The father and five children aged 18 to 25 were found in the small village of Ruinerwold in the northeastern province of Drenthe, the Dutch police said CNN. A sixth adult child had left and was the one who alerted the police.
Although the village has only a few thousand inhabitants, nobody knew that the family was there.
The police arrested a 58-year-old man who had rented the property. The authorities said they still try to establish the connection between the detained man and the family, and they believe the family has been living there since 2010.
The son visited one of the few cafes in the village, the Café de Kastelein unkempt, according to NOS. He came in and tried one night to order a beer, but went to RTL Germany when the café wanted to close. He came in again on another night but left quickly.
Westerbeek described him as long hair, long beard and in old clothes, NOS reported.
"Last Sunday he came and he sat here on the terrace and he asked for help," Westerbeek told RTL Germany. "So I tried to talk to him, did not get enough information about him, so I asked him to call the police so we could help him."
The son told Westerbeek that he had never been there According to NOS, he had not visited a hairdresser for nine years.
"It is unclear whether they have remained voluntary," it says in the police statement.
The family was not on the government records. They have now been examined by doctors, the police said.
"Part of our investigation is to find out what the people in this house were doing, there are still many questions that we have no answer to, the investigation is not yet complete. People were taken to a home where they are being cared for and where they are being asked to tell their story, "police spokeswoman Nathalie Schubart told RTL Netherlands.
Dutch police said they found them in a small room on the ground floor. Aerial views of the property show a landscaped garden with vegetables planted in orderly rows, and NOS reported that the family also had a goat.
Aerial views of the property show three roofs. Some of the doors of the buildings are overgrown with overgrown greenery.
The house was accessible via a bridge over a river that neighbors said was always locked and locked, NOS reported. A neighbor told NOS he once tried to get closer, but turned around when he saw several cameras on the property.
A man is being held
"We understand that everyone has many questions, and we have them," the Dutch police said in a statement, adding that they had a large team to investigate How the family came to be there and whether any violations were committed.
The 58-year-old man was arrested for failing to cooperate with the investigation, the Dutch police said.
CNN's Rosanne Roobeek, Stephanie Halasz, Karley Wilson, Valeria Longhi and Philip Victor contributed to this story.