A summer residence designed by En Route Architecture for a single person, Diagonal House is situated within a secluded natural area of the Greek Cycladic Island of Kea. The strong topographical changes of the surrounding landscape represent, at the same time, the conceptual source and guideline for the architectural idea.

The house—placed diagonal to the topography slope—is organised as a subdivided square with alternating enclosed and open spaces. Windows and structural elements create cinematic frames of the surrounding landscape, the Aegean Sea and neighbouring islands. Due to the steep topography, the backside of the house is submerged within the landscape, while its front is oriented towards the main view. The home is therefore protected from harsh climatic conditions, such as high temperatures and strong winds. All house support functions, such as the bathrooms and kitchen, are placed towards the backside and transitioning corridor. Through this free-flowing layout, the inhabitant can enjoy multiple views within the house and towards the landscape.

Exposed concrete as a structural and visual frame is intended as a contemporary interpretation of a traditional stone house. This reinterpretation of conventional building intelligence is also related to the integration of passive techniques for heating, cooling, and storing water. Such techniques thereby allow for a higher degree of energy efficiency and independence. Small windows oppose large openings with operable shutters that are located in the alternating courtyards and rooms. This juxtaposition does not require air-conditioning but allows for cross ventilation from the prevailing winds through the enclosed and open spaces. Thereby keeping the house cool throughout the year and warm during the colder periods. The house's roof is used to collect rainwater in submerged tanks and re-used in the house as filtered water.

Diagonal House is part of an architectural trilogy consisting of Parallel, Diagonal, and Perpendicular houses. This trilogy is based on a study on how summer residences with the minimum necessary spaces can relate to their immediate context simply and beneficially. The study, therefore, represents an iteration of three houses that each have an optimal yet different relationship to their specific topography and landscape. The Diagonal House and Parallel House have been successfully completed, while the Perpendicular House is currently preparing for construction.

Similar to the design of its neighbouring houses, Diagonal House was conceived and constructed as a solid yet minimal contrast to its idyllic surrounding landscape.