What most people call Hong Kong consists of three major areas: Hong Kong Island, Kowloon, and New Territories.
Hong Kong Island (where majority of people live) was given to UK with no lease limits. Kowloon and New Territories were leased to UK for 99 years in 1898. So, in theory UK only needed to return Kowloon and New Territories. Of course, this wasn’t practical. It’s like giving Manhattan to another country but keeping Queens and Brooklyn. So in 1994 UK agree to return all 3 (Hong Kong Island, Kowloon, and New Territories) in 1997.
Kowloon south of Boundary street was also ceded in perpetuity to the UK in 1860. Kowloon north of boundary street was leased for 99 years in 1898. https://en.wikipedia.org/wiki/Boundary_Street
Hong Kong Island (where majority of people live) was given to UK with no lease limits. Kowloon and New Territories were leased to UK for 99 years in 1898. So, in theory UK only needed to return Kowloon and New Territories. Of course, this wasn’t practical. It’s like giving Manhattan to another country but keeping Queens and Brooklyn. So in 1994 UK agree to return all 3 (Hong Kong Island, Kowloon, and New Territories) in 1997.