Ten people were forced to leave the surrounding homes after the "total collapse" of the two-storey building in Sumatra Road, where properties fetch over £1 million.
A spokesman for the LFB said firefighters were making the scene safe while working with the local authority's structural engineer.
"One fire engine and one fire rescue unit and 10 firefighters have been called to house collapse on Sumatra Road in Hampstead.
"There has been a total collapse of the two storey property from the roof to basement level," a spokesman said.
Fire crews from West Hampstead and Battersea fire stations were called to the scene on Monday.
The house appeared to be undergoing renovations at the time of collapse.
No-one was injured.
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