Holzbox House / HGA Henning Grahn Architektur

Architects: Henning Grahn Architektur
Area: 165 m²
Year: 2022
Photography: David Schreyer
Structure Engineers: Thomas Bassier Baustatik
Design: Henning Grahn
Materials: Wood frame construction, solid wood ceilings, treated MDF panels, larch wood windows
City: Mainz
Country: Germany

Holzbox House is a residential renovation and extension project in Mainz, Germany, designed by Henning Grahn Architektur. The intervention reimagines a small family house with a pitched roof by introducing a contemporary timber volume that expands the living space for a young family. The new addition is elevated in coordination with the raised floor level of the existing structure, improving internal accessibility while addressing flood risks posed by a nearby brook. Constructed using a sustainable system of timber framing and solid wood ceilings, the extension is wrapped in a distinctive facade of specially treated MDF panels with concealed fixings. The project carefully integrates new and existing elements through material continuity, particularly with larch wood windows, while creating a dynamic interior sequence culminating in a double-height gallery space. Holzbox demonstrates a thoughtful balance between preservation and transformation, combining spatial innovation with environmental responsiveness.

We welcome the current transformation in the field of architecture, including a renewed appreciation for existing structures and the use of single-material construction methods.
Interview with Henning Grahn of Henning Grahn Architektur

The Holzbox House project approaches the renovation of a modest pitched-roof house not as a simple extension but as a spatial and environmental recalibration. By elevating the original floor level, the architects establish a new datum that unifies the existing structure and the added volume, resolving both circulation challenges and the site’s vulnerability to periodic flooding from the adjacent brook. This intervention transforms a constraint into a defining architectural move, positioning the extension as both a protective and connective element.

Externally, the composition negotiates the relationship between continuity and contrast. The newly introduced timber volume aligns with the geometry of the original roof while subtly overlapping the rear facade, creating a layered condition that blurs the boundary between addition and inheritance. This dialogue between old and new becomes more pronounced inside, where spatial intersections replace formal distinctions. The interior unfolds as a calibrated sequence, moving from lower, more intimate spaces to increasingly generous volumes, culminating in a double-height living area that acts as the spatial and social core of the house. The gallery above extends this experience vertically, linking the new living space with the circulation of the existing upper floor.

The facade strategy reinforces the project’s conceptual clarity. The extension appears as a solid, almost monolithic box punctuated by a single window per elevation. Yet these openings reveal a more complex organization, each comprising paired windows that serve multiple interior spaces while framing varied views and enhancing daylight penetration. This controlled ambiguity contributes to the building’s refined exterior expression.

Material choices further consolidate the integration of old and new. The use of larch wood for both the replacement windows in the original house and the flush-mounted openings in the extension establishes a visual continuity, while the experimental MDF facade panels lend the new volume a distinct yet restrained identity. The construction system, based on timber frames and solid wood ceilings, underscores a commitment to sustainability without compromising architectural precision.

The project concludes in the landscape, where the contrast between front and rear conditions is carefully articulated. Entry is marked by solid steps at the street-facing side, while the more private garden opens onto a sequence of new steps descending toward the brook, shaded by a walnut tree. This duality reflects the overall approach of the project, which balances clarity and subtlety, pragmatism and spatial richness, in the transformation of a modest domestic structure.