The last of the five luxury units in the Richelieu Flats sold out last week. Two 2,650-square-foot full-floor spreads on the third and fifth floors of the 7-story masonry building, at 318 S Michigan Avenue, were the last to go after 18 months on the market.

The pricier half the pair went for $1,430,000, which was $65,000 above the asking price. Sure, that sounds astonishing. But Millie Rosenbloom, the Baird & Warner agent for the whole building, says, "90 percent of the time when you think you’re looking at a full list price for a new-build or developer conversion, you’re not seeing costs like parking and custom finishes.”

When I snooped around the building last September, model unit 300 was the only space with finishes, so buyers of the other four spaces got the opportunity to customize the look, which often runs up the closing price. They “sat down with the developer to discuss options for cabinetry,” says Rosenbloom, part of what made the process “intimate and emotional…unlike the normal way of doing things.”

Richelieu’s crown jewel, a duplex penthouse with 4,300 square feet of interior space, extra-high ceilings, and a 1,000-square-foot Grant Park-facing roof deck, was such a compelling piece of real estate that it sold completely raw for $2,300,000 in October 2013, five months after listing.

This process all started in March 2013, when Chicago-based LG Development Group purchased of the vacant structure from Louis D’Angelo's family firm, Metropolitan, which held it since the 1970s. It was known as the the Karpen furniture building at the time, but, more than a century before, it was the Richeleu Hotel—"the famous hotel, endeared to epicures the world over," as it was described in 1895. It seems fitting that some form of residential use would return at this address, although credit must be given to S. Karpen & Bros for updating the exterior with its sculpted, silky terra cotta.

Working from the wide-open floor plans of a defunct furniture warehouse and showroom made it easy for the developer, also practiced in interior design, to minimize the number of walls in each layout. The model unit presents an open central space that merges living room, dining room, and a marble-clad kitchen. Two bedrooms attach to the common area and a third is down a short hallway, paired with a family room and bathroom. All five units—penthouse included—are drawn up with three-and-a-half bathrooms.

Now that these units are out of the equation, are there other Loop prospects for well-healed buyers hoping for a small-scale, custom find? Nothing compares to the Richelieu project, says Rosenbloom, but nearby is the more formidable but still boutique-y 888 S Michigan, from titans of the early skyscraper Holabird & Roche. Most of the slighter structures of Richelieu’s generation—the early 1880s—have vanished from Michigan Avenue and the Loop’s other desirable drags.