Since the late 1800s, one of Sydney's grandest sandstone landmarks has been closed to the public, used for little more than government offices.

Fast forward to 2020 and every Sydneysider - public servant or otherwise - will be able to enjoy a rooftop drink at the site, which is set to become a "world-leading hotel".

The heritage-listed 'Sandstones,' or the Lands and Education buildings on Bridge Street in Sydney's CBD, will undergo a dramatic transformation from the end of 2017, when Singaporean developer Pontiac begins a $300 million redevelopment.

Designs unveiled on Thursday show ground-level retail, dining and a rooftop salon in the Lands building, to be open to the public, while more than 250 hotel rooms and suites are included in the plans for both buildings.