the exclusivity, a nominal payment is common, or the agreement can instead be executed as a deed, which does not require consideration to be enforceable. Third, and most important, the drafting should avoid language that drifts back into the territory Walford v Miles ruled out: phrases like "negotiate in good faith" or "use best endeavours to agree terms" reintroduce the uncertainty that makes an agreement unenforceable, even if a fixed period is also present elsewhere in the document. The safest drafting states plainly what the seller will not do (approach, negotiate with, or accept an offer from anyone else) for a stated period, full stop.
Why lock-outs fit direct negotiations but are rare through agents
Lock-out agreements are not legally barred in an agent-run, marketed sale. In practice, though, they are rare there, for a structural reason rather than a legal one. Section 21 of the Estate Agents Act 1979 requires an instructed agent to pass on every offer received to the seller right up to exchange of contracts. An agent operating under that duty has little room to help a seller commit to genuine exclusivity with one buyer while the property remains actively marketed to others, since new offers keep arriving and must be disclosed regardless of any informal exclusivity understanding. A direct, off-market negotiation removes that structural friction: with no agent instructed and no live marketing process running in parallel, a seller and buyer are free to agree, and mean, a fixed-term lock-out without the tension of a duty pulling in the opposite direction. It is rare in practice through agents, not a legal impossibility, but the practical effect is the same: lock-outs are a tool that belongs naturally to direct negotiation.
When a lock-out makes sense
A lock-out is most useful where due diligence, survey, or finance arrangement will take real time and cost real money, and where a buyer needs assurance the seller will not use that window to shop the property elsewhere. It is a lighter-weight tool than an option agreement or a subject-to-planning contract: it does not fix a price, or bind either party to complete a sale, it simply buys a defined, protected period to negotiate and investigate. Because the remedy for breach is limited to wasted costs, buyers relying on a lock-out should still move quickly and keep due-diligence costs proportionate to the protection the agreement actually offers.
In a typical direct, off-market negotiation, a lock-out might run alongside the early stages of agreeing a subject-to-planning contract or an option: the buyer asks for two to six weeks of exclusivity to commission surveys, review title, and confirm finance, in exchange for a modest payment or simply the seller's willingness to see the process through with a serious, credible buyer rather than field competing approaches. Because the remedy for breach is limited to wasted costs rather than forcing the sale through, a lock-out works best as a short bridge into a more binding structure, an option, a conditional contract, or exchange itself, rather than as a long-term substitute for one.
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Is a verbal promise of exclusivity enforceable?
Generally not reliably. Courts look for a clear, fixed time period and, ideally, identifiable consideration or a deed. A vague verbal understanding is far more likely to be treated as an unenforceable agreement to negotiate rather than a binding lock-out.
Can a lock-out agreement force a seller to complete the sale?
No. Following Tye v House, the remedy for a breach of a valid lock-out is damages, typically wasted due-diligence costs, not specific performance. A lock-out cannot be used to force a sale through.
How long should a lock-out period be?
It depends on how long due diligence genuinely needs. Two weeks was the period upheld in Pitt v PHH Asset Management, but the right length depends entirely on the complexity of the property and the checks the buyer needs to complete, provided it is a fixed, stated period rather than open-ended.