The front of a house has collapsed "from the roof to basement level".
Firefighters were called to West Hampstead, north-west London, at 12:06 GMT and requested the help of the police to cordon off Sumatra Road.
Ten people were evacuated from neighbouring properties, London Fire Brigade said.
The London Ambulance Service sent an ambulance crew and an incident responder, but said there were no patients to treat at the scene.
London Fire Brigade (LFB) said firefighters were making the scene safe and working with Camden Council's structural engineer.
A spokesperson added: "There has been a total collapse of the two-storey property from the roof to basement level."
The Met Police said it received no reports of injuries.
Planning permission to renovate the basement of the property was granted by Camden Council in April 2014.
However, an application to convert the home into two one-bedroom flats and two two-bedroom flats was refused on 11 August 2017.
A subsequent appeal, made by Jeremy Stein against Camden Council, was dismissed by the Planning Inspectorate on 10 October 2017.