The alarm reporting a fire at the Kehlsteinhaus in Obersalzberg was raised at 9 p.m. on Friday, but because of the steep path up the mountain, the local rescue service did not reach the summit above Berchtesgaden until 9:52 p.m.
A spokesperson for the local police told Bild that the damage to the building was extensive.
It is unlikely that the house can be saved.
Bavarian Interior Minister Joachim Hermann told AP that reconstruction plans were already in place.
"We will not let it remain as a ruin. I think it will probably be possible to come up with a sensible solution. It is important that we continue the work of remembering history," he told AP.
Police have arrested a man suspected of starting the fire, a 33-year-old with ties to Berchtesgaden who does not currently live there, according to the prosecutor. The man has been questioned and is said to have shown signs of mental illness, but it is unclear whether there was any political motive.
According to police, there are no signs that anyone else was involved in the fire.
The building, called the Kehlsteinhaus, is at an altitude of 1,834 metres in Obersalzberg, near the border with Austria, and is a popular tourist destination.
The house was built in 1938 as a “tea house” for Adolf Hitler on behalf of Nazi leader Martin Bormann, but Hitler rarely visited the Kehlsteinhaus. He preferred to stay farther down the mountain, in the giant Berghof complex, which served as the Nazis’ second government headquarters.
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