When Powerhouse Company first moved into rented office space at 399 Westzeedijk as part of a live-in guardian scheme, the building was neglected and unloved. But the 1965 design by J. W. C. Boks clearly had huge potential, and Powerhouse Company founders Nanne de Ru and Nolly Vos set their hearts on transforming it – with an eye to living at the top of the building, with its wonderful views of the Rotterdam skyline. Although West 399 started life as a high-quality Modernist building, it was much altered in the 1980s and 1990s. The renovation began with stripping away these additions to reveal the original beauty of the interior, complete with lofty high ceilings, round columns and mahogany-framed strip windows. The exceptionally rich palette of materials was restored to its former glory: warm brickwork outside, with slate, travertine and solid hardwood inside. Rich materials and high-quality detailing similarly define the glass-box penthouse structure on the roof, a new addition that develops the European Modernist idiom of the building with a thoughtful contemporary interpretation of its transatlantic incarnation. The 450m2 penthouse apartment now comprises a one-layer new-build perching on the roof and surrounded by a roof garden, and a portion of the existing building below that roots it in the original structure.