The bilateral promise contract in Spanish property: Article 1451 of the Codigo Civil and a mutual obligation to buy and sell (2026)

A bilateral promise contract (contrato de promesa bilateral de compraventa) in Spain is a precontract under which a buyer and a seller, having already agreed the property and the price, mutually bind themselves to complete a sale at a future date. Article 1451 of the Codigo Civil governs it, giving each party the right to demand reciprocal performance. The Tribunal Supremo has treated the bilateral promise as a pactum de contrahendo since its Sentencia of 15 March 1945, and its settled doctrine holds that the essence of the figure is to defer the perfection and entry into force of the projected sale while the parties remain bound by the precontractual bond.

What legal article governs the bilateral promise in Spain?

The operative article is Article 1451 of the Codigo Civil, not Article 1254 or 1255. Article 1254 defines contract formation in general, and Article 1255 sets out party autonomy, but Article 1451 is the specific provision that the Tribunal Supremo applies to the promise of sale. Its text reads: the promise to sell or buy, where there is agreement on the thing and on the price, gives the contracting parties the right to reciprocally demand performance of the contract; where the promise cannot be fulfilled, the rules on obligations and contracts in the same book apply to the seller and the buyer as the case may be.

Two features of this text matter for property practice. First, Article 1451 covers both the unilateral promise (one party bound to keep an offer open) and the bilateral promise (both parties bound to a future sale). The Spanish civil law commentary on Article 1451, reflected in the BOE consolidated text, treats the bilateral variant as the default reading because the article gives rights to “los contratantes”, the plural, and frames the remedy as “reclamar recíprocamente”, a reciprocal claim. Second, the second paragraph of Article 1451 sends the parties to the general obligations regime when performance is impossible, which is the bridge to Article 1124 (resolution for breach) and to the Tribunal Supremo doctrine on dificultad extraordinaria discussed below.

This is the distinction that the published purchase option guide covers from the option side. The purchase option sits on Article 1255 because it is an atypical contract with no dedicated article in the sale chapter; the bilateral promise sits on Article 1451 because the sale chapter expressly addresses it. Confusing the two articles is the single most common error in Spanish property precontract analysis, and it changes the remedy: under an option only the concedente is bound, while under a bilateral promise both parties are irrevocably bound.

How does a bilateral promise differ from a unilateral promise and from an option?

The three precontractual figures in Spanish property law, the unilateral promise, the bilateral promise, and the opcion de compra, are easy to conflate but produce very different obligations. The table below sets out the distinctions a buyer or seller actually needs.

Feature	Unilateral promise (Art 1451)	Bilateral promise (Art 1451)	Opcion de compra (Art 1255)
Who is bound	One party only (the promisor)	Both parties irrevocably	Only the concedente
What the other party holds	Right to demand performance	Reciprocal right to demand performance	Right to decide whether to buy
Can the bound party withdraw	No, not unilaterally	No, neither party can	No, the concedente is locked in
Can the holder walk away	Yes, no obligation to proceed	No, walking away is breach	Yes, the optante loses only the premium
Price and thing	Must be agreed	Must be agreed	Must be fixed in the option
Remedy for breach	Cumplimiento especifico or resolution under Art 1124	Cumplimiento especifico or resolution under Art 1124	Loss of premium, plus damages in some cases
Registry treatment	Not inscribed as a sale; anotacion preventiva de demanda available	Not inscribed as a sale; anotacion preventiva de demanda available	Registrable under Reglamento Hipotecario Art 14, four year maximum

The bilateral promise is the only one of the three that creates a reciprocal obligation to complete. A seller who has signed a bilateral promise cannot accept a higher offer from a third party during the term, because doing so is breach of an obligation already perfected, not merely a withdrawal of an offer. A buyer who has signed a bilateral promise cannot walk away because they found a cheaper property, because their signature is an undertaking to purchase, not an option to consider it.

How does the bilateral promise differ from arras?

Arras are a deposit placed on a sale that has already been agreed. Article 1454 of the Codigo Civil governs the penitential variant, under which either party can withdraw by forfeiting the deposit (the buyer) or returning it doubled (the seller). The bilateral promise is the instrument that creates the obligation to reach the sale in the first place, before any deposit changes hands. The two figures often appear together in a Spanish property transaction, but they do different work.

The arras reservation contract typically takes a property off the market for a short period while due diligence and financing are completed. The bilateral promise is used for a longer horizon, when the parties have agreed the commercial terms but completion depends on a future event: a licence being granted, a plot being segregated, a mortgage being discharged, a development being finished. The arras sit on top of an agreement; the promise is the agreement to agree, binding the parties to reach the formal deed.

A common structure on the Costa del Sol, particularly for off-plan or pre-construction purchases, is a bilateral promise followed by arras at the point of starting construction, followed by the property purchase contract and finally the escritura publica. Each stage tightens the obligation and moves the transaction toward the notary. The bilateral promise is the earliest binding instrument; the arras are the middle-stage commitment; the escritura is the transfer itself.

What does the Tribunal Supremo say about enforceability?

The Spanish Supreme Court has built a steady line of doctrine on the bilateral promise since 1945. The Sentencia of 15 March 1945 first admitted the figure of the precontract, the bilateral promise, and the pactum de contrahendo into Spanish civil jurisprudence, reading Article 1451 as the statutory basis. The Sentencia of 3 June 2002 (STS 521/2002) restated the doctrine: the essence of the bilateral promise of sale, to which Article 1451 refers, is to defer to a later moment the perfection and entry into force of the projected contract, leaving the parties bound in the meantime by the peculiar bond that the precontract produces.

