Set into the foothills at the base of Mount Sanitas, 3555 4th Street is a study in organic modernism, where limestone, steel, & expanses of glass blur the line between home & landscape. Designed by Fanas Architecture, the home is a work of architectural integrity, realized at the highest level of craft. A smooth limestone spine wall threads through the home, with rough-hewn limestone as a counterpoint. In the great room, exposed steel I-beams frame 24-foot retractable walls of glass, opening to north & south-facing patios. European white oak with radiant heat runs underfoot, beneath a floating wood-plank ceiling washed in cove lighting. The chef's kitchen by EKD features custom walnut cabinetry, a hand-laid mosaic backsplash, Dornbracht fixtures, & Gaggenau appliances. Built-ins flow through the butler's pantry, mudroom, & laundry. A main-floor flex suite serves as primary, office, or guest, complete with walk-out patio, walnut walk-in closet, & Japanese-inspired bath in Vola fixtures. The primary suite opens to the second-story deck through wall-to-wall sliding glass. Behold a coved wood plank ceiling, pin lights, and sculptural pendants. The primary bath is a masterful blend, anchored by a freestanding MTI resin tub, with a steam shower, bespoke cabinetry, alabaster sconces, & a custom walk-in closet. A loft with a wet bar adjoins two ensuite bedrooms. Over 3,200 square feet of finished exterior living space spans three levels, redefining outdoor living in Boulder. The south terrace pairs an overhang & electronic louvres with a fire pit & water features. Steel stairs rise to the upper deck, unveiling a hot tub, cantilevered overhang, and Sanitas to the west. Above, a rooftop deck opens to panoramic views. Lutron lighting on Control4, radiant heat across nine zones, full solar array, & two EV-charging rough-ins. Steps from the Goat trail and tucked into one of Boulder's most iconic neighborhoods, 3555 4th Street is a triumph of detail, intention, & place.