Firefighters were called to 43-year-old Kathryn Neale's home in Little Hay Lane, Lichfield, after a passer-by spotted her cottage on fire at 7.45am on January 26.
Investigation officer Lee Richards told the inquest at Cannock that the fire had started in the dining room after a spark from her coal fire fell onto a nearby armchair.
A toxicology test conducted on Miss Neale's dog, Monty, found the animal had high levels of a drug similar to ecstasy in his blood, although police say the readings could have occurred as a result of natural processes.
He said: "A small smouldering fire happened over several hours and it ensued, reaching ceiling joints and floorboards, before progressing to the bedroom and along the hallway into the roof space above.
"Miss Neale was probably in her bed when she discovered the fire but she was found in the spare bedroom in the same place as Monty."
The inquest heard Miss Neale had been relaxing with her friend Richard Myatt the previous evening, who had stoked the fire before leaving her home at around 1.30am. He said: "Katy and I had known each other on and off for many years. We became close just before Christmas and I visited her during the evening of January 25 at 8.30pm.
"There was a coal fire in one of the rooms and prior to leaving I stirred it up to ensure that it would stay alight all night. I made the fire very high but I made sure it was stable.
"Myself and Katy's friends have been very concerned that Monty did not raise the alarm as he was very lively."
Detective Inspector David Johnson said there was no medical explanation why such a high reading of drugs had been found in the dog's system but added that it can occur naturally.
He said: "There was no way of seeing whether Katy or Monty succumbed to smoke inhalation."
Staffordshire Coroner Andrew Haigh concluded Miss Neale's death was accidental.