Vets for Pets says the incident is "a day we will never forget".
The 15-year-old appears in court following the huge blaze at Nene Valley Retail Park in May.
Northamptonshire Fire and Rescue is handing back Nene Valley Retail Park after a blaze on Saturday.
Two other boys are also on bail after being arrested in connection with a blaze at a retail park.
Businesses are told they can open as normal on Monday after Saturday afternoon's blaze.
The fire is thought to have started outside, possibly from a discarded cigarette, the fire service says.
The cause of the fire, which was in the store's roof space, is being investigated.
The fire is thought to have started behind the building before spreading to the roof.
Dunelm is set to open shortly before Christmas in Beverley's Flemingate shopping centre.
