The Ask The Librarian service emailed me with his team’s findings, and found the most likely site of Tan Seng Poh’s mansion:
You might find this useful.
Title: One hundred years’ history of the Chinese in Singapore / Song Ong
Siang ; introduction by Edwin Lee.
Author: Song, Ong Siang, Sir, 1871-1941.
Publisher: Singapore : Oxford University Press, 1984.
“Tan Yeok Nee’s mansion was the last of the four residences built in
typical Chinese architectural design and style. The earliest was Tan Seng
Poh’s in Hill Street, erected in 1869 and for many years used as the
Chinese Consulate and now the site of the block of shophouses facing Hill
Street and Loke Yew Street.” (p. 335-6)
I think this would be the “Seng Poh’s new building”. Looking at SLA’s
streetmap website(http://www.map.gov.sg/StreetMap/), I think the building
would be the one with the address 30 Hill Street.
On streetdirectory.com, it says it’s the former Malaysian High Commission building, but there’s no indication what the site is now used for, and I can’t be arsed to drive there to find out yet. I’ve put the google map of the place up instead of a streetdirectory.com one because, you know, the friendly folk at streetdirectory.com used to take offence at people copying and pasting their maps, and may take such action that may render me lying at the back of Seng Poh’s new house smoking a pipe.
Also, according to the librarian, the building’s probably been demolished, but that’s no surprise.
Thank you, Mr Librarian.