The D.C. Council is scheduled to vote Tuesday on whether to allow the Lab School of Washington, a private school for students with disabilities, to extend its lease on an old public school building in Northwest Washington.

The proposal would allow Lab to lease the old Hardy School off Foxhall Road — not to be confused with the still-operating Hardy Middle School in Burleith — for 25 years, with an option for an additional 25 years. It has drawn broad support and passed out of the council’s Committee on Economic Development on a unanimous vote.

But parents at nearby Key Elementary are asking the council to delay a decision until the city finalizes new school boundaries in September. The boundary overhaul is meant in part to address severe overcrowding at Key Elementary and other Northwest schools, they argue. What if it turns out that the city could use an extra building in that part of the city?

“The old Hardy school should be factored into the boundary review process currently underway,” said Key parent Bill Slover, adding that it’s premature to execute a long-term lease on that building until the city can “provide a clear road map that will solve the existing overcrowding, not to mention future issues.”

Two dozen of Key’s former PTA officials and other parent leaders echoed that argument in a letter to Gray administration officials and council member Mary Cheh (D-Ward 3), who has pressed to resolve school overcrowding in the neighborhoods she represents.

Cheh said school system officials have assured her that they don’t want or need the old Hardy School. School system officials declined to comment.

Lab is well-known for serving children with disabilities and currently enrolls nearly 360 students, including about 70 whose tuition is paid by D.C. taxpayers because their needs cannot be met in the city’s public schools.

The Hardy building serves as Lab’s elementary-school annex, home to about 80 students in grades one through four. Lab’s current lease would allow it to continue using the school for another decade, until 2023. But Lab officials say they need a longer lease to make needed renovations and secure the future of their school.

“The Old Hardy Building has become a major asset to our program,” Head of School Katherine Schantz wrote in testimony to the council. “Losing this building would place our school in a precarious situation.”

The Hardy School has not been used as a public school since the mid-1990s. In 1998, the city leased it to Rock Creek International, a private school that declared bankruptcy in 2006. The Lab School acquired Rock Creek’s lease and has used the building for about the last five years.

Lab currently pays $80,000 per year for the property, which includes a land area of 50,000 square feet and a building of more than 17,000 square feet. The proposed lease would charge $16.50 per rentable square foot, but the city would discount up to $10 per square foot to help offset the school’s operating costs.

Lab’s expenditures on building renovations would also count as a credit toward the rent charge; for every dollar Lab spends on construction, it would pay the city one less dollar in rent.

After 25 years, the base charge would be reset according to fair market value.

Deputy Mayor for Education Abigail Smith said the Gray administration is “comfortable moving forward with the lease.”

“The Lab School serves an important role as part of the continuum of services available to students in the District, and we believe there are a number of viable options to address the over-enrollment issues at nearby schools,” Smith wrote in an e-mail.