In the Philippines, the surname of a child is not merely a label but a reflection of legal status and filial recognition. Historically, illegitimate children were strictly required to use the mother’s surname. However, with the enactment of Republic Act No. 9255 in 2004, the law provided a clear mechanism for illegitimate children to bear their father’s surname, provided certain legal conditions are met.

1. The General Rule and the Exception

Under Article 176 of the Family Code, illegitimate children shall use the surname of their mother. However, the law allows the use of the father's surname if:

The father has expressly recognized the child through the Record of Birth appearing in the civil register.
The father makes an admission of paternity in a public document or a private handwritten instrument.

2. Modes of Recognition

To change a child's surname to that of the father, the father’s recognition must be established through one of the following:

Affidavit of Admission of Paternity (AAP): This is a legal document executed by the father, usually at the time of birth, admitting that he is the biological father of the child.
Private Handwritten Instrument: A document written and signed by the father acknowledging paternity.
Affidavit to Use the Surname of the Father (AUSF): Under RA 9255, this is a mandatory document executed by the mother (if the child is 0-6 years old), the child with the mother’s assistance (if 7-17 years old), or the child alone (if 18 or older).

3. The Procedure Based on the Child’s Age

The process for registration and the documents required vary depending on whether the child’s birth has already been registered and their current age.

Case A: At the Time of Birth Registration

If the father is willing to recognize the child at birth, he must sign the Birth Certificate (specifically the "Affidavit of Admission of Paternity" section on the back). An AUSF must also be submitted to the Local Civil Registrar (LCR) to allow the child to immediately carry the father's name.

Case B: After the Birth has been Registered (Late Recognition)

If the child was initially registered under the mother's surname, the following steps are taken:

Preparation of Documents: Execute the AAP (if not yet recognized) and the AUSF.
Filing: Submit these documents to the LCR where the child’s birth was recorded.
Registration: The LCR will record the documents in the Register of Legal Instruments.
Annotation: The original Birth Certificate will not be replaced, but a marginal annotation will be added stating that the child has the right to use the father's surname.

4. Required Documents

When applying for the change of surname at the Local Civil Registry Office, you typically need:

Original/Certified True Copy of the Birth Certificate.
Affidavit of Admission of Paternity (AAP) (if paternity wasn't admitted at birth).
Affidavit to Use the Surname of the Father (AUSF).
Certificate of Registration of the AAP/AUSF from the LCR.
Valid IDs of the affiants (Mother, Father, or Child).

Important Note: If the father is deceased or refuses to sign an admission of paternity, the child cannot use the surname through this administrative process. In such cases, a judicial petition for compulsory recognition may be necessary.

5. Legal Effects of the Change

Changing the surname does not grant the child "legitimacy." The child remains "illegitimate" in legal status unless the parents subsequently marry (and were not disqualified from marrying at the time of conception). However, the change does solidify the child's right to:

Use the father's surname on all official records.
Provide a stronger basis for claiming support and successional (inheritance) rights.

6. Summary Table

Scenario	Primary Requirement	Document to File
Birth not yet registered	Father's presence/signature	AAP + AUSF
Birth registered (under Mother's name)	Proof of Paternity	AAP (if needed) + AUSF
Father refuses to recognize	Evidence of Paternity	Judicial Petition (Court)

Would you like me to draft a template for an Affidavit to Use the Surname of the Father (AUSF)?