Image copyright Treehouse Image caption Alan Naiman saved and invested most of his earnings throughout his life

No one who knew Alan Naiman could have guessed he had amassed a fortune.

The frugal social worker bought his clothes at Costco, cut coupons and liked to take thrifty road trips for his holidays.

But when he left $11m (£8.7m) to local charities, friends say they were not surprised by his generosity.

Naiman died in January 2018 of cancer at age 63 in Seattle, Washington.

"I think everyone was shocked, I mean really shocked that he had so much money to give away," says his friend Mary Monahan, who worked with Naiman at Washington state children's services.

Soon after being diagnosed with cancer, he told her that he would donate everything he had to charity upon his death.

"People will be surprised at the amount," she recalls him saying.

"Yeah, people were - I'm sure - very surprised," she told the BBC.

Naiman left a lucrative banking career to work with children 30 years ago. Ms Monahan believes his experiences working with children in social services, many of whom had disabilities, inspired him to put others first.

"I guess he thought he had what he needed, and a lot of other people didn't have what they needed," she says. "The kids that came into care got into Alan's heart."

Naiman inherited some of his wealth from his parents, but he also saved and invested most of his earnings throughout his life.

Ms Monahan says he worked two or three jobs and hated to have debt.

Six local organisations were recipients of his legacy, including Childhaven, Little Bit Therapeutic Riding Center, Make-A-Wish Alaska and Washington, Odessa Brown Children's Clinic, Treehouse and WestSide Baby.

Jessica Ross, chief development officer for Treehouse, says the first time she became aware of Naiman was a few months before he died, when he donated $5,000 to the organisation that helps children in foster care graduate from high school.

Ms Ross says the average gift to the organisation is a couple hundred dollars.

"That's a big deal for us, we were even surprised by that," she says.

But when they found out that Naiman had left Treehouse $900,000 in his will, they were truly "shocked".

Once the number had sunk in, Ms Ross says the staff and volunteers with the organisation were really struck by his story, and how he had lived so frugally so that he could give his wealth to others.

"Our organisation was just overcome with that kindness and that love. I don't want the number to outweigh the experience of being a part of this, that was really about the way he wanted to leave the world."