Prewar buildings have long had cachet in New York, but with decades of wear and tear, apartment layouts that have been cut up over time, and systems that are often outdated, living in them doesn’t always stand up to the romanticized ideal.

Still, for buyers like William and Nazak Savitt, living in a postwar building would be out of the question. “I like a beautiful building,” said Ms. Savitt, like the Beaux-Arts structures she admired while growing up in Paris.

So to take advantage of New York’s prewar history while offering the conveniences of a 21st-century condo at the top of the market, developers are using some of the city’s historic housing stock to create what are essentially new buildings in old shells. The resulting projects aim to appeal to contemporary tastes and expectations while holding their own against a flood of glassy new-construction luxury condos built from the ground up.

The Savitts, for instance, fell for the Chatsworth, a 1904 Beaux-Arts apartment building at 344 West 72nd Street, facing Riverside Park, with a brick-and-limestone facade resplendent in cherubs, elk heads, human busts and garlands, which is being converted to a co-op with condo-like bylaws by HFZ Capital Group.

“I came to the lobby, and it was so impressive,” said Ms. Savitt, 51, a graphic and interior designer. The wide columns and pilasters topped with Ionic capitals caught her eye, as did the carved walnut paneling and detailed plasterwork, both restored and recreated. Where there were once boarded up skylights, the space had been reimagined with backlit LED glass panels, along with sleek modern furniture and enormous glass-rod chandeliers from Italy.

Upstairs, she found an apartment that seemed ideal, with hefty crown molding, paneled doors and a foyer with a tracery ceiling. It was both historic and brand new — a freshly polished take on prewar living that didn’t exist when the building was constructed more than a century ago.

“I thought this layout was very intelligently done, and I just fell in love. And the bathroom and closet,” she said, pausing for a breath while showing a reporter her home, “wait until you see them.”

The reinvented spacious master bathroom had heated herringbone Carrara marble floors, a long lacquer vanity with a Bardiglio marble top, a windowed shower and a free-standing soaking tub by another window. The large walk-in closet in the master suite had three windows, one offering a glimpse of the park.

By the time Ms. Savitt heard about the building’s planned 10,000 square feet of new amenity spaces, including a fitness center, yoga room, games room, media room, wine room and library, there was no turning back.

She and Mr. Savitt, 52, an attorney at Wachtell, Lipton, Rosen & Katz, closed on the four-bedroom, three-and-a-half-bath apartment for about $5.4 million last December. They moved in this past June with their 16-year-old daughter, as construction continues elsewhere in the building.

The building very deliberately combines early 20th-century and contemporary design. The thick exterior walls, beefy floor slabs and lavishly detailed facade are original, but most of the rest — many interior walls and floors; the heating, cooling, plumbing and electrical systems; the extensive amenity spaces — is new.

“These moldings, people may think they’ve been here forever, but of course they haven’t,” said Andrew Sheinman, president of Pembrooke & Ives, the design firm that conceived the Chatsworth’s refreshed interiors. “It’s about recreating people’s perceptions of what this building should be.”

Competition in new construction is fierce, as the city’s building boom continues. The real estate appraisal firm Miller Samuel expects 2,800 new condo units to have entered the Manhattan market this year, and 3,000 more to follow in 2018. Those pile on top of 5,400 new condos in 2015 and 4,000 in 2016.

Among sales of new Manhattan condos above $2 million over the 12 months ending in September, new-construction projects fetched an average of $3,116 a square foot compared to $2,686 a square foot for new prewar conversions, said Jonathan J. Miller, president of Miller Samuel.

Most of that price difference, he said, can be attributed to the higher floors in today’s vertiginous skyscrapers, which command a premium for each additional story that rises into the sky.

Although prewar buildings often can’t compete on height, one of the reasons developers are completing such elaborate Sheetrock-to-steam-shower overhauls in prewar conversions is to appeal to the largest pool of upscale buyers possible.

“You have particular buyers who will not live in a renovated building because they want to be in a new glass tower, and you have people who only want to live in prewar or historic, architecturally significant buildings,” said Ziel Feldman, the chairman and founder of HFZ Capital Group, which is developing the Chatsworth as well as converting the nearby Astor and Belnord apartment buildings to condos. “If I can somehow combine that attraction people have to a brand-new tower, but create it with a historic turn-of-the century building, I think you’ve got the best of both worlds.”

Part of the challenge in updating older buildings is that their original layouts catered to a different lifestyle, and a time when many wealthy people had live-in help and the kitchen was considered service space rather than a family gathering area.

“A lot of these prewar buildings had a lot of little rooms,” said Nancy J. Ruddy, a founder and managing principal of the architecture and design firm CetraRuddy. “The living rooms were grand, and the foyers were grand, but then they had little maids’ rooms, tiny bathrooms, a lot of bedrooms without closets and kitchens that were mostly designed for a cook.”

Such was the case when CetraRuddy began designing the renovation of 360 Central Park West, a limestone apartment building designed by Rosario Candela in 1929 that is being converted by Argo Real Estate, where available units start at $2.995 million.

To move and expand the kitchens, enlarge the bedrooms, and add closets and laundry rooms, the development team is combining roughly two old units for every new one.

“Originally, there were 146 units, and we will probably end up with around 70,” said Jeffrey Stockwell, an associate broker with Stribling Marketing Associates, which is handling sales and marketing, noting that the development team is working around rental tenants who remain in some of the apartments.

