According to 香港歷史散步 "A walking history of Hong Kong," in 1842, the Governor built a two-storey mansion, as the official residence and offices. In the 1870s, the building was expanded into a three-storey building. . . .
We can see what the two photos have in common, but before the expansion what was the building like? Was it really a two-storey mansion ?
From uWants - via my guesses of the Google Translation
Formerly called "Johnston House", and originally built in 1843, it became the first Governor's residence. Governors Pottinger and Davis lived there. Davis in 1846 moved to his Caine Road residence. The building has changed hands several times.
In the 1880's it was owned by E. R. Belilios, and rented by HSBC for their canteen, and also the Russian Consulate.
The building was owned by HSBC director Belilios since 1879
(Google translate isn't perfect :)