This year's 42nd annual Decorator Showcase — the annual event in which a bevy of the city's top designers each take a room in a whole-house makeover — debuts for invited guests tonight and for the public on Saturday. And SFist sat down with 27-year-old Scott McMahan of the newly named Scott Robert Design, selected this year as one of 43 designers tackling rooms at the grand 1904 Presidio Heights mansion known as Le Petit Trianon.

McMahan is the youngest designer ever selected to design a room in a Showcase house — he was 26 back when he learned the news last fall — and the proposal that was accepted was for a small hallway or anteroom on the house's lowest level. The room, formerly a forgotten servant's passage between the kitchen and the maid's quarters, has been transformed into an elegant space that McMahan is calling "Classically Modern."

Built back in the first Gilded Age, the 40+ room Petit Trianon at 3800 Washington Street is modeled on Marie Antoinette's "cottage" on the property of Versailles. And designers this year were tasked with creating a variety of spaces out this abundance of space, including several studies and sitting rooms, a third-story breakfast room, a flower-arranging room, and a "wo-man cave." Also on the partially subterranean level with McMahan's anteroom is a grand ballroom that's been reimagined by Applegate Tran Interiors as an elegant, art-filled nightclub space.

Many SF firms return multiple years to the Designer Showcase — it remains an excellent venue for showing off one's aesthetic and finding new clients — and each proposes two room designs that have to be laid out only as mood boards. This year's renovation of Le Petit Trianon (the second time it's been a Showcase home, the first time being in 1982) comes as the current owner is just listing it for $30 million and you can see many of the room designs here via that listing.

For McMahan, a former dancer with the Oakland Ballet, this was a first big break as he's just launching his design career — he works a day job currently at Zeterre Landscape Architecture, which was selected this year to design the rear garden of the house.

"I wanted to create a classical-feeling room at first glance that had a modern take with materials," McMahan tells SFist. He points to the oak and steel chevron floor, which formed much of McMahan's inspiration — custom, patina'd and textured steel panels by Iron Roots Welding are interspersed with gray-stained oak, and the pattern is repeated up the rear wall, framing an 18th Century Louis XVI gilt mirror from C. Mariani Antiques.

The study in contrasts also includes an imposing 18th Century oil painting, and a custom steel console table designed by McMahan and made by Iron Roots featuring rectangular inlays of glass. McMahan also wanted to recreate the moldings found everywhere else in the house, but instead of strictly traditional moldings, his are subtly inlayed with strips of steel.

McMahan's is also probably one of the greatest transformations of any of the rooms. "This room had ducts running through it, pipes everywhere. It was basically this mangled mess," McMahan says of the forgotten servant's passage. Ducts and pipes had to be re-routed and put up into the ceiling before McMahan's design could even take shape. The result is a balanced and welcoming space in which once can imagine party guests gathering for conversation on their way between getting snacks from the kitchen and returning to the ballroom. And the room now feels as if it's a part of the rest of the house.

None of this transformation came easily, though, over the last five months since McMahan's work began.

"It's absolutely, mind-bogglingly stressful," McMahan says. "The majority of the people who are in the Showcase have their own businesses and work for themselves every day, whereas I work for Zeterre 9 to 5 all week long, and had to email with contractors and manage this whole project from afar, mostly."

The public will get to walk through the reimagined Le Petit Trianon as Decorator Showcase starting on Saturday, and it will be open Tuesday to Sunday through May 27. Find tickets here ($40 for general admission).