A son your father never acknowledged appears and claims the estate

The case, in five lines

What is brought

Claim to establish non marital filiation (articles 131 and 133 of the Código Civil), followed or accompanied by the succession claims that filiation opens up: preterición under article 814 if there is a will that omits him, or the petición de herencia (claim to the estate) and division if there is none, with the payment provided for in article 1080 where the estate has already been divided.

Before which court

The filiation claim goes before the Tribunal de Instancia (the first-instance court) through the special procedure of articles 764 onwards of the Ley de Enjuiciamiento Civil (the Civil Procedure Act): in its sección de familia, infancia y capacidad (the family, children and capacity section) where one has been set up and, if not, in the sección civil (the civil section), with the Ministerio Fiscal (the public prosecutor) as a party. The inheritance claim goes before the sección civil of the deceased's last domicile (article 52.1.4 of the same Act).

Deadline

Filiation has no time limit for the child: without posesión de estado (open treatment as a son), article 133 gives him the claim for his whole life, and where there was such treatment article 131 sets none either. The financial side does have a clock: preterición, a four year lapse period applied by analogy from article 1301, with a disputed starting point when filiation is declared after the death; the petición de herencia, a thirty year limitation period (article 1963) from when the heirs possess the assets as their own.

Who can bring it

Claimant: the child and, should he die before the periods in article 133 have run, his heirs for the time remaining; where there was posesión de estado, anyone with a legitimate interest (article 131). Defendants: under article 766 of the Ley de Enjuiciamiento Civil, the heirs of the deceased father and also the claimant's mother if she is alive, unless she is the one bringing the claim.

Financial risk

If you oppose the filiation and the biological test comes back positive, a costs order against you is to be expected, unless the court finds serious doubts of fact or law. On the financial side, losing means paying him his share in money if the division was made in good faith, or redoing it if there was bad faith or fraud, plus the cost of the experts. Opposing everything costs more than choosing carefully what to dispute.

No time limit runs against paternity, and your father is no longer needed

It is worth accepting this early, because it shapes the whole strategy. Article 133 of the Código Civil provides that the claim to establish non marital filiation, where there is no posesión de estado (the child was never openly treated as such), belongs to the child for his whole life. There is no lapse or limitation period to plead: it makes no difference whether he is thirty eight or sixty, nor that your father has died. If your father also treated him publicly as his son, article 131 opens the claim to anyone with a legitimate interest, again with no time limit.

The parent's death does not extinguish the claim; it only changes who is sued. Article 766 of the Ley de Enjuiciamiento Civil (the Civil Procedure Act) provides that, where a parent has died, his heirs are the defendants: you and your siblings take your father's place in the proceedings, alongside the claimant's mother if she is alive, and the Ministerio Fiscal (the public prosecutor) always takes part because a person's civil status is at stake. Arguing that the claimant is only after money rarely leads anywhere: the courts do not examine why someone investigates his paternity, but whether the evidence confirms it.

The case is heard by the Tribunal de Instancia (the first-instance court), in its sección de familia, infancia y capacidad (the family, children and capacity section) where one has been set up and, if not, in the sección civil (the civil section), and no prior attempt at negotiation is required: article 5.2 of LO 1/2025 expressly exempts filiation, paternity and maternity from that admissibility requirement. That is why the first you hear of it may be the claim itself. If a burofax arrives first, it is a courtesy or a tactic, not an obligation on the claimant, and it is answered with care, without admitting any facts.

Before the DNA: the claimant's birth record and the threshold evidence

The defence's first check is not biological but a matter of the register. Article 113 of the Código Civil warns that a filiation is not effective while a contradictory one is proven. If the claimant was born while his mother was married, or another man acknowledged him, he already has a registered father, and to claim his biological father he must challenge that filiation in the same proceedings, which article 134 allows him to do. A claim that does not do so cannot succeed as framed.

There is one situation in which the door is shut completely: article 764.2 of the Ley de Enjuiciamiento Civil requires the court to reject a claim seeking a filiation that contradicts another established by final judgment, and to close the proceedings if that judgment comes to light once they have begun. That is why the claimant's full birth certificate, with its marginal notes, is obtained before anything is answered, either directly or by asking the court to call for it. It is a document few heirs think of requesting, and it sometimes settles the case without reaching the biological evidence.

The second filter is the principio de prueba, the threshold evidence. Article 767.1 of the same Act bars the admission of a filiation claim unless it is accompanied by initial evidence of the facts on which it rests. Do not raise your hopes: the courts treat it as a serious indication, not as finished proof, and a match on a genealogy website, some photographs or the mother's testimony are usually enough to open the proceedings. What the filter does achieve is that the claimant shows his hand from the outset, and that lets you prepare the defence knowing what you face.

