Updating its beloved historic building was always going to be fraught, but when New York's Frick Collection announced renovation and expansion plans, it elicited a chorus of wails and breast-beating reminiscent of a Greek tragedy.

The much-beloved music room, destroyed? A desecration. The small green space (intended to be a "viewing garden") designed by Russell Page bulldozed? Practically criminal. Even the suggestion that the objectively insufficient 1970s-era reception hall should be updated was savaged by one critic as "a terrible mistake".

The ferocity of preservationist backlash comes at least partially from the reality of doing business (or rather, keeping business from being done) in New York City, where often the only thing holding wrecking balls at bay is relentless public outcry.

But it also speaks to the precious alchemy of the Frick's luscious interior - not quite a historic home, not quite a museum, but something singular and better - and the understandable fear that any intervention would destroy its magic.

After the Frick's first plan was abandoned in 2015, a compromise of sorts was reached via a 2018 proposal by the architect Annabelle Selldorf. Her plan, which was approved by the Landmarks Preservation Commission, entailed a subtle piece of new construction, along with a slight extension to the library itself - 27,000 sq ft of new space.

Today, it's finally done, at a final cost of US$220 million ($297 million) and opened to the public on April 17. It is with no small amount of relief that, after a pre-opening tour, I can report that the Frick is better than ever before. There is more to explore and appreciate in the old building, while the new construction, which is lovely and elegant, never gets in the way.

To be clear, the plan was never to alter Henry Clay Frick's original 1914 mansion, home to a dazzling array of rare French furniture and heavy silk curtains, not to mention three Vermeers and dozens of other spectacular paintings. Instead, the goal was always to update the building's antiquated mechanicals, replace the leaking, clouded skylights, open up the second floor to visitors, and add better spaces and flow for exhibitions.

In the end, the garden stayed, although the music room did disappear, replaced by a larger temporary exhibition space whose first show will be devoted to Vermeer. New construction was limited to a two-storey addition between the old building and the library, along with an expansion of the library itself via a 20ft bump-out of its south facade into a space that had previously been occupied by mechanical equipment.

And as planned, the upstairs of the mansion is now triumphantly open: What had once been bedrooms, and then for decades offices, is now yet more exhibition space. (This was achieved by moving many of the offices into the two-storey addition built between the mansion and the library.)

By making these and other tweaks, Selldorf's interventions repurposed 60,000 sq ft of space. Beyer Blinder Belle Architects & Planners was the executive architect on the project.

Old made new

Visitors still enter via the same entrance hall as before. Like nearly every other piece of wood in the building, its panelled ceiling has been cleaned judiciously; the same goes for the limestone walls, which give the impression of being slightly newer and more refreshed.

The rest of the ground floor will be much as visitors remembered it, with the notable exception that the cabinet room, which once boasted boiserie - decorative wood panelling - to accompany the eight paintings by Boucher, has been turned into a temporary exhibition space presently devoted to displaying work on paper from the Frick's permanent collection.

That's because the decoration was originally designed for the upstairs sitting room of Frick's wife, Adelaide. With the reopening of the upstairs rooms as gallery spaces, the Bouchers, along with the boiserie and even the original parquet floor, were returned to their original position.

Moving through the Frick's ornate ground-floor rooms, it's hard to pinpoint what exactly has been cleaned, what's been replaced, and what's been untouched. All of it looks ever-so-slightly less shabby. New interventions are subtle; even most of the lighting is the same, although all of the chandeliers were cleaned, restored, rewired, and brought up to code.

Downstairs, the biggest change (and to be clear, you really have to look) is to the walls. When the renovation began, the Frick contacted Prelle, the factory in Lyon that had made the original silk velvet and silk damask wall coverings for the Gilded Age mansion. That factory had the original swatches and receipts, and so the old fabrics, which had been removed at least 40 years ago, were replaced.

The best example of the change is in the cavernous West Gallery, where the (not original) monochrome wall covering was replaced with an extraordinarily subtle variegated green.

But the most visible change is the absence of stanchions blocking the public from the upstairs rooms. Suddenly, visitors can simply walk down the hall, past two Vermeers and one of the world's best Bronzinos, and ascend the staircase to what was once the Frick family's private quarters.

For some, it might be a letdown. The upstairs is noticeably less grand, at least in comparison with the rest of the house. But the Boucher room is certainly not to be missed, as is Frick's own wood-panelled bedroom, where the collection has moved its spectacular painting of a woman in a blue dress by Ingres (Comtesse d'Haussonville), an image that often graces the covers of its catalogues and press material. That room also contains Romney's Lady Hamilton as 'Nature', which Frick once had mounted so that he could see it from his four-poster bed.

Thanks to this newly public wall space, the amount of the Frick's permanent collection on display has nearly doubled, to about 47% from 25%.

New showpiece

Like before, visitors will first turn right in the entrance hall and enter the reception hall, where they can visit the ticket desk. Cavernous, with walls clad in Indiana limestone and floors that use marble from the same quarries as the Frick's original building, the hall either leads back into the main house or down to the brand-new auditorium, which is located underneath the Frick's garden.

Anchoring the hall is the showpiece of the new construction: an exquisite staircase in Breccia Aurora Blue marble leading to all levels.

Should visitors ascend that staircase, they'll arrive in yet another vestibule. Although once again clad in a lavish amount of marble, this space feels less grand; it's a glorified hallway, albeit one flooded with light. It does serve an important purpose, namely as an entryway to the Frick's new store, along with a pass-through to the cafe. From that same second-floor vestibule, visitors can reenter the historic home through a passage.

Impressive as these spaces might be, the new auditorium, which seats 218, eclipses them all. Intimate without being claustrophobic, the space is extremely contemporary, with trim leather seats and a minimal, white proscenium, whose sinuous sculpted form is as much of a departure from the Frick's original aesthetic as you can get.

The new construction is everything one would expect from Selldorf, a noted perfectionist. Everything - from the light fixtures (which she designed) to the mural in the cafe (which opens in June) to the beautiful vitrines in the museum store - is well-made, beautifully installed, and, in its own way, luxuriously understated.

In that respect, despite their aesthetic differences, the old building and the new have that critical, unifying thing in common: Each bears the timeless sheen of no expense having been spared.

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