Under the original provision of Article 176 of the Family Code of the Philippines, illegitimate children were mandated to use the surname of their mother. However, with the enactment of Republic Act No. 9255 on March 19, 2004, the law was amended to allow illegitimate children the right to use their father’s surname, provided that the father has expressly recognized the child.

This legal framework balances the child's right to identity with the requirement of voluntary or proven recognition by the biological father.

1. Legal Basis for Recognition

For an illegitimate child to use the father's surname, the father must recognize the child through any of the following means:

Admission of Paternity in the Birth Certificate: The father signs the "Affidavit of Admission of Paternity" located at the back of the Certificate of Live Birth (COLB) at the time of birth.
Public Document: A separate notarized admission of paternity, such as an Affidavit of Admission of Paternity (AAP).
Private Handwritten Instrument: A document written and signed entirely by the father in his own handwriting, clearly admitting paternity.

2. The Role of the AUSF (Affidavit to Use the Surname of the Father)

Recognition of paternity alone does not automatically change the child's surname in the civil registry. To trigger the change from the mother's surname to the father's, an Affidavit to Use the Surname of the Father (AUSF) must be executed and registered.

Who Executes the AUSF?

The signatory of the AUSF depends on the age of the child at the time of filing:

Child is 0 to 6 years old: The mother or the guardian executes the AUSF.
Child is 7 to 17 years old: The child executes the AUSF, but it must be accompanied by an Attestation of the Mother.
Child is 18 years or older: The child executes the AUSF personally; no mother’s attestation is required.

3. Procedural Requirements

Case A: Registration at the Time of Birth

If the father is willing to recognize the child at the moment of birth:

The father signs the Affidavit of Admission of Paternity on the back of the COLB.
The mother (or person authorized) executes the AUSF.
Both documents are filed with the Local Civil Registrar (LCR) of the place where the birth occurred.

Case B: Registration After the Birth was Already Recorded

If the child was originally registered under the mother’s surname:

Preparation of Documents: Secure the AAP (if not yet recognized) and the AUSF.
Filing: Submit these to the LCR where the birth was recorded.
Annotation: The LCR will not issue a new birth certificate but will instead annotate the original birth certificate. The annotation will state that the child is now authorized to use the father's surname pursuant to RA 9255.
PSA Copy: Once the LCR processes the local copy, the documents are forwarded to the Philippine Statistics Authority (PSA) for the adjustment of the birth record in the national database.

4. Documentary Requirements

Common documents required by the Local Civil Registry Office include:

Original and certified copies of the Child’s Birth Certificate.
Affidavit of Admission of Paternity (if not signed on the COLB).
Affidavit to Use the Surname of the Father (AUSF).
Valid Government IDs of the signatories.
Attestation of the Mother (for children aged 7–17).

5. Effects and Limitations

No Change in Status

It is a common misconception that using the father's surname "legitimizes" the child. Under Philippine law, legitimation only occurs if the parents were not disqualified from marrying each other at the time of the child's conception and subsequently enter into a valid marriage. RA 9255 only changes the surname; the child’s legal status remains "illegitimate."

Use, Not Mandatory

The Supreme Court has clarified (in cases such as Grande v. Antonio) that the use of the father's surname is a right granted to the child, not a duty. If it is proven that using the father's surname would be detrimental to the child's best interests, the court may maintain the use of the mother’s surname.

Successional Rights

Recognition via RA 9255 is a strong piece of evidence for compulsory succession. By admitting paternity through the AAP or the COLB, the father effectively acknowledges the child as a compulsory heir, entitling the child to a legitime (inheritance) under the Civil Code.

6. Where to File

If the birth occurred in the Philippines: File at the Local Civil Registry Office (LCRO) of the city or municipality where the birth happened.
If the birth occurred abroad: File at the Philippine Foreign Service Post (Embassy or Consulate) having jurisdiction over the place of birth.
If the father is deceased: Recognition can no longer be made through RA 9255 if the father did not leave a signed admission of paternity during his lifetime. In such cases, a judicial special proceeding to prove filiation may be necessary.