More than 20 firefighters battled the blaze, which tore through the roof of the home on Raglan Street in Mosman after 2.30pm today.
The elderly owner was inside at the time the fire broke out but escaped unharmed.
Neighbours were shocked by the fire after the city saw a rather wet day.
“Very surprising that there is a fire in Mosman in a house with a tile roof on a day where it’s pouring with rain,” one resident said.
9News understands the owners of the home had only just started renovating.
An investigation into the cause of the fire is underway.