Anyone who’s ever rented in New York City knows how hard it is to get reliable information about an apartment. How much has the unit rented for in the past? Does the building’s history entitle tenants to rent-regulated leases? If so, for how long?

These are important questions that every renter should be able to answer before they sign a lease. Except that there’s no one place to find the information.

But there could be. And, in fact, according to New York law, for many apartments, there should be.

Since 2007, there’s been a statute on the books that requires the city’s Department of Housing Preservation and Development to collect and make public key data on the thousands of new apartments built each year under the city’s single-biggest housing subsidy. But the provision has been ignored by HPD: The agency simply hasn’t done it.

The requirement is part of section 421-a of New York’s property tax law — the legislation that can slash landlords’ tax bills by upwards of 90 percent. In exchange, owners of the estimated 83,000 city apartments covered by the tax break must agree to limit rent increases on all their tenants and set aside a portion of the units for “affordable” below-market rents in certain high-priced city neighborhoods.

The housing department’s decision to ignore the law’s reporting requirement has deprived New Yorkers of vital information on the apartment buildings that benefit from the subsidy, which costs taxpayers $1.2 billion a year in lost revenues.

The missing information is particularly important in a city like New York, where policymakers use subsidies to try to keep a lid on rents but tenants are often in the dark about their rights.

An example: In January, the state launched what it called a “major initiative” to help tenants who had been wrongly denied rent-stabilized leases by landlords who collected another city tax break known as J–51. As we reported, the state isn’t directly informing tenants they could receive rent adjustments or refunds. Instead, it’s asking landlords to pass the word.

Public reporting requirements like those set out in the 421-a law are meant to empower tenants by giving them access to important information about their apartments. That’s why Ben Kallos, a City Council Member representing the Upper East Side, sponsored a bill that would beef up housing data collection and reporting by HPD.

But when the Council held a hearing on the bill in February, HPD said Kallos’ bill was “unnecessary” and a “waste of taxpayers’ resources” because the agency was already doing a great job keeping track of the city’s taxpayer-subsidized housing.

So in February, right after the agency testified against Kallos’ proposal, ProPublica asked HPD to make a copy of the 421-a reports available for inspection, as required under the law.

Six weeks after the request, the agency’s records access officer wrote back to say “HPD does not possess or maintain any records responsive to your request.”

Kallos, whose bill hasn’t gone anywhere, wasn’t pleased.

“It is time for HPD to release the data it has, share which data it doesn’t have and start following the state law,” Kallos said in an interview.

So what kind of information is the public missing out on? A lot, it turns out.

The law governing the 421-a program requires HPD to produce a report “no less than annually” which “at a minimum” contains the following information “for every building” receiving the tax break: The building address, the total number of residential units, the number of “affordable” units, the rent for each unit in the building, and the beginning and end date of the tax benefits. The latter is important for tenants because — once the tax benefits end — so do the rent protections attached to the property.

Once completed, the reports “shall be available for public inspection” in a format that doesn’t compromise tenant privacy, according to the law.

That public disclosure requirement is crucial: Apartment-level data gathered by the state, such as the rent charged by landlords, normally can’t be disclosed under the state’s Freedom of Information Law, thanks to a long-standing exemption in New York rent law.

When they required HPD to publicly report on rents in 421-a buildings, state legislators effectively created an exception to this non-disclosure rule, allowing New Yorkers a rare glimpse into an important chunk of the city’s taxpayer-subsidized apartments.

We asked HPD whether its records officer was correct in asserting that the legally-required public reports did not exist. HPD spokespeople ignored those inquiries.

And so we took our questions to Mayor Bill de Blasio, who is pushing to build tens of thousands of new affordable apartments through the tax subsidy.

We asked whether the Mayor was concerned that his housing agency was not tracking basic information about apartments already benefitting from the subsidy. We also asked if the Mayor intends to order HPD to follow the law and report the information.

Austin Finan, a spokesman for de Blasio, declined to address either question. In a statement, he said only that “we’ve taken unprecedented steps to strengthen requirements and oversight of 421-a, to protect and expand affordable housing.”

That assertion is accurate but incomplete. While it’s true that Mayor de Blasio has pushed for significant reforms to the 421-a program — such as expanding its affordability requirements citywide — his proposed reforms ultimately did not become law.

On the other hand, the requirement to track the rents paid by tenants in each unit — and to report when the tax benefits will end, so that renters know when their protections will disappear — remains on the books.