After the reunification of Hong Kong with Mainland China in 1997, although the common-law system was preserved, the abolition of the automatic right of abode by birth was retained. The present law is complicated. Essentially, being born in Hong Kong does not of itself confer any rights. Additional qualifications must be met and are summarised below:

Persons who were born in Hong Kong who also have Chinese nationality are entitled to Hong Kong permanent resident status, regardless of where their parents are from. This was established in a landmark decision by the Court of Final Appeal in Director of Immigration v Chong Fung Yuen.

Persons who were born in Hong Kong who are not of Chinese nationality may be entitled to permanent residence if one of their parents is a permanent resident. In such cases, the entitlement to permanent residence is only valid up to the age of 21, and thereafter these persons must apply for qualification on their own (article 24(2)(5) of the Basic Law).

Persons who were born in Hong Kong who are neither Chinese nationals nor have a parent who is a Hong Kong permanent resident acquire no legal rights by virtue of being born in Hong Kong. If the parents of such persons are living in Hong Kong , they must seek a dependant visa for the child.
http://www.clic.org.hk/en/topics/immigration/hk_permanent_residence/q5.shtml