Boulogne Dassault — Métamorphoses

In 2019, the year of the competition, the offices on Rue Dassault had already been vacant for several years.

They are typical late-1970s structures—an architecture strictly driven by maximum envelope volume (gabarit), later criticized for its enclosed, impenetrable feel, but highly appealing at the time. Across France, there is a vast amount of recent heritage just like this that is calling out to be renewed. Today, the challenge is finding beauty in it once again and turning it into a playground for experimentation.

Cardif (BNP Paribas Group), the owner, chose to invest and transform the building into housing.

At the time, converting offices into residential units was still a relatively recent phenomenon. However, given the density of the Paris region’s urban grid, rising material costs, and environmental challenges, it was only the beginning of a long-term trend. Today, it has become a major discipline in its own right—and a blueprint for the future. By adapting past architecture to make it resilient, we learn and anticipate the architecture of tomorrow. Transforming an existing building presents unique challenges that make it far more complex than new construction. It is a Rubik’s Cube balancing the project’s program against the constraints of the existing structure.

The goal is to repurpose a building that was never designed for residential use, bring everything up to current standards, deliver high-quality finishes, and forge a new identity—all while minimizing structural interventions as much as possible.

It is about integrating into the existing fabric without erasing it.

In the case of Rue Dassault, the rehabilitation was particularly heavy. It required an immense effort from our teams, far beyond the initial scope of the competition.

First and foremost, this type of project demands dedicated preparation time to truly understand the building and its structural framework before deciding what to remove or preserve. This painstaking exploratory phase took weeks of meticulous work. The change of use also makes technical execution far more demanding: where do you run utilities and HVAC systems without compromising the structure or volumetric integrity? How do you insulate the existing shell while opening it up to the outdoors? It requires diving much deeper than usual into the needs of various technical engineering consultancies (BET), stepping into the engineers’ shoes, and working with utmost precision.

Finally, technical constraints inevitably lead to adjustments. The final outcome cannot match the exact standards of a brand-new build across every detail, nor is it necessarily more economical. Behind the scenes, this calls for a significant educational effort with the client.

“A major challenge of the project was adding floor area, but the local urban plan (PLU) restricts densification in the area. There is, however, a legislative tool to solve this equation: the ELAN law. Article 9 allows for a derogation from the PLU of up to 30% of the existing building’s total volume. Qualifying for this framework requires meeting several conditions. One of them is justifying the creation of new floor area. Here, the case was straightforward: creating housing in an area facing a severe rental shortage. Thirty percent of these units must be social housing—a threshold we successfully met. Another condition requires harmonious integration into the urban context—a somewhat vague legal concept. We addressed this by blending into the diversity of the surrounding urban fabric, which mixes multi-family housing and single-family homes, and by realigning the facade with the street using balconies.

In parallel, we had to negotiate with the city. From the competition stage through to several months after the award notification, we spent extensive time exchanging ideas and working alongside legal counsel to ensure the project was legally viable. It was a long journey, but there was genuine political will from the municipality to bring the project to fruition. To unlock the additional square meters, we designed a step-by-step modular plan (projet à tiroirs) that opened up multiple possibilities. We had to embrace new paradigms that had not necessarily been accepted until now. The only footprint added to open ground was for the townhouses. For the rest, we built upon what was already there. On the east side, the existing structure was too restrictive for housing, so we planned an extension—this section will accommodate the social housing units. On the roof, another set-back extension on either side of the technical equipment zone allows for spacious duplexes while staying within PLU limits. Finally, converting offices into residential units reduced overall parking needs. This allowed us to reclaim part of the first basement level (R-1) and incorporate it into garden-level split-level apartments (souplex). Thanks to openings onto sunken gardens, these homes capture a distinct townhouse feel.