Gustavo Nunes, the owner of Nunes Tiling, said the mistake was proving to be a tough business lesson.
He had not thought anything like that was going to happen, he said.
However, despite the setback, Mr Nunes remains upbeat, continuing to service clients and determined to meet the challenge head-on.
"Some things are just beyond our control and if we get upset about it, it just makes it worse.
"I think we’ve just got to look at the positive side — no-one got hurt, and we still have work coming in," he said.
Fire and Emergency New Zealand (Fenz) specialist fire investigator John Smalls attended the scene on Friday morning, where the cause of the fire quickly became obvious.
"We found what was likely a faulty light on the floor.
"It was one of those big drop-down lights," he said.
"It was underneath some of the debris, so we worked out that probably happened earlier in the fire than later."
The theory fitted with burn patterns both outside and inside the building that showed "the fire started high".
Mr Nunes had CCTV operating in the building that offered another important clue.
"You can see the lights flashing like crazy before the camera went off," he said.
Firefighting crews from Cromwell, Clyde and Alexandra attended the blaze.
Cromwell Volunteer Fire Brigade chief Neil Gillespie said the fire was "efficiently and effectively handled on the night by the team".
"They did a fantastic job and dealt to it very quickly."
Initial reports from Fenz said crews acted under the assumption there might have been people inside the building.
Mr Nunes said although his vehicle was parked outside at the time of the fire, he had already left the building and no-one else was inside.
He had been operating from the building on the corner of McNulty Rd and Barry Ave for close to a year, with plans to open a showroom from the leased space, he said.
Meanwhile, it is business as usual for Central Otago Powder Coating, which operates from an adjacent space in the building.
Mr Smalls said the fire was a reminder to check lights for faults.
"It you do notice something faulty, get it checked out by a sparky.
"Don’t ignore it."
