Daniel Beckwitt, 27, had been building the bunker because of his fear of "international threats, including from North Korea", his lawyer said.
In September, 21-year-old Askia Khafra died after a fire broke out in a network of tunnels under the house.
Mr Beckwitt was charged with involuntary manslaughter last week.
Officials in Bethesda, Maryland - a wealthy suburb of Washington DC - have also charged Mr Beckwitt with "depraved heart second-degree murder" because they say he acted with wanton disregard for human life.
The 200ft (60m) tunnel system, which was concealed 20ft beneath Mr Beckwitt's basement, was fitted with a "daisy chain" of electrical power cords, prosecutors say in charging documents.
The court papers, filed on 25 May in Montgomery County, cite dangerous hoarding conditions inside the home, and say the accused was "specifically aware of the increased likelihood of a fire breaking out", but did nothing to prevent it.
Police also described "immense piles of garbage and discarded items strewn throughout the entire home" of the stock trader, and "narrow maze-like pathways", which made it difficult to move about within the single-family home.
When the fire broke out on 10 September 2017, Mr Beckwitt was able to escape the home and tell emergency officials that another person was still inside.
Mr Khafra was later discovered dead from smoke inhalation and burns.
Officials say that Mr Beckwitt went to extraordinary lengths to prevent the victim - whom he had met online - from knowing where he was working, reports the Washington Post.
He would make him wear blacked-out goggles after picking him up from his home in a rental car.
Mr Beckwitt would then drive around for one hour so that his worker did not know the location of the home.
Mr Khafra would work for days at a time in the basement, eating and even going to the toilet down there, prosecutors say.
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