Matthew Lee, 41 and the Severn Vale Housing Society, which commissioned him to carry out plumbing work at the home of Christine Goodall, pleaded guilty to offences under the Health and Safety at Work Act 1974.
Mrs Goodall, 65, and her dog Kelly, died in 2007 when a boiler behind her fireplace, which should have been decommissioned, overheated and exploded.
The iron back boiler in Mrs Goodall's house at Nut Orchard End, Twyning, Gloucestershire was improperly decommissioned by Lee in 1999.
After her death, Severn Vale visited 1,700 homes in the area to ensure residents were safe, an inquest heard last year. It also triggered a warning from the Health and Safety Executive who told home owners landlords and housing associations to decommission back boilers.
Three other similar incidents were found to have occurred, but none were fatal.
Magistrates in Gloucester heard that Severn Vale, of Ashchurch, Tewkesbury, was at fault as it failed to ensure Mrs Goodall's safety in her home. The society, along with Lee, of Mendip Close, Cheltenham will be sentenced at Gloucester Crown Court.