Portland City Hall and Multnomah County have estimated how much money is needed to create 2,000 units of supportive housing, the kind of housing that comes with services and is designed to address the needs of chronically homeless people.

It's a big number: somewhere between $592 million and $640 million for the first 10 years.

Then they say a sizable investment will be required to keep those services going: $43 million to $47 million a year.

Those estimates are part of a report put together by the New York-based nonprofit Corporation for Supportive Housing at the request of the county and city.

More bad news: The need has grown since the county and city started analyzing the number. Over the next 10 years, the county and city probably need 2,400 units, not the 2,000 estimate they started with.

But the good news is that the city and county have already opened or started to create 517 units of supportive housing.

The next step will be figuring out how to bring more units online and pay for services.

"We've set a stretch goal," says Commissioner Nick Fish. "We're really saying this is the critical next step in figuring out how we get there and how we pay for it."

They're hoping that setting up a supportive housing system will make it easier to add units as needed.

Not everyone on the streets is chronically homeless or has major health, mental health or addiction issues, but serving that part of the homeless population is the most expensive and most intractable part of the homelessness problem.

"We've made tremendous progress in the past four years helping people get back into housing," says Joint Office of Homeless Services spokesman Denis Theriault. "But we see more and more people who have been [on the streets] for a long time or have a disability, and this kind of radical, major intervention is what they need to get somewhere stable and stay somewhere stable."