He entered it through an open doorway and started a fire, which burnt a large hole in the floor, damaging the floorboards.
After lighting the woolshed fire, Thomas continued driving towards the coast, stopping to light another fire at the former home of Mr Neilson.
The Tokomaru Bay fire crew were called to it on their way home after putting out the woolshed fire.
Mr Neilson died in his house in December 2019 after being brutally assaulted there by two men, who are serving life sentences for his murder.
One of them is also serving a concurrent sentence for arson after attempting to burn down the house with Mr Neilson’s body still inside.
That time, the fire was put out before the blaze could take hold and Mr Neilson’s body was discovered.
However, the fire lit by Thomas in the early hours of Saturday, July 9, was already well involved when firefighters arrived.
With his female passenger still passed out in the car, Thomas started a fire under the house near the stairs up to the front deck.
The deck and lounge room off it — where the assault on Mr Neilson took place — were extensively damaged. There was smoke damage to the rest of the house.
When first spoken to by police, Thomas declined to make a statement. He denied any involvement and said he was at home the whole night.
Reparation for damage to the properties will be sought at sentencing.