CetraRuddy also convinced the city’s Landmarks Preservation Commission to allow workers to punch 34 new windows in the facade facing Central Park. “It enabled us to have corner living rooms with great windows on both 96th Street and Central Park West,” Ms. Ruddy said. “It was a game changer.”

At the same time, the building’s operating systems are being replaced. Mr. Stockwell said a new “four-pipe” central heating and cooling system will eliminate the need for window-mounted air-conditioners and allow the rooms to have independent temperature control.

Even with changes like these, apartments in prewar buildings typically have a different character than those in new buildings because they have traditional double-hung or casement windows rather than floor-to-ceiling glass, and a progression of defined rooms rather than wide-open spaces.

Jodi Balkan and Chris Dickerson were surprised at the difference when they bought an apartment at 88 & 90 Lexington Avenue. A conversion project from HFZ, it combined a 1927 limestone building at 88 Lexington with a 1958 building at 90 Lexington; available apartments there range from about $2 million to $5.875 million.

Designed by Workshop/APD, the renovation retained the facade of the older building but replaced that of the 1958 building with a new one featuring full-height windows, giving it the feel of a new-construction tower.

At first, Ms. Balkan, 50, the founder of the beauty-focused public relations company Bold PR, and Mr. Dickerson, 43, an actor, went into contract on a one-bedroom, two-bath apartment in the newer tower in September 2015. But a few months later, they upgraded to a larger two-bedroom, two-and-a-half-bath unit in the older tower, after falling for its traditional charm. They closed for $3.4 million last June.

“The layout in 88 is just more homey and has that prewar feeling, as opposed to new construction, which has adjoining rectangles,” Ms. Balkan said. “You walk into a rectangle, and then there’s just a rectangle for one bedroom and another rectangle for the other bedroom — that’s what most new builds look like.”

Their apartment, by comparison, is more rambling, with an entrance hall leading to a kitchen that is only partially open to the living and dining room, a separate hallway leading to the bedrooms and a study tucked in a corner.

“The prewar building is pretty special and has great proportions,” said Matthew Berman, a principal of Workshop/APD, noting that 88 Lexington has expansive floor plans with room to spare. “You end up with a lot of really generous spaces, like big foyers and bonus spaces.”

Nothing else about the project is old fashioned, however. On the ground floor, Workshop/APD has connected the buildings with a new shared lobby that is thoroughly contemporary, with perforated metal wall panels, neon signage and a faceted terrazzo-and-wood reception desk.

A staircase leads down to 8,000 square feet of new amenity spaces, including a large gym, screening room, lounge, playroom and spa with a lap pool that was carefully dug out between existing footings.

A similar mix of old and new is apparent at 100 Barclay Street, where Magnum Real Estate Group and CIM Group have carved out 157 condos on the top 22 floors of a 1927 Art Deco office building designed by the architect Ralph Walker (Verizon occupies the lower floors). Available units range from about $4 million to more than $14 million.

To reach the apartments, residents pass through a cozy new residential lobby designed by Jeffrey Beers International, with marble herringbone floors and salon-style displays of contemporary art, before emerging into the cavernous landmark lobby, which has ceiling murals depicting the history of communication and original bronze elevator surrounds blossoming with flowers and vines.

“You enter here and begin to see all the craftsmanship. Look at all these creatures,” said Stuart Marton, the executive vice president of Magnum, gesturing toward pewter squirrels adorning ornate elevator doors. “That’s something that can’t be replicated.”

Upstairs, contemporary corridors have carpets and wallcovering with Art Deco-inspired patterns, and lead to apartments with white oak floors, Calacatta Gold marble counters and Sub-Zero, Wolf and Miele appliances.

The building offers a dizzying 40,000-square-foot spread of amenity spaces, split between the full 18th floor, where there are four outdoor terraces, lounges and playrooms designed for residents of various ages, music practice rooms and a wine room, and two lower levels where there are two new pools, a fitness center, spa treatment rooms and yoga and Pilates studios.

“It’s a brand-new building within an amazing existing structure,” Mr. Marton said.

“It’s much, much easier to build a building from scratch,” he continued. With conversion projects, “all the electrical conduit, water pipes, steam pipes, gas lines are brand new, but you have to deal with existing slabs and take out the old systems.”

How much more difficult are these elaborate conversions compared to new construction? “About 10 times more,” said Robert Gladstone, the chief executive of Madison Equities, which is converting the 1912 neo-Gothic commercial building at 212 Fifth Avenue into condos designed by Pembrooke & Ives and Helpern Architects, in partnership with Thor Equities and Building and Land Technology. Mr. Gladstone recounted how parts of the building’s facade had become so unstable that they were lying in pieces on one of the upper floors when the project started.

On a recent morning, he pointed up at the restored facade from the sidewalk across West 26th Street. “Do you see the frieze of faces?” he asked. “Every single one had to be cleaned.”

And because the steel structure underneath that facade didn’t rise with straight lines, he said, the layout of some bathrooms had to be tweaked on almost every floor.

Mr. Gladstone said it was his company’s first — and last — conversion project: “I’m not running to do it again.”

But there was at least one recent turn of events that promised to make it all worthwhile. The triplex penthouse, which has five bedrooms and five full and three half bathrooms across 10,079 square feet, along with 5,730 square feet of outdoor space, was finally completed. The price: $73.8 million.

A sale of that size might be enough to make any developer reconsider.