Kamar, of Wembley, north-west London, had been one of four men lifting the lintel on to a wall on a Scottish Amicable development at 33 Old Broad Street, Moorgate, in the City of London on 21 February 1997, Southwark Crown Court was told.
The court heard how, as the lintel was being lifted, a metal-framed bandstand he was standing on "juddered and wobbled", then sheered, causing two men to fall. Kamar was struck on the head.
Richard Matthews, for the prosecution, said: "It appears this was a rush job, and because of the lack of planning, inexperienced workers were asked to carry out a dangerous task that was wholly unnecessary." He said the circumstances leading to the death were "at best confused, at worst a failure to assess or train operatives".
Richard Ferguson, counsel for the contractor, said: "We accept responsibility and face up to it fairly and squarely."
The judge adjourned the case to consider the sentence, which will be announced this week.
HBG Southern was known as Kyle Stewart at the time.