Green’s Dictionary of Slang

(US black) an apartment used for parties, the illegal sale of liquor, possibly for commercial sex.

1924		Magie Jones ‘Good Time Flat Blues’ 🎵 Miss lizzie Green in New orleans runs a good time flat. / Sellin’ booze and singing blues down where she’s at.
1929		Frankie ‘Half Pint’ Jaxon ‘It’s Heated’ 🎵 I went to a good time flat last saturday night, / The cops knocked on the door, everybody made their flight.
1990		(ref. to 1920s) P. Oliver Blues Fell This Morning 163: When they frequented the ‘good-time flat’ they forgot for a while the realities of the congested ghettoes of the city; when they went to the rent party they helped others whose plight was similar to their own.
1997		(con. late 1920s) D.H. Edwards The World Don’t Owe Me Nothing 40: [W]e played at a good-timing house on Avenue F. [...] Women was flocking [...] and we kept the house lively [...] 126: I stayed at that good-timing house on Avenue F, Lula Spencer's. Lula was a half-white woman. She had a great big house, almost like a hotel, and five or six girls working for her.