Architects designing an expanded version of the Vashon library will unveil their latest concept at a public forum Thursday night — a 10,000-square-foot structure that will boast a so-called living roof and other green features.

The new branch will also build off of the existing one: Rather than tear down the current structure, architects have designed an addition to it that will bring in more light and provide more space for books, a larger meeting room and other amenities.

“I think we’ve come up with a scheme that’s fairly solid,” said Robert Miller, an architect with Bohlin Cywinski Jackson, a Seattle firm hired by the King County Library System to design the branch. “I think it represents Vashon quite well.”

But the design is still a work in progress, he said, and architects hope Island-ers will attend Thursday night’s session and offer up their input.

“It’s still fairly fluid,” Miller said. “We wanted to get back to the community while we still have a chance to affect the architecture and design.”

The open house is the second one KCLS has held to garner feedback from the community. The first one — held in April — gave Islanders a chance to view three possible footprints for the expanded structure and comment on the different versions. Miller said he and his colleagues looked over the 200 or so comments they received from that meeting. What they’ve come up with, he said, is one footprint with “two variations on the skin of the building.”

Most noteworthy, he said, is their intention to have a living or green roof — where asphalt shingles or other conventional roofing materials are replaced by a membrane supporting a thin layer of soil and plants such as sedums, mosses and grasses.

The firm designed the Ballard branch library, which also has a green roof. Miller said such roofs last much longer, provide greater insulation, slow rainwater run-off and provide habitat for songbirds.

The design is also sensitive to Ober Park’s popular berms, Miller said. He and his colleagues have looked at historic photos of Vashon’s Strawberry Festivals and have attended recent ones themselves in an effort to understand how Islanders use the space. The new design would not impinge on the berms, he said. It would also provide greater drainage; the bowl formed by the berms currently collects water and is often soggy.

The library sits on land owned by the Vashon Park District. Wendy Braicks, the park district’s executive director, and Mike Collins, who chairs the park district’s board of commissioners, recently got a preview of the proposed design. Both said they were pleased with the way the expanded branch is situated on the park.

“What they presented to us was attractive,” said Collins. “It builds off the existing library and looks fine.”

The design they saw, Braicks added, includes a lot of glass. “It feels like you’re a part of the park.”

The firm still has a long way to go before KCLS can break ground. After Thursday’s meeting, Miller said, “assuming we’re on the right track — and I certainly hope we are,” the firm will begin the next phase, considered the actual design and development of the new structure, which will lead to what’s called the construction documents.

Once those documents are completed, the team can begin applying for permits from the county’s Department of Develop-ment and Environmental Services.

Contractors likely won’t be able to break ground until the end of 2011 or early in 2012.

“There’s a lot of work left,” Miller said.

Architects will present the design concept for an expanded Vashon library from 6 to 7:30 p.m. Thursday, Oct. 7, at McMurray Middle School