Greta Gerwig makes her solo writing-directing debut with “Lady Bird.” (Scott Gries/Invision/AP)

“Anybody who talks about California hedonism has never spent a Christmas in Sacramento.”

That’s the Joan Didion quote that precedes “Lady Bird,” Greta Gerwig’s solo writing-directing debut in which she chronicles a year in the life of a 17-year-old high school senior — the title character — who longs to escape her home town.

Didion, whom Gerwig calls her patron saint, looms large in “Lady Bird.” “That was the first time I read a writer who wrote about where I was from,” Gerwig recalled at the Middleburg Film Festival in Virginia in October, of discovering Didion as a teenager. “And she’s not just a writer, she’s a great writer. She’s one of the great writers of the 20th and 21st centuries. I just couldn’t believe it when I read it. Because I always thought writers had to be from New York. And this woman was writing about the exact place I was from and where she was from, and it was so meaningful.”

Didion’s 1961 Vogue article “Self-respect” exerts a particular pull on Gerwig’s movie, which she says is about “learning how to answer for yourself and what you are and what you do.” The movie more than deserves the comparison to one of the finest essays written in the English language, the struggles of its pimply teenage heroine every bit as thoughtfully limned as Didion’s cooler but equally measured appraisals of identity and character.

“Lady Bird” makes the case for reframing female stories as epics on a par with genres usually coded as male: Our heroine’s crucible might be a snug middle-class home and the sleepy streets of Sacramento, but her movie is just as big and canonical as a film about young men evacuating Dunkirk or the ponderous existential crisis of a blade runner in 2049 Los Angeles.

Patronizing assumptions having to do with gender and genre are already dogging “Lady Bird.” Because Gerwig set the film in her home town, and because it roughly tracks with her own chronology (it takes place during 2002-2003, just a few years after she graduated from high school), it’s being called “autobiographical,” a sneaky way of erasing the intentionality and superb craftsmanship behind a film that feels both spontaneous and utterly inevitable.

As Gerwig said during a question-and-answer session after the Middleburg screening: “You put such a tremendous amount of work in, such a tremendous amount of shaping and writing and working with these actors to invent these characters and make them whole people and the way it looks and the way it feels. . . . You worry when someone says, ‘Oh, that’s just your life.’ ”

Saoirse Ronan in “Lady Bird.” (Merie Wallace/A24)

The “that’s just your life” problem is akin to the obstacles female filmmakers encounter within an overwhelmingly white and male industry. Gerwig spent a decade amassing 350 pages of material for the film, which she carefully honed and refined while starring in and sometimes co-writing and co-directing such “mumblecore” classics as “Hannah Takes the Stairs” and “Baghead,” and later “Greenberg,” “Frances Ha” and “Mistress America.” She co-wrote the latter two films with Noah Baumbach, but while doing media events for the films she noticed that journalists asked her if she “helped” write them. “It was like, ‘Did you get to be there with him when he was writing?’ ” Gerwig recalled at a panel of female filmmakers at Middleburg, as if “this entire story comes from a 46-year-old man and it’s about girls who are best friends.”

Even during the rollout of “Lady Bird,” Gerwig said, the thinly veiled doubts about her expertise are staggering. During an interview for a cinematography magazine, her director of photography, Sam Levy, was asked if he made all the technical decisions and she “just worked with the actors.” There’s an assumption, she said, “that the guy would be the one to figure out the look of the film, and that I would just go along with whatever he did.”

Like the work of Claire Denis, Claudia Weill and Chantal Akerman — all of them important influences — “Lady Bird” and its maker are easily underestimated, the daily tasks and quotidian moments they contain minimized in comparison with the Heroic Acts of Great Men. Gerwig noted that Akerman’s “Jeanne Dielman,” about a woman going about her day cooking, cleaning and doing routine chores, constitutes “the lowest image on the cinematic totem pole . . . and she elevates it.”

The same could be said of Gerwig, who has made a movie that shouldn’t be called “small” because it’s about a young girl, or “just your life” when it’s the product of meticulous, assured decision-making about everything from structure and visual language to casting, camera placement, costumes and exquisitely calibrated emotional tone. The days of Greta Gerwig being underestimated as mere muse, helpmeet, mumblecore queen or erstwhile America’s sweetheart are as safely ensconced in the rearview as Sacramento itself. “Lady Bird” is the work of a filmmaker — a great filmmaker — saying goodbye to all that.