Moira's flat. In Diamond Dogs (novel), Bill directly refers to it as Moira's flat at least twice. However, a redirect to Bill Potts' flat could be kept, for easier searchability.
Talk about it here.
Later, Bill returned to her flat to hear the shower running. When she received a call from Moira, she was surprised to hear no one was staying over. Upon investigating further, she found out that the sentient oil had followed her to her flat. (TV: The Pilot)
A shadow version of Bill Potts brought s girl called Penny to her flat to have a drink with her. However, the Twelfth Doctor landed his TARDIS here, distrubing their date. The real Bill was making tea when the Doctor phoned her. (TV: Extremis) It would similarly occur when the real Bill invited the real Penny inside. (TV: The Pyramid at the End of the World)