Thieves burned down a barn in Ringstead, causing tens of thousands of pounds worth of damage, after stealing items from inside.
The incident happened in the early hours of Saturday morning.
Firefighters were called to the scene in Gladstone Street in Ringstead at about 2am and spent hours battling the blaze.
Adam Jones, whose grandfather Pete Crowhurst owned the barn and was in the midst of constructing it, said the fire was ‘devastating’.
He added: “There were 828 bales of hay in there which have been destroyed, two mini-tractors, and a hay baler.
“My grandad thinks about £50,000 worth of damage has been caused.”
Mr Jones said his grandfather had not yet been able to insure the building as it was under construction and had not they been checked by an electrical engineer – something which had been due to happen in the next few days.
The thieves made off with an angle grinder and copper cable before starting the fire.
He added: “For them to go inside and steal the items was bad enough, but to set it on fire afterwards is really upsetting.”
Mr Jones said the blaze was so hot that parts of the metal barn had completely melted.
All that has been salvaged from the wreckage is a sit-on lawn mower.
Northamptonshire Police have sent leaflets to nearby properties in Ringstead to ask if anyone saw anything suspicious, including vehicles around the village late on Friday night or very early on Saturday morning.
Anyone with information is asked to phone Northamptonshire Police on 101. Alternatively, phone Crimestoppers on 0800 555111.