Around 40 neighbours had to leave their homes after two multimillion-pound mid-terrace houses which were being renovated collapsed "from the roof to ground level" in west London.
The two properties on Durham Place in Chelsea, which neighbours said were undergoing basement development projects, collapsed shortly before midnight last night, causing emergency services to evacuate nearby homes.
Nobody was thought to be hurt, according to emergency services.
A man in his 30s who witnessed the building collapse and was the first to call police returned to the scene the next morning.
The man, who did not want to be named, told PA news agency: "I live in the area and was just passing by when the building suddenly collapsed to the ground.
"It was extremely loud, there was dust everywhere.
"There were a lot of people coming out of their homes in the surrounding area to see what was going on, they were unharmed.
"It was quite bad, really surreal.
"The police were amazing, I was still on the phone with them when the first patrol came to the scene," he said.
"The evacuation was done in a Covid-secure way and they did a good job," he added, as he thanked a police officer at the scene on Tuesday morning.
Planning permission submitted by real estate company Seabrook Properties Ltd was given the green light by Royal Borough of Kensington and Chelsea council in August last year.
A council document detailed the works to be carried out - a rear extension to lower ground floor with a terrace on the upper level - which was an amendment to a previous application granted in July 2018.
The council warned in the application letter that "if not properly managed, construction works can lead to significant negative impacts on the local environment, reducing residential amenity and the safe function of the highway".
A planning application for the neighbouring property was made less than two weeks ago, with a council letter dated October 23 detailing "basement excavation, demolition of extension and roof terrace and replacement with a two-storey rear extension with roof terrace, and a single storey extension at the lower ground floor".
It said a decision from the council was expected on November 19.
One seven-bedroom house a few doors down from those that collapsed sold last year with a value of around £16 million, according to figures on property website Rightmove.
Large-scale basement developments, often including the construction of entire new floors under the footprint of the house and garden, have become relatively common among London's wealthy.
Some of the more lavish constructions include multiple basement levels and can go as deep as 18 metres underground, according to a 2018 report from researchers at Newcastle University.
The study, Mapping Subterranean London: the Hidden Geography of Residential Basement Developments, found of 4,650 basements constructed across seven London boroughs between 2008 and 2017, 374 developments included a pool, 242 a sauna or steam room and 115 incorporated accommodation for staff.
There were also a number of subterranean parking garages as well as 42 bars, 40 libraries, 29 music rooms, 18 art galleries, 15 pilates or yoga studios, two golf simulators and two gun rooms.
The quiet road opposite Royal Hospital Chelsea was cordoned off on Tuesday morning with police and surveyors at the scene to determine what caused the two houses to fall into ruin.
A man and a woman from the house next to the collapsed buildings arrived the following morning to pick up some belongings and two budgies.
The man, who did not want to be named, said he first heard the scaffolding fall down, adding: "It was loud, and then the rest of it kept coming down.
"There was dust in the house, we knew something has happened so we just left.
"We left through the front door and the first floor came down after that."
A caretaker who lives on the end of the road said there were construction works being carried out at the properties for around two years.
Speaking anonymously, she told PA: "I jumped out of bed and ran down the stairs.
"The police were saying 'come on, you've got to get out'.
"We stayed at a family friend's and were told we could come back at 12pm (on Tuesday afternoon)."
London Fire Brigade described the scene as a "total collapse".
Station commander Jason Jones, who attended the scene, said: "There was a total collapse of the building from the roof to ground level.
"Firefighters worked to make the scene safe and our drone team carried out a search.
"A police search dog also carried out an external search of the building and at this stage, there are no reports of any injuries."