Kial Ahmed, 34, of Tongwynlais, pleaded guilty to murdering 38-year-old Nadia Jones, whose body was found at the property in Tremorfa on 11 September 2015.
Roxanne Deacon, 26, of Grangetown, denied murder but pleaded guilty to manslaughter and robbery at Cardiff Crown Court.
They will be sentenced later in June.
The court heard the pair stole a quantity of Class A drugs and a roll of bank notes from Ms Jones's home.
Deacon was due to go on trial for murder but prosecutors accepted her plea to manslaughter on Wednesday.
Ahmed pleaded guilty to murder in April but his admission can only now be reported.
Judge Neil Bidder told the court: "This was a homicide killing which was committed in the course of a robbery.
"It was committed in order to steal drugs and cash from her."
A man and woman have admitted killing a mother during a robbery at her Cardiff flat.