Rei Mitsui Architects Sinks a Weekend Home into the Forest Floor in Karuizawa

Most mountain retreats share a reflex: get as high as possible, cantilever over the slope, and frame the view. Rei Mitsui Architects went the other direction with Tull Weekend Home, a 101 m² retreat in Karuizawa that hunkers down among birch trunks and conifer roots rather than rising above them. The two steeply pitched volumes sit at an angle to each other, their rooflines tracing the kind of steep pitch you associate with sheltering, not spectacle. The result is a house that feels less like a lookout and more like a burrow.

What makes the project genuinely interesting is its conviction that proximity to the ground is a richer spatial experience than elevation above it. The sunken living area, the low sills framing undergrowth rather than skylines, the pale green alcoves that compress and release: these are decisions that reject the scenic postcard in favor of something more tactile and intimate. Rei Mitsui cites ancient structures as inspiration, and you can feel it in the way the building meets the earth, not perched on it but pressed into it.

Two Wings in the Trees

From outside, the house reads as two angled timber volumes that have settled among the trees like a pair of folded hands. The steeply pitched rooflines are clad in the same pale wood as the walls, making the whole form register as a single material mass. In fog or snow, the pitched silhouettes dissolve into the canopy. The geometry is deliberate: the angle between the wings creates a sheltered threshold while allowing each volume its own orientation toward different groves of birch and conifer.

The forest does most of the contextual work. Birch trunks stand close enough to the facade to cast dappled shadows directly onto the glazing, and the gravel apron around the foundation blurs any hard line between building and terrain. There is no manicured garden, no cleared approach. You arrive through the woods, and the house is simply there.

The Sunken Heart

The central living area sits below grade, its concrete floor dropped just enough to change your relationship to the landscape outside. Windows at this level frame the base of tree trunks and the mossy forest floor rather than the middle canopy. It is a subtle move with a powerful effect: you feel grounded, literally enclosed by the terrain. Exposed timber rafters rise steeply overhead, compensating for the floor's descent with generous vertical space.

The double-height volume is the hinge between the two wings. Standing in it, you can look up toward the skylights or out through full-height glazing into the trees. A dog wanders the concrete floor, a person reads at the table, and the scale feels exactly right for a weekend retreat: large enough to breathe, small enough to be warmed by a single fireplace and a stack of birch logs.

Sage Green and Plywood

Inside, the palette is reduced to two main materials: pale plywood ceilings that follow the rake of the roof, and mint or sage green walls that define the kitchen and dining alcoves. The green is a confident choice. It could easily tip into kitsch, but Mitsui uses it sparingly and pairs it with arched openings that give each alcove the feeling of a small chapel or a cave mouth. The curves soften what could be an aggressively angular interior.

Integrated cabinetry in the same green tone keeps the kitchen walls clean. Angled skylights slice light across these surfaces, producing the kind of shifting stripe patterns that make you aware of the time of day without looking at a clock. The material restraint matters: with only two dominant tones, every shift in light registers clearly.

Framing the Forest at Every Scale

The fenestration strategy is one of the project's quiet strengths. Stacked windows in the kitchen frame birch trunks like vertical paintings. A deep timber-lined window seat turns a corner opening into a reading nook where the sitter is nearly flush with the tree line outside. A low corner window in the bedroom captures gravel and roots. Each opening is sized and positioned to deliver a specific fragment of the forest rather than a sweeping panorama.

The clerestory windows and skylights serve a second role beyond light: they pull the canopy into the interior at unexpected angles. Looking up through a narrow skylight to see green leaves overhead reconnects you to the vertical dimension of the forest, which ground-level glazing alone cannot achieve.

Light as Material

Several of the most compelling moments in this house are produced not by structure or finish but by light falling across simple surfaces. Slanted sunlight through the skylights paints diagonal stripes on the sage walls and the concrete firewood bench below. In the bedroom wing, a low window catches morning light that barely clears the undergrowth. These effects are seasonal and time-dependent, which is precisely the point for a weekend retreat: each visit reads differently.

Thresholds and Transitions

A large pivoting glass door opens the plywood-lined interior directly onto a timber-framed porch, collapsing the boundary between inside and forest in a single gesture. A white sofa sits in dappled light that could be interior or exterior. The staircase, open and lined with glass railings, threads between the two levels with the same transparency, letting you see across and through the house as you move vertically.

The arched openings between rooms function as another kind of threshold. They are not doors but soft transitions, each one framing the next room as a picture. Looking through a green arch to the dining table, through the glazing beyond, and into the trees, you pass through three layers of enclosure in a single glance. It is a spatial trick that makes 101 square meters feel much larger than its footprint.

Winter and the Deck

Karuizawa winters are serious, and the house transforms under snow. The timber deck, railed with simple timber frames, becomes a white platform looking into bare branches. The laminated timber roof supports and cylindrical columns that look almost decorative in summer reveal their structural logic when loaded with snow. Inside, the bedroom with its sloped plywood ceiling, corner window, and white bedding becomes a cocoon, a scale and warmth calibrated for the cold months.

Plans and Drawings

The site plan confirms the relationship between the two angled wings and the surrounding tree canopy. The lower level plan shows the wings flanking a central curved living room, the geometry driven as much by the existing trees as by any abstract composition. The upper level distributes bedrooms to each wing, each with an exterior deck that engages a different aspect of the forest.

The sections are revealing. The stepped foundation follows the sloping terrain, confirming that the sunken living area is not an excavation but an embrace of the existing grade. The sloped roof pitches vary between the dining wing and the master bedroom wing, producing different ceiling heights and spatial characters in each. Exposed rafters and the double-height volume over the bedroom are clearly legible in section.

The exploded axonometric separates the staircase, roof planes, and structural components to show how the cylindrical columns carry the steep roof independently of the wall planes. It is a useful drawing because it clarifies what the photographs suggest but never fully explain: the roof is the primary architectural gesture, and everything beneath it is organized in service of its steep, sheltering pitch.

Why This Project Matters

Weekend homes in mountain forests tend toward two clichés: the glass pavilion floating above the slope, or the rustic cabin thick with nostalgia. Tull avoids both. By sinking into the ground, pitching its roof steeply, and framing the forest floor rather than the horizon, it offers a third model: the retreat as inhabited landscape. The house does not observe nature from a safe distance. It participates in it, letting tree shadows fall across its walls, letting the grade determine its section, letting fog erase its outline.

Rei Mitsui's claim that the design draws on ancient structures is not mere rhetoric. There is something genuinely archaic about a building that prioritizes shelter and closeness to the earth over transparency and view. In a residential market saturated with look-at-me weekend retreats, Tull is a quiet argument that the most compelling relationship between architecture and landscape is not visual dominance but physical intimacy.

Tull Weekend Home by Rei Mitsui Architects (lead architect: Rei Mitsui), Karuizawa, Japan. 101 m², completed 2024. Photography by Yoichi Onoda and Jérémie Souteyrat.

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