Users of an adult autism centre are "heartbroken" after an arson attack destroyed a sun house they had fundraised to build.
The £1,500 garden room at Sandringham House in Middlesbrough was burnt down in the early hours of Thursday.
It was installed last summer after centre users carried out various fundraising efforts including a sponsored walk.
Cleveland Police called the fire a "mindless act of damage".
Community support officer Val Llewellyn said: "We are heartbroken, all just very sad.
"The service users who worked so hard to raise the funds for it just cannot understand why this has happened."
She said it was aimed at users who don't like open spaces and was a quiet space to eat lunch or do other activities.
"We were really looking forward to using it this summer and have a lot of activities planned around it," Ms Lewellyn said.
She said a couple of weeks ago they noticed someone had been in the summer house during the night and smoked cannabis.
Ms Llewellyn said CCTV shows someone throwing something at the sun house at about 01:30 GMT on Thursday quickly igniting the structure.
"We do not think this was an accident and we just cannot understand it," she said.
"People must know what kind of centre this is and who uses it."
Police are appealing for information including CCTV or dash-cam footage from the area.