The biological evidence: refusing does not protect you, controlling it does

Article 767 allows every kind of evidence in these proceedings, biological evidence included, and with the father dead there are three routes. Exhumation of the remains, which the court orders when there is no other reasonable way to obtain a sample. Samples that already exist: a hospital where your father had surgery or a biopsy usually keeps the tissue in its pathology archive, and the court can call for it. And the DNA of the acknowledged children, that is, yours and your siblings', which is the least invasive route and the one the claimant will propose first.

Paragraph 4 of article 767 is the one that decides the strategy: an unjustified refusal to undergo the biological test allows the court to declare the filiation provided there are other indications and paternity has not been proven by other means. The Tribunal Supremo (the Supreme Court) has held that such a refusal is not equivalent to a confession, but is an especially valuable indication. Whether a refusal by a child of the deceased has the same effect is debated, because the rule has the parent in mind; but refusing without reason only pushes the judge towards exhumation, which will produce the result anyway, and leaves the court with the image of someone who has something to hide.

What you can control is how the test is carried out. A kinship analysis between half siblings is statistically weaker than a direct paternity test, and its result depends on how many relatives take part. The Y chromosome shared by the men of the same paternal line proves the lineage, not the individual: it would match just the same if the claimant's father were one of your father's brothers. Insisting on an accredited laboratory, a documented chain of custody and a party expert who reviews the calculation of the kinship index is where this phase is won or lost.

What he receives if he proves to be a son: the intentional preterición crossroads

Once filiation is declared, its effects are backdated. Article 112 of the Código Civil states that filiation takes effect from when it arises and that its legal determination is retroactive whenever retroactivity is compatible with the nature of those effects, as succession effects are. Article 108 adds that marital and non marital filiation have the same effects. The claimant was your father's son on the day he died and, therefore, a forced heir with the same legítima as you.

With a will that appoints the children by name, what opens up is preterición under article 814, which has two very different regimes. If the omission was intentional, because your father knew he had this son and still did not name him, the claimant can only demand his legítima by cutting back the appointment of heirs, and on the majority reading that is his share of the third reserved as strict legítima: with four children, one twelfth of the estate. If it was unintentional, because your father did not know about him, the appointment of heirs is voided and, save for legacies and mejoras that are not excessive, the estate is shared equally: one quarter each.

This is what nobody tells you: both sides have the same problem, in reverse. It suits you that your father knew, because that reduces the claim to the strict legítima, yet proving he knew is the best indication of paternity. It suits the claimant that he did not know, yet without contact or knowledge his indications grow weaker. The line of defence is chosen before answering the filiation claim, because whatever is asserted there will bind you in the inheritance dispute. We cover preterición in general in its own guide; what matters here is this crossroads.

A division already signed is not undone unless there was bad faith

Many heirs believe the claim forces them to hand back the properties and start again. Article 1080 of the Código Civil says otherwise: a division made omitting one of the heirs will not be rescinded unless it is proven that the other parties acted in bad faith or with fraud, but they are obliged to pay the omitted heir the proportional share that corresponds to him. If you signed without knowing he existed, each flat still belongs to whoever received it and what is owed is money.

The practical consequence is that the date on which you learned of him becomes the most important date in the file. What was signed before is protected by good faith; what you do afterwards will be judged in the knowledge that the claim existed. The same applies to the rent from the let apartment: article 451 of the Código Civil lets a possessor in good faith keep the fruits received until possession is legally interrupted, so what was collected before the court claim is not repaid. The date at which the assets are valued to calculate the payment is another of the disputes in the case.

Selling is not prohibited, and a buyer who acquires in good faith and registers at the Registro de la Propiedad (the land registry) is protected, but the sale does not release you from the debt and may lead the claimant to seek interim measures, such as a precautionary entry of the claim in the land registry or a preventive attachment securing his share. Before signing the sale it is worth deciding how much money to set aside. And if any of you knew about this son before the division (a letter, a conversation with the mother, a transfer in the bank statements), bad faith stops being a hypothesis and the division can indeed be rescinded.

The time limits on the money and what can be agreed

The fact that filiation has no time limit does not mean the financial side has none. If there is a will, case law applies to preterición the four year lapse period of article 1301 of the Código Civil, and when filiation is declared after the death the starting point is disputed: you will argue for the opening of the succession; the claimant, that he could not act as a son until the judgment. Had your father died without a will, the route would be the petición de herencia, to which the Tribunal Supremo applies the thirty years of article 1963 from when the apparent heirs possess the assets as their own, and between co-heirs the action to seek division of the estate never becomes time barred (article 1965).

