Conley Thompson, who told his mum, Paula Thompson, that he was going out to play with friends at about 6pm on July 26, 2015, was reported missing to police at 10.10pm when he did not return to his home on Underwood Avenue, Worsbrough.
It sparked a huge search effort - spearheaded by more than 50 officers and dozens of concerned residents - which continued through the night.
He was found dead inside a 23cm-wide plastic pipe on the building site, off Bank End Road, at 8.40am the following morning.
It is unknown how the schoolboy became stuck in the pipe - placed vertically into the ground to house fence posts - but he must have descended into it feet-first, either by slipping or lowering himself, an inquest into his death found.
In March this year, principal contractor Howard Civil Engineering pleaded guilty at a hearing - held at Barnsley Magistrates’ Court - for failing to prevent access to the site and admitted failing in its duty under health and safety laws to protect the public from risk.
Having been told to expect ‘at least’ a six-figure fine, bosses from the Leeds-based firm will appear before Sheffield Crown Court to hear their sentence having been prosecuted by the Health and Safety Executive (HSE), who took the case on from South Yorkshire Police.
A statement, read out in court, said: “The focus of the Health and Safety Executive was the significant sub-standard perimeter fencing around the site.
“Either it wasn’t of a suitable height or indeed it was non-existent, or where it did exist it was poorly maintained.”
The plot had been under 24-hour surveillance by security firm Security Guards UK until July 16 when the cover was stopped - just 11 days before Conley’s body was discovered.
Prosecutors on behalf of the HSE argued children were subsequently afforded easy access to the site due to ‘inadequate’ safety fencing - in mitigation the firm said it acquired fencing it believed met standards, but the land’s topography made it ‘more difficult to create a secure perimeter’.
“That obvious risk was known to the company, and they failed to take all reasonably practical precautions,” the statement added.
“Measures that were taken post-incident were reasonably practical measures that could have been in place from the start.”
The firm will be sentenced for two counts - breaching regulation 13(4)(b) of the Construction (Design and Management) Regulations 2015 and Section 3 (1) of the Health and Safety at Work Act 1974.