The operational consequence is that the bilateral promise is not itself a sale. It is a precontract that converts into the sale when either party demands performance and the conditions for perfection, the thing and the price, are met. The Sentencia of 24 October 2008 (STS 1006/2008) characterised the precontract as a promise to sell or buy regulated in Article 1451, confirming that the Tribunal Supremo treats Article 1451, not Article 1254, as the statutory home of the figure. A party who treats the bilateral promise as already perfected and tries to enforce a transfer before the agreed term runs the risk of being met with a defence that the precontractual bond, not the sale, is what currently exists.

The remedy the promisee can claim is cumplimiento específico, specific performance. The first paragraph of Article 1451 gives the contracting parties the right to reciprocally demand performance, which the Tribunal Supremo reads as a right to compel the other party to appear before a notary and execute the deed. This is stronger than a damages claim. It is the right to the actual property, not the right to compensation for not getting it.

What happens when performance becomes impossible?

The second paragraph of Article 1451 sends the parties to the general obligations rules when the promise cannot be fulfilled. Article 1124 of the Codigo Civil gives the aggrieved party the right to resolve obligations that are reciprocally binding, and the Tribunal Supremo has held that the same remedy applies to bilateral promises when performance has become impossible or extraordinarily difficult.

The Sentencia of 5 June 2014, resolving the Jubing versus Urbem case, illustrates the line. A property developer had signed a reciprocal promise of sale to deliver parking spaces in a basement it would build on a plot in Valencia. The city council denied the building permit, making the construction legally impossible. The Tribunal Supremo applied the doctrine from its Sentencia of 30 April 2002, which equates impossibility with extraordinary difficulty, and held that the contract should be resolved under Article 1124 rather than enforced. The promisee could not compel performance that the promisor could not legally deliver, but it could claim resolution and the damages that flow from it.

This is the limit on cumplimiento específico that buyers and sellers should understand before signing a bilateral promise. Specific performance is the default remedy, but it gives way to resolution where the obligation has become objectively impossible. A seller who loses the property to an expropriation, or a buyer whose financing is legally blocked, is not in mere breach; they are in the regime of the second paragraph of Article 1451, and the remedy is Article 1124 resolution with damages rather than forced completion. The condicion resolutoria guide covers the Article 1124 mechanism in full.

Can a bilateral promise be registered in the Land Registry?

A bilateral promise is not inscribed as a sale, because the sale has not been perfected. The Land Registry records transfers and real rights, not precontractual bonds. The practical protection for a promisee who wants to secure their position against a third party is the anotacion preventiva de demanda, the preventive notice of a lawsuit, provided for in Article 42 of the Ley Hipotecaria.

The mechanism works as follows. The promisee files a performance claim in court and, at the same time, applies for an anotacion preventiva to be noted on the registry entry of the property. The note warns any third party who later acquires the property that there is a pending lawsuit, and the third party takes subject to the outcome of the litigation. If the court orders performance, the resulting deed is inscribed in the normal way and the anotacion is cancelled. If the court denies performance, the anotacion is cancelled and the third party takes free of the claim.

This is a weaker protection than the purchase option enjoys under Article 14 of the Reglamento Hipotecario, which permits the option itself to be inscribed as a real right for up to four years. The bilateral promise does not get that treatment because Article 1451 creates a personal obligation to complete, not a real right over the property. The promisee holds a claim, not a property right, until the court converts the claim into a deed.

The pacto de retro guide covers a different but related mechanism, the right of repurchase, which is registrable because it attaches to a sale that has already been perfected. The bilateral promise is earlier in the timeline than the pacto de retro, and its registry treatment is correspondingly lighter.

When is a bilateral promise the right instrument?

The bilateral promise fits the cases where both parties have agreed the commercial terms but completion must wait for a future event that neither controls. Three scenarios are common on the Costa del Sol.

The first is a property that is still in the planning or segregation process. The buyer and seller agree the price now, but the sale cannot complete until the plot is legally segregable or the licence is granted. A bilateral promise locks both sides in: the seller cannot accept a better offer while the paperwork is processed, and the buyer cannot walk away because they found a cheaper unit. The buying property as a foreigner guide places the promise in the overall purchase timeline.

The second is a development where the buyer wants to commit to a specific unit before the building is finished but after the commercial terms are settled. This overlaps with the off-plan regime, but the bilateral promise is used when the parties want a precontractual bond rather than the bank-guaranteed stage-payment structure of a full off-plan contract.

The third is a sale contingent on the seller discharging an existing mortgage or resolving a boundary issue. The bilateral promise obliges the seller to clear the encumbrance by a set date and the buyer to complete once it is cleared. If the seller fails to clear it, the buyer has the Article 1451 right to demand performance or, if that has become impossible, the Article 1124 right to resolve with damages.

In each scenario the bilateral promise does what neither an option nor arras can do: bind both parties irrevocably to a future sale while preserving the right to walk away only if performance becomes legally impossible. The option binds only one party; the arras sit on a sale already agreed and allow a penalty walkaway. The bilateral promise is the instrument for the middle ground, where the deal is struck but the paperwork is not.

This guide is general information, not legal or tax advice. Rules change and individual circumstances differ. Verify current requirements with an independent lawyer (abogado) or tax advisor (gestor/asesor fiscal) before acting.