The financial claim, unlike filiation, does require a prior attempt at negotiation: article 5 of LO 1/2025 imposes it in declaratory proceedings and in the special proceedings of the Ley de Enjuiciamiento Civil, judicial division of an estate included, and filiation is the only matter in this dispute left outside it. A money claim filed without that attempt is not admissible, and whoever refuses to negotiate without cause risks the court taking it into account when deciding on costs. Answering the proposal with your own figures is a way of protecting yourself, not of giving in.

What cannot be agreed is filiation. Article 1814 of the Código Civil forbids compromise on a person's civil status, and article 751 of the Ley de Enjuiciamiento Civil deprives waiver, acceptance of the claim and settlement of any effect in these proceedings: he cannot waive being a son, nor can you acknowledge it by a written statement. What can be closed is the financial content, through a settlement on his share of the estate that holds good whatever is later declared. And if your father's vecindad civil (regional civil law status) was Catalan, Aragonese, Navarrese, Basque, Galician or Balearic, the legítimas and preterición are governed by that region's own civil law and the figures change.

How we run the case, step by step

1. 1

   Freeze the estate and keep everything from today

   The Gandía sale is not signed and the funds are not moved without a prior calculation. We gather the will, the deed of acceptance and division with its date, the burofax with its proof of delivery, your father's papers and bank statements and his medical records. Nothing is destroyed or tidied up: whatever disappears after the claim is turned against whoever made it disappear.

2. 2

   Obtain the claimant's full birth certificate

   It is requested from the Registro Civil (the civil registry) with its marginal notes, or the court is asked to call for it, to find out whether he has another registered father or a filiation fixed by final judgment. With that and with the indications he has already put forward, the real strength of his claim is measured.

3. 3

   Choose the line of defence before writing anything

   It is decided whether paternity is contested or the defence is concentrated on the inheritance, and whether it is worth maintaining that your father knew this son existed. The reply to the burofax is drafted with that decision taken, admitting no facts and without closing the door on a test carried out with safeguards.

4. 4

   Answer the filiation claim within twenty days

   Service of the claim opens a twenty day period to answer it. The register issues are raised, the threshold evidence is examined and a proposal is made on how the biological test should be carried out: which samples, which laboratory and with what chain of custody, preferring the less invasive routes to exhumation.

5. 5

   Have the genetic report reviewed by your own expert

   The party expert checks the identity of the samples, the method, the markers analysed and the statistical calculation. If the result is conclusive, the biology is no longer disputed and all the effort moves to the inheritance: continuing to deny the obvious only adds costs.

6. 6

   Close the financial side with figures

   Once filiation is declared, what he is owed is calculated according to whether the preterición was intentional or not, the lifetime gifts to be brought into account, the fruits since the claim and the expenses you bore. It is negotiated through the dispute resolution mechanism the law requires and, failing agreement, the position defended before the sección civil is that the division stands and only the payment under article 1080 is owed.

The law that applies

• Art. 133 CC. Without posesión de estado, the claim to establish non marital filiation belongs to the child for his whole life. If he dies within four years of coming of age or of the removal of his support measures, or in the year after discovering the evidence, it passes to his heirs for the time remaining. Parents have one year from learning the facts. BOE-A-1889-4763
• Art. 134 CC. Bringing the claim to establish filiation, by the child or the parent, allows the contradictory filiation to be challenged in every case. BOE-A-1889-4763
• Art. 108 CC. Filiation may arise by nature, marital or non marital, or by adoption, and all have the same effects under the Código. BOE-A-1889-4763
• Art. 814 CC. Omitting a forced heir does not harm the legítima. Unintentional omission of children or descendants voids the financial provisions if all are omitted and, if not, the appointment of heirs, leaving non excessive legacies and mejoras standing; an appointment in favour of the spouse is voided only in so far as it harms the legítimas. BOE-A-1889-4763
• Art. 1080 CC. A division made omitting one of the heirs is not rescinded unless bad faith or fraud by the other parties is proven, and they must pay the omitted heir the proportional share that corresponds to him. BOE-A-1889-4763
• Art. 1963 CC. Real actions over immovable property become time barred after thirty years, without prejudice to the rules on acquiring ownership or real rights by prescription. BOE-A-1889-4763
• Art. 767 LEC. A filiation claim is not admitted without threshold evidence; every kind of evidence is allowed, biological evidence included; filiation may be declared from acknowledgement, posesión de estado, cohabitation with the mother at the time of conception or other similar facts, and an unjustified refusal to undergo the biological test allows it to be declared where there are other indications. BOE-A-2000-323
• Art. 5 LO 1/2025. Requires recourse to an appropriate dispute resolution mechanism before suing in declaratory and special civil proceedings, with a closed list of exceptions that includes filiation, paternity and maternity. BOE-A-2025-76

Each article checked against the consolidated text published in the BOE (the Spanish official gazette).

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