What began as a renovation ended as a surprise demolition for a 70-year-old Mississauga house on Wednesday morning.

Ontario Ministry of Labour investigators and Mississauga's building department are investigating how a two-storey estate at 3447 Cawthra Rd. suddenly collapsed into a heap of rubble around 8 a.m.

A construction crew had been working in the basement when the structure began to crumble, though officials said everyone escaped unharmed.

The old brick structure has stood since 1940, according to Emilio Lavignasse, owner of Olympia Windows & Doors Ltd., which used the house as a showroom.

"Funnily enough, this is our 30th year — our 30th anniversary," Lavignasse told reporters, surveying the mess around him. "Thirty years in business and this is our gift."

Crew was digging in basement

Nobody lived in the house, which was built during the Second World War. Olympia employees also managed to avoid the disaster.

"We were doing an addition and like I said, underpinning the basement," Lavignasse said. "They were digging the basement to lower the floor and make it the same height as the new extension. They had to dig up and pour cement."

A Google Street View shows the structure at 3447 Cawthra Rd. before it crumbled into a pile of debris Wednesday. The Olympia Windows company used the property as a showroom. (Google Street View)

That's when the structure began to give way, he said, adding that to his knowledge, the old building was in "excellent" condition and that the remodelling companies possessed the required permits.

Mississauga Fire's district chief, Paul Kernan, said the property had already been completely levelled by the time the first units arrived around 8:30 a.m. to respond to a report of a gas leak.

"We received a call for initially a gas leak. The first arriving crews [saw] obviously it was a collapsed building, and so we had a full response for that type of call," he said.

Nearby homes on the residential street were temporarily evacuated and hydro and gas lines were shut down while investigators toured the scene.

Plan to rebuild 'from scratch'

Neighbours said they were awoken by the sound of sirens on the street, and were stunned to learn the well-known landmark in their area had been reduced to a pile of debris.

"That's terrible," said Nancy Hamilton, who bought her living-room window from Olympia several years ago and believed the property was kept in good condition.

'What are we going to do? We have to build a new house'—Emilio Lavignasse, owner of Olympia Windows & Doors Ltd.

"Oh, it was very nice. It was an old house, probably maybe 75 years old, and it was brick and it was two-storey and they had it all set up as their showroom."

As bad as the collapse appeared, it could have been fatal. Lavignasse said his brother-in-law was meant to be on the second floor filling out some paperwork at the time, but had been running late.

He said the business will continue to sell windows at another location, and that the next plan is to "build from scratch," possibly on the same site.

"My wife was talking to the architect and the house is going to start as soon as they pull the debris out," he said. "We're talking about it. What are we going to do? We have to build a new house."