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. Just in March, the administration of Mayor Bill de Blasio won approval for a vast citywide plan that would encourage sleeker, more affordable developments.

Yet many of New York’s buildings remain stuck in the past.

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.

A New New York Would Be Less Dense

19 Jones Street in Greenwich Village. Pablo Enriquez for The New York Times

On the cover of “The Freewheelin’ Bob Dylan,” there on the right side, its cornice almost grazing the N in Dylan, stands 19 Jones Street. It is one of the thousands of buildings in Manhattan with too many dwelling units for its size.

Built in 1910 as a tenement, 19 Jones Street predates the zoning code by six years. It belongs to a special family of tenements known as dumbbell apartments, so named because of the way the buildings are squeezed in the middle, creating air shafts. Such openings were a requirement of the Tenement Housing Act of 1879, meant to make tightly packed apartments a little bit more livable.

19 Jones Street If built today Current building 55-foot height limit Setback from street 15-foot setback Nonconforming JONES ST. 19 Jones Street If built today Current building 55-foot height limit Setback from street 15-foot setback Non- conforming

Were 19 Jones built today, it would have to be significantly smaller. The number of apartments would fall sharply, to just eight from 24. The building’s total dimensions would be nearly halved, and a story or two would have to be chopped off.

New York’s zoning rules were intended to create less cramped quarters, but they also have consequences for the number of aggregate apartments in the city. Such limitations can quickly decrease the supply of housing, and most likely drive up rents. If every tenement in the city were reconfigured in these ways, they would be less crowded, but there would also be fewer apartments to go around.

A New New York Would Be Shorter

720 Park Avenue on the Upper East Side. Pablo Enriquez for The New York Times

Problems persist at the other end of the real estate spectrum.

Take 720 Park Avenue, a Rosario Candela classic from 1928 that rises 17 stories and has only 29 units — one of which, a 7,000-square-foot duplex, recently came on the market for $22.5 million. All that grandiosity is too much for modern zoning, which now constrains such boxy, bulky buildings in this part of town.

The city’s first zoning code was enacted as New Yorkers began to worry that tall buildings would cast the city into eternal darkness. People feared the spread of bulky skyscrapers like the Equitable Building, at 120 Broadway in the Financial District. Rising 42 stories and 538 feet straight up from the street in a hulking limestone slab, it spread a seven-acre shadow over downtown when it opened in 1915.

720 Park Avenue Current building If built today Nonconforming 60-foot height limit 15-foot setback 70TH STREET PARK AVENUE 720 Park Avenue Current building If built today Nonconforming 60-foot height limit 15-foot setback 70TH STREET PARK AVENUE

In response, the city included a setback rule in the zoning code the next year, which required buildings to step back as they rose. It helped create that familiar saw-toothed shape beloved in so many prewar skyscrapers as well as the ersatz ziggurat in shorter structures like 720 Park.

Yet changes brought about with the 1961 zoning overhaul and tweaks since would create a very different 720 Park today. First, it would have to be much shorter on the 70th Street side. And, since this building reflects a previous era’s rules on bulk and density, it would have to slim down along Park Avenue, as well.

The rules were dreamed up by planners in part to ensure that historic buildings would not be replaced with something totally out of context, like the skinny towers now springing up on 57th Street. Yet they also ensured that many of the existing structures that made up that context would eventually be out of context themselves.

But a New New York Will Still Look a Little Old

Both 19 Jones Street and 720 Park Avenue belong to a vast group of buildings in New York City that are treasured for their architectural value and historical significance. They persevere out of genuine appreciation but also the zoning quirks that determine the fate of almost every building, new or old.

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.

Not all buildings are worth keeping. In Midtown East, many nonconforming structures have low ceilings and columns that make them unappealing to new businesses. Some developers have gone so far as to demolish all but the bottom quarter of their buildings, and then build up from there, allowing them to retain the old zoning for their plots so as not to sacrifice a single square foot. The city is currently reconsidering a proposal that would allow these buildings to be rebuilt to their original size and possibly even larger.

It does not have to be this complicated. In honor of the code’s 100th anniversary, the Municipal Art Society of New York has called on City Hall to consider overhauling the code in a way that would make it intelligible to all.

“To understand zoning, you have to have a law degree, it’s so convoluted and so dense,” Mike Ernst, director of planning at the civic group, said. “The whole process of how buildings get built these days is so confusing and opaque to people. There really should be more transparency, so people can have an understanding of what the future holds for their city.”