A small crack appeared and the whole structure collapsed within minutes.
More than 50 people attending a nearby centre for the disabled were evacuated, many using wheelchairs and walking frames. One was wheeled out on a trolley after being caught half-way through a reflexology treatment. A six-year- old girl had to be rescued by firefighters from the house next door after rubble blocked the exits.
Stephen Willis, the owner and a company director, said: "We were very happy there. We loved the area."
His three children took the news "surprising well" but his wife, Victoria, was very upset. The 1830s house was insured and the family hoped to rebuild on the same spot, Mr Willis added.
The firm responsible for the building work was G&D Building Services, of Northolt, west London. Peter Doherty, the company's contracts manager, said he had no idea what made the property collapse on Friday. Mr Doherty said: "We are sure it was a fault with the existing structure." The builders had been shaken. "It doesn't happen every day."
The firm had delegated the underpinning work to specialist subcontractors Stress UK Ltd, of Slough.
David Kitching, the head of Stress UK, said the property's age and condition were likely to have caused the collapse.
But Martin Runnacles, of Winkworth's estate agents in Notting Hill, said in his whole career he had never known a property of the same kind to collapse. "People do have problems with London clay, but I have never heard of anything like this. They are fantastic properties and are highly sought after. They usually fetch at least pounds 750,000."
Howard Nash, of architects Nash Parker, acting as contract administrators for the family, said until an investigation was completed by Westminster council the cause of the collapse would not be known.