The boy was named as Sean Williams, whose parents Lynn and John and sister Sarah, 13, live at The George public house in Park Road, Finchley, north London. Mr Williams is the licensee.
Forensic tests on the body were being completed today.
A police spokesman said it was not know what Sean may have been doing in the flats. The boy had been riding his bicycle during yesterday afternoon. A relative found the bicycle in the stairwell of a block of flats at 5.35pm. About 13 minutes later, a resident of the flats called the lift to the sixth floor. The doors opened to reveal the boy's body.
Police said that a man was arrested in the block of flats shortly before midnight.