After separating in 2011, they continued to work on the house together to complete it.
Ms Flower said, on the day of the fire, the pair were working on the house and his ex-partner, who lives elsewhere, left about 3pm.
"The defendant had been drinking alcohol and in the early hours of the morning decided to burn the house down."
He began pouring kerosene around the house and into the cavities of the walls, and then "lit a roll of toilet paper and threw it in the window".
This failed to ignite anything and for the next 40 minutes, he set fire to various things in the house, including newspapers and curtains, until something took hold in the dining room.
It triggered a fire alarm which alerted his ex-partner, who sent him a text, to which he replied "got it".
"The defendant then changed his clothes and sat on the lawn drinking wine until police arrived."
Fire crews arrived at 5.30am and contained the blaze, which badly damaged the home. Nobody was injured and police arrested and charged Grindlay that morning.
Ms Flower said when the defendant was questioned about why he did it, he said his reasons "encompassed a lot of things".
Mr Stevens said his client had been stressed trying to finish the house.
Grindlay also had alcohol issues, which he was now getting help with from CareNZ, an addiction treatment provider, he said.
He added that his client removed the gas bottles from the house to prevent anybody from getting hurt.
The house, which was uninsured, had since been sold and Grindlay was working two jobs to pay for the remainder of the mortgage.
Judge Pat Grace remanded Grindlay on bail again until March 26, to await a section 38 report which assesses a person's mental health.
