The short answer
If you are older than 18, you don't need your parents' permission.
The whole question
I was born out of wedlock and so I was given my mother's maiden name. I am now 23. My mother has since gotten married and now wants me to change my surname to that of my stepfather instead of my biological father with whom I've been living since Grade 3. Can I change my surname to that of my biological father without my mother's permission?
The long answer
If you were a minor (under 18 years old) you would have to get your mother’s permission to change your surname, but as you are 23 years old, you have the right to change your surname without permission from your mother.
You need to fill in Form B1-196 from the Department of Home Affairs. Form B1-196 is “Authority to assume another surname.” You can also download the form online.
On the application form, there is a section where you must fill in “a sufficient reason” for changing your name.
You will also need to submit Form DHA-9, which is the full fingerprints form. This form cannot be downloaded but is obtainable at Home Affairs or at police stations.
You will also need to submit your birth certificate with both parents’ names. If your father’s name is not on the birth certificate, he might need to submit an affidavit explaining the circumstances of your birth, that you have been living with him since Grade 3 and that he is in favour of you taking his surname.
All this must be submitted to Home Affairs. The officials there will examine the forms and scrutinise the reason given for wishing to change your surname. If they are satisfied that it is a valid and sufficient reason, they will send your application on to the Director-General (DG) who will authorise the change of surname.
After authorising it, the DG will publish the amended name in the Government Gazette. Then the DG will issue a Certificate from the Population Register on the change of name.
All this usually takes about three to four months but with the lockdown it could take a lot longer.