Long Island faces “an enormous gap” between its supply of rental apartments, co-ops and condominiums and the coming demand for that type of housing, a new study finds.

The region could gain as many as 158,000 additional households in the next 15 years, according to a Long Island Index study released Tuesday. The study, “Long Island’s Need for Multifamily Housing: Measuring How Much We Are Planning to Build vs. How Much We Need for Long Island’s Future,” was commissioned by the Index, a project of the Garden City-based Rauch Foundation. That predicted influx far exceeds the 64,000 new homes planned under “the most optimistic scenario,” the study found.

What’s more, roughly two-thirds of the new residents — or 104,000 households — are likely to favor walkable neighborhoods with multifamily housing, rather than single-family neighborhoods, according to the study. With developers planning to construct 26,000 new multifamily homes on the Island under current proposals, many young adults and retirees could be left in the lurch, the study found.

The current supply of housing “is not enough to meet the demands of Long Island’s future population,” the report stated. However, by rezoning downtowns and the areas around Long Island Rail Road stations to allow more multifamily housing, local communities “can improve the overall health of Long Island’s housing market, providing more affordable housing options to all residents.”

One of the report’s authors, Shuprotim Bhaumik, a partner at Manhattan-based consulting firm HR & A Advisors, acknowledged that developers have been constructing more multifamily homes on Long Island in recent years, but he said it still isn’t enough.

“The multifamily housing that we’re producing is at a price point that is really unaffordable to the vast majority of the people who need that multifamily housing, whether it’s singles or young families or even empty-nesters who are living on a fixed income and looking to step down,” said Bhaumik, a Syosset resident.

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The 86-page report focuses on three communities — Hicksville and Valley Stream in Nassau County, and the village of Babylon in Suffolk County — and predicts how many more multifamily housing units developers could build in downtowns if the communities allowed larger buildings.

The report relied on data from the U.S. Census, the federal Bureau of Economic Analysis, opinion surveys and other sources. The New York City region is expected to add 1.9 million jobs by 2040, which “creates a demand for a certain amount of housing,” said Christopher Jones, senior vice president with the Manhattan-based Regional Plan Association and an author of the report.

In the communities that were studied officials had varied reactions.

Rezoning Hicksville’s downtown has been a topic of discussion for years, but “it needs to be done exceedingly cautiously, and in conjunction with the residents,” said Rebecca Alesia, a councilwoman in the Town of Oyster Bay, which includes the hamlet of Hicksville.

Babylon Mayor Ralph Scordino said his village offers nearly 1,000 rental apartments and hundreds of condominiums.

In Valley Stream, Mayor Ed Fare said a newly opened, 92-unit rental complex near the train station has been a boon to downtown shops. Mom-and-pop businesses that were struggling a few years ago “are now renovating and improving their businesses because we brought them people, so I think it’s a very positive thing.”