Forty percent of Manhattan could not be built today:

New York City’s zoning code turns 100 this year. That may not sound like cause for celebration — except maybe for land-use lawyers and Robert Moses aficionados. Yet for almost every New Yorker, the zoning code plays an outsize role in daily life, shaping virtually every inch of the city.

The bays and cliffs of the Empire State Building come from zoning, as do the arcades and plazas of Park Avenue. The code gave us Zuccotti Park and Billionaire’s Row, the quietude of Greenwich Village and the bustle of the High Line, the glass towers now lining the formerly industrial waterfront and the portion of subsidized apartments that fill them.

New York’s zoning code was the first in the country, meant to promote a healthier city, which was then filling with filthy tenements and office towers. Since it was approved in 1916, the ever-evolving, byzantine code has changed many times to suit the needs of a swollen metropolis. …

Whole swaths of the city defy current zoning rules. In Manhattan alone, roughly two out of every five buildings are taller, bulkier, bigger or more crowded than current zoning allows, according to data compiled by Stephen Smith and Sandip Trivedi. They run Quantierra, a real estate firm that uses data to look for investment opportunities.

Mr. Smith and Mr. Trivedi evaluated public records on more than 43,000 buildings and discovered that about 17,000 of them, or 40 percent, do not conform to at least one part of the current zoning code. The reasons are varied. Some of the buildings have too much residential area, too much commercial space, too many dwelling units or too few parking spaces; some are simply too tall. These are buildings that could not be built today.

It is important to note that these estimates rely on public records that can be imperfect. Still, while the data may at times be imprecise, it allows for an insightful view of zoning in New York.

Many buildings in distinctive Manhattan neighborhoods like Chinatown, the Upper East Side and Washington Heights could not be erected now: Properties in those areas tend to cover too much of their lots (Washington Heights), have too much commercial space (Chinatown) or rise too high (the Upper East Side). Areas like Chelsea, Midtown and East Harlem, on the other hand, would look much as they do already.

“Look at the beautiful New York City neighborhoods we could never build again,” Mr. Smith said. “It’s ridiculous that we have these hundred-year-old buildings that everyone loves, and none of them ‘should’ be the way they are.”

As the zoning code enters its second century, it is worth considering the ways it has shaped the city; whether and where it is still working; and how it might be altered so the city can continue to grow without obliterating everything New Yorkers love about it. …

Nearly three-quarters of the existing square footage in Manhattan was built between the 1900s and 1930s, according to an analysis done by KPF, an architecture firm based in New York. In a way, the zoning code helps to preserve such architectural diversity. The laws have gotten more restrictive over time, giving an edge to properties built in earlier eras.

Astounding to realize that the vast majority of Manhattan’s square footage came into being in thirty years. Even stranger to realize how much of what’s beloved is impossible to replicate today, in an allegedly more enlightened time. If we want greater affordability, a liberalization of the codes to achieve something between the codes of the present and the tenement-friendly practices of the past seems reasonable.