Tauranga Detective Senior Sergeant Greg Turner said the fire was being treated as an arson.
However, it was too early to determine whether the man would face charges.
"The occupant of the house, we are not able to speak to him for a few days for medical reasons," Mr Turner said.
A Waikato Hospital spokeswoman said the man was last night in a serious but stable condition in the hospital's high dependency unit.
Police and fire investigators at the scene yesterday wore dust masks and used shovels to help sift through the remains. Fire investigator Jon Rewi was unable to comment yesterday, as the matter was being dealt with by police, he said.
Police could not be reached for further comment.