The harbor was glistening. The ornamental grasses were swaying. The misting station was ready to mist. Down on the streets of Boston people were sweltering, but 18 floors above, on the “Sky Deck’’ of the Watermark Seaport, a luxury apartment building, life was perfect.

Well, almost.

Elie Sader, 30, gazed across Seaport Boulevard toward a rival luxury building, the Benjamin, and a dark thought intruded: The Benjamin’s deck has a pool and his does not.

“I try to ignore it,’’ said Sader, a neurology resident at Boston Medical Center.

Such is the state of Boston roof decks, where even a 3,000-square-foot deck — with an outdoor flat-screen TV, Wolf grills, a music system, heat lamps, a lawn, an ice maker, and that misting station — can be one-upped.

It was not always thus. Hard though it may be for millennials and fancy new Bostonians to believe, there was a time when a Boston “roof deck’’ usually didn’t even have a deck.

If you were lucky enough to have roof access, you’d climb worn stairs, struggle to open a heavy hatch, and emerge onto bumpy tar. If you wanted a chair, you schlepped it up yourself. Good luck getting back intact after a few beers.

But the new Boston? We’re a city of roof decks. The Inspectional Services Department counts 2,500 decks — on residential towers, office buildings, smaller multi-unit buildings, and single-family homes — and that doesn’t include decks done without proper permitting.

The city doesn’t have comparative numbers from past years, but Brian Ronan, the chief building inspector, described the growth. “I used to see them once in a while,’’ he said. “Now every inspection job you go out on potentially has a roof deck.’’

Back when Boston was Boston, the deck was not part of an “amenity package.’’ It didn’t have a trellis or a motorized retractable awning or accent lighting or an entire kitchen. You could not shower on your deck. You did not invite friends over to sit in the hot tub, or gather round the fire pit. The sparkling rosé came from your apartment downstairs, not the rooftop dual-zone wine cooler.

But as millennials – and just about everyone else — have become more demanding, what might be called “lifestyle decks,’’ with yoga classes and cafe tables, have become a must in upscale buildings.

“I don’t think you’ll find a deck going up in Boston that doesn’t have the new approach,’’ said Patrick McMahon, director of development at Federal Realty Investment Trust, the firm developing a 20-story, 447-unit apartment building at Assembly Row, in Somerville.

The city is so deck mad that many buildings have two “roof’’ decks — one on the actual roof, another on a lower floor, and office buildings have them, too, lending a high-end wedding vibe to the workaday experience.

For a certain segment of the population, deck culture has so come to define Boston that Albany transplant Brian Rivers, 38, said his new building — the two-deck Benjamin in the Seaport — feels more Boston than the South End brownstone he called home when he first moved here three years ago.

“That was so old,’’ Rivers, an auditor with PwC, said as he finished a glass of wine poolside.

The city’s best people-watching is no longer at ground level, but rather in the sky, where people on a higher deck or terrace judge the tops of the heads of the people below.

“You can watch people partying on the roof of the Envoy hotel,’’ said Matt McKenna, creative director of garden design at Winston Flowers, a high-end florist increasingly called in for what might be called deckscaping.

Decks have gone so high-end that prices once associated with condos, or in the Back Bay, parking spots, are now what it costs to build a private roof.

By the time you’ve paid the architect, the structural engineer, the electrician, the florist, the interior designer and the builder, brought up water and gas, and bought your eco-friendly fire pit and installed an irrigation system, you can easily spend $250,000, or more, said Loren Larsen, a real estate agent with Compass, in Boston.

“People feel so squeezed by the pricing [of condos] that if they can get that extra real estate outside it helps soften the blow,’’ she said. “Truly a lot of people don’t use it that much, but they really want it.’’

Alas, with progress come challenges.

In Back Bay, residents are having their views of the Charles River interrupted by neighbors planting trees on their roofs, said Susan Prindle, co-chair of the Neighborhood Association of the Back Bay’s architectural committee.

“It’s the Hanging Gardens of Babylon syndrome,’’ she said.

In Somerville, Federal Realty is considering adding personal gardening plots to its apartment building’s deck, but there are risks.

On the plus side: it’s a nice perk for residents eager to grow their own kale. On the downside: the developer knows from experience, at a building in Bethesda, Md., that not everyone who signs up to garden actually gardens, leading to the potential for unsightly dead plants.

“Most people fashion themselves having a green thumb,’’ McMahon said.

Meanwhile, back on the deck of the Watermark, in the Seaport, humans were not the only ones enjoying the amenity.

Romeo, a teacup Maltese, had arrived on the deck wet following a bath in the building’s on-site pet spa. “The blow dryer was blowing cold air,’’ his owner, Adriana Provetti, 16, explained.

“He’s fussy,’’ she added, as the petite pooch looked around, perhaps appreciating the panoramic view, or maybe wondering why he was a dog with the bad fortune to live in a building without a pool.

Beth Teitell can be reached at beth.teitell@globe.com. Follow her on Twitter @bethteitell.