The alarm about the fire came just before midnight.
Several motorists had then seen what was perceived as a forest fire a bit south of Södertälje.
- When we arrive at the site, it turns out that it is a summer cottage that is on fire, says Åsa Holmberg, alarm operator at Södertörn's fire service.
The cottage was then already overheated and the rescue service focused on letting the house burn down under controlled conditions and prevent it from spreading.
A man in his 50s is feared to have died in the fire, the police confirm just before 5 o'clock on Saturday morning.
- We have been in contact with his relatives, says Carina Andersson, duty officer at the police's regional command center.