The smell of smoke remained at the scene on Thursday morning as a police forensic team examined the house.
A policeman at the house said the investigation was ongoing and would not comment further.
A neighbour who wished to remain anonymous said she was "quite close with the owners".
She was home when the fire broke out next door and called the emergency services.
"I'm just glad they're both safe," she said.
"The fire crews arrived really quickly and managed to get the fire under control fast."
Police were urging anyone with more information to come forward and contact either their local police station or call Crimestoppers anonymously on 0800 555 111, quoting file number 190220/3662.