Dominic Elliott, 23, was high on ecstasy and cocaine when he walked into a bathroom at the house in Bridlington, east Yorkshire, and apparently began drinking from a bottle of bleach filled with a 95% concentration of sulphuric acid.
Hockney's former long-term partner, John Fitzherbert, drove Elliot to hospital in Scarborough, but he is thought to have died during the 45-minute journey.
Fitzherbert, in his 60s, told the hearing in Hull how Elliott spent the day before his death watching films, taking drugs and drinking alcohol.
At one point the young man got out of bed, laughed hysterically and threw himself 10ft off a first-floor balcony.
"I've no idea why he did it," said Fitzherbert, who said he was in a nine-month relationship with Elliott before the latter joined Hockney's inner circle at his home-cum-studio.
"He'd been snorting cocaine and he seemed agitated. I just saw his feet go over. He went head-first. Afterwards he got up. He seemed OK.
"I calmed him down and he came back to bed and smoked more cannabis. I asked him what it was all about, but I never got to the bottom of why he'd done it.
"I fell asleep in his arms. He seemed calm and relaxed".
Tributes have been paid to David Hockney's assistant Dominic Elliott by members of the rugby club where he played, as a second row, in the seaside town of Bridlington following his reported death.
Dominic was a valued member of our club, a good player with great potential. With his size and ability he could easily have established himself as a first team regular. He was a popular lad and had so many friends at the club.
– Bridlington Rugby Club chairman Gordon Morric, speaking to the Bridlington Free Press
HUMBERSIDE Police were contacted by Scarborough General Hospital, shortly after 0600hrs yesterday morning (Sunday 17 March ), after a Bridlington man had been taken to the hospital by a friend in a serious condition.
The 23-year-old man was sadly pronounced dead at hospital and the police notified.
The circumstances of the man's death are not clear and officers are currently undertaking enquiries to establish events leading to his death.
There were no signs of violence and a post mortem examination is due to take place tomorrow (Tuesday 19 March) which is hoped may provide further information.