Emergency services were called to Ilikai Place in Dee Why at 7.11am where they found a 40-year-old man’s body in a residential car park.
Officers set up a crime scene and launched an investigation.
For four hours the riot squad, detective and paramedics tried to enter one of the units to speak with the 41-year-old man inside.
He was eventually arrested about 12.30pm.
He has been taken to Manly Police Station and is assisting with inquiries.
Police said he is known to them and he also knew the dead man.
Acting Commander Traci Watt from the Northern Beaches Police Area Command confirmed the death is being treated as suspicious.
It is not being treated as murder.
One line of inquiry police are looking into is that the man suffered a drug overdose before his body was dumped in the garage downstairs.
“We will be waiting on the results of the autopsy to understand how this gentleman died,” Watt said, adding there were no signs of injury.
No charges have been laid.