Flat 27 was a flat in the Park Hill council estate in Sheffield in which Anna lived.
It was the two doors down from Flat 34, where Hakim, Najia, Yasmin and Sonya Khan lived.
The Thirteenth Doctor, Ryan Sinclair, and a concerned colleague of Anna's searched the flat, where they discovered Anna had been killed there by a giant spider. (TV: Arachnids in the UK)