The roof of a semi-detached house in Arden Road, Finchley, north London went up in flames at about 9.40am today.
Dozens of firefighters arrived almost immediately to tackle the huge blaze.
Pictures and video posted on Twitter showed bright orange flames and thick, black smoke billowing into the sky above the house.
Six fire engines and 35 firefighters were sent to tackle the fire, London Fire Brigade said.
Crews from Finchley, Hendon Hornsey and West Hampsted fire stations were on ground.
An LFB spokesman said: “The roof of the semi detached property, which is under refurbishment, is alight.
“The cause of the fire is not yet known.”