The cost of letting public housing deteriorate until it is uninhabitable would be just as much as fixing it, a report released Tuesday by a nonprofit argued.

If the New York City Housing Authority continues to be underfunded and mismanaged—especially at the hundreds of buildings at risk for future flooding—then some of its units will begin degrading beyond repair, with huge consequences for the city, the Regional Plan Association said.

"[Public housing] is not a historic relic, but is actually part of the vital infrastructure of the city," said Tom Wright, president of the planning and advocacy organization.

The report explored a number of hypothetical situations, the most drastic of which was a third of the authority's apartments becoming unfit for tenants. Because the average income of households in public housing is $25,000—too little for occupants to afford an apartment on the open market and even in many of new units deemed affordable by the de Blasio administration—residents and officials would be left with few good options.

If every public housing resident stayed in the city, the association predicted, an additional $3 billion annually would be needed to house them in shelters. The Housing Authority's total capital needs at the moment are $45.2 billion.

Alternatively, the city would need to dedicate all of the new units created in its housing plan to displaced public housing residents. That would require vouchers to supplement tenants' rents and would take away opportunity for other New Yorkers who otherwise would have qualified for the apartments.

If public-housing residents who could find work outside of the city moved to more affordable areas of the country, the report predicted, the workforce would drop by 20,000 and the local economy would lose out on $1 billion in spending. Even in this scenario, the administration would need to spend an additional $1 billion annually on shelters.

Workforce participation in public housing is 60%, roughly on par with the city as a whole. Its residents primarily work in health care, retail and hospitality. Given public housing's cultural resources—open space, senior centers and sports fields—and the fact it is the largest source of affordable housing in the city—the report argued that public housing is too important to the economy, housing market and cultural fabric to fail.

The repair bill for the Housing Authority was recently determined to be $31.8 billion. The de Blasio administration rolled out a plan this month that it claimed would ultimately fund $24 billion in repairs, three times more than it had previously identified.

Some 7,000 of the agency's 176,000 units are already in such bad shape that fixing them is not cost-effective, and the slow pace of repairs had raised the risk that tens of thousands more will be beyond repair by the time work could be done.

The official population of Housing Authority properties is a bit more than 400,000 people, but as many as 600,000 appear to be living in them, based on the amount of trash the Department of Sanitation collects. Many tenants remain off the leases so their income does not trigger a higher unit rent, which is on average $508 a month.