The Oak Bluffs planning board is set to decide next week on a project at the Oak Bluffs Tabernacle that is the last phase of a restoration spanning the past two decades.

The board opened a public hearing last week on the plan to replace the roof and put a 1,300-square-foot addition on the back of the historic open-air building that sits in the center of the Camp Ground. Named to the National Register of Historic Places in 1979, the Tabernacle has long been a center of Island cultural, religious and civic life in the summer months. A week from Sunday, high school graduation ceremonies will be held there.

The roof project aims to replace asbestos roofing that dates to the early 1930s. The planned addition on the back of the building would include 10 bathrooms, a back stage area and a multipurpose room with climate-controlled storage space for a grand piano. Currently, portable toilets are the only public bathroom facilities on site.

The two projects are expected to cost $6 million, with $4 million raised to date, Martha’s Vineyard Camp Meeting Association president Andrew Patch told the Gazette by phone this week. The funding includes public monies, including a total of roughly $950,000 in Community Preservation Act funding approved by voters in Island towns last year, and again this spring.

Private fundraising for the remainder is under way, Mr. Patch said.

The first four phases of the project included refurbishing the cupola, restoring old benches and structural work on the iron-frame building.

Earlier this year the Martha’s Vineyard Commission decided not to review the roof replacement and addition as a development of regional impact, sending it back to the town for site plan review by the planning board.

But some neighbors are objecting to the accessory building, raising concerns about the visual impact on the architectural integrity of Tabernacle. The planning board has received a recent flood of letters about the project from Camp Ground residents, most of them opposing it.

At the planning board hearing last week, select board member Gail Barmakain, who is also a member of the town wastewater commission, questioned whether the bathrooms would be allowed by the town wastewater commission, a recording of the meeting shows.

The town’s small wastewater treatement facility is nearly at capacity and cannot be expanded until a comprehensive wastewater planning study is complete and an expansion plan is approved by voters.

Ms. Barmakian cast doubt on whether the hookup would be allowed, noting that a request for two bathrooms was denied by the wastewater commission some years ago.

But Mr. Patch took a different view.

“I’m hopeful and expectant that we’ll be able to work it out,” he said.

The planning board hearing resumes June 8 at 5 p.m., when the board is expected to deliberate and vote.