Emergency services were alerted to the blaze at a detached property in Crovie, near Gardenstown in Aberdeenshire, at about 7.15pm on Tuesday.
Four fire crews, who found the blaze "well established" when they arrived, used jets and breathing equipment to bring the flames under control by around 9.40pm.
A woman in her 60s, believed to be Joan Rennie, and her dog are said to be unaccounted for and it is feared they were inside the seafront property at the time of the fire.
The property was too unsafe for emergency services to examine in the hours immediately following the incident but an investigation into the circumstances surrounding the fire is under way.
A police spokesman said: "Police Scotland are investigating the circumstances of a house fire that was reported about 7.15pm on Tuesday.
"Four appliances from Scottish Fire and Rescue (SFRS), stationed at Macduff, Banff, Fraserburgh and Turriff, attended, where on their arrival the blaze was well established.
"The fire was brought under control."