Anne Marie Cropper, 47, was found dead with head and chest injuries in the property on Royal Terrace, Southport, in September.
Ian Gordon, 52, also of Royal Terrace, changed his plea to guilty at Liverpool Crown Court, four days into his trial for her murder.
Gordon, was remanded in custody for sentencing on Tuesday.
A man who denied murdering his girlfriend in her flat has changed his plea to guilty.