Vienna-based Franz Architekten designed the extension for an 80-year-old family home in Wienerwald and covered it with grey larch boards, contrasting with the white rendered facade of the existing house.
Connected to the main building via a glazed corridor, the boxy two-storey extension provides more space to accommodate the children and grandparents of the family.
The partially submerged lower level provides a home for the grandmother, but could be converted for other uses in the future, while the upper level is used as a playroom for the children and could be subdivided to create four smaller rooms.
"The shape is caused by the desire to create four equivalent rooms, which specified the dimensions of the house," architect Erwin Stättner told Dezeen.
A rectangular section appears to have been cut from the facade to allow for sliding doors and windows at the front of the playroom.
This room is lined with spruce panels and features a child's swing suspended from the ceiling.
The level below has an open-plan layout, with kitchen worktops and cupboards running along the far wall, while a yellow-painted bathroom creates a box in the centre of the space and separates the living area from the bedroom beyond.
Clerestory windows bring extra light into the space from above, while the playroom overhead features a row of square skylights.
Photography is by Kurt Kuball.
Here's a project description from Heimspiel:
After, the young family were taken by the idea of a single-family house in the countryside. In the Wienerwald they found a little house in need of rehabilitation with a large garden. Prospective possibilities of enlargement were already investigated before buying the property. The lean budget required a high portion of own steps in renovating the old building and undertake thermal sanitation. Upon completion of sanitation, the second child had already been born, and thoughts regarding an annexe became more concrete. Also, the grandmother was taken into consideration.