The Tour Montparnasse is one of the best-known buildings in the French capital. Built in 1973 and standing 209 metres high, the tower in the 15th arrondissement is hardly one of Paris’s most beautiful structures. Quite the opposite, in fact – many detractors regard it as an architectural eyesore. That is set to change, however. In December 2021, the relevant administrative tribunal gave the green light for a long-planned complete refurbishment of the property. Based on plans by architects Nouvelle AOM, the Tour Montparnasse will get a more appealing façade and also be vertically extended by 20 metres, resulting in a new height of 229 metres.

This makes the Tour Montparnasse a prominent example of a growing type of construction project: adding extra storeys to existing buildings. There are multiple advantages, as highlighted by the Technical University of Darmstadt (TU Darmstadt) and the Hanover-based Pestel Institute in their Germany-wide study (“Deutschlandstudie”) published in 2019. Vertical extension not only increases the usable floor space of a building and therefore its cost effectiveness, it also has a positive impact from an ecological perspective. “The supply of new sites is finite and increased development cannot be reconciled with our climate targets,” explains Karsten Tichelmann, a professor in the Department of Architecture at TU Darmstadt. “So why not promote and support the creation of new living space that avoids concreting over more land?”

Vertical extensions for attractive residential and commercial space

The study by TU Darmstadt and the Pestel Institute clearly demonstrates how huge the potential is for adding extra storeys, particularly when it comes to residential use. Adopting this approach would allow the creation of between 1.1 and 1.5 million additional apartments in Germany on top of residential buildings dating from the 1950s to the 1990s. According to the authors of the study, a further 560,000 residential units could be built by extending administrative and office buildings upwards. “Adding extra storeys to existing buildings can make a crucial contribution to solving the housing shortage, especially in urban areas,” notes the Federal Association of German Housing and Real Estate Enterprise Registered Associations (Bundesverband deutscher Wohnungs- und Immobilienunternehmen – GdW), which has launched a “hundred thousand roofs” initiative in conjunction with trade bodies in the building materials industry. “Vertical extension of residential buildings is absolutely a sensible solution,” confirms Monika Fontaine-Kretschmer, a director of Nassauische Heimstätte | Wohnstadt (NHW), a group of companies which owns close to 60,000 apartments in the German federal state of Hesse. Having said that, it is important to carry out a case-by-case assessment as to whether adding extra storeys is economically viable and technically feasible. 

The 1950s Fritz-Kissel-Siedlung estate in Frankfurt passed this test, allowing NHW to create 82 additional one- to four-room apartments by vertically extending 14 apartment buildings. According to Fontaine-Kretschmer, adding extra storeys was made considerably easier due to the strictly rectangular shape of the buildings. Another advantage was the generous distance between the individual rows of buildings, which means the extra storeys don’t cast a shadow on the neighbouring apartments. Prefabricated timber modules were chosen for the extensions to avoid impacting structural integrity. None the less, the foundations had to be reinforced somewhat in places. Structural strength is one of the crunch points that must always be considered when exploring the feasibility of vertical extension. “Critical issues typically include fire safety and escape routes, alongside the structural strength of the existing building,” says Simon Dietzfelbinger, a partner at consultancy firm Drees & Sommer. In his opinion, there is a lot to be said for working with wood when adding extra storeys. This material has the advantage of being lighter than steel and also offers a comparatively good level of thermal insulation. “Timber structures can also be prefabricated easily, which saves time and money,” adds Dietzfelbinger.

Critical issues typically include fire safety and escape routes, alongside the structural strength of the existing building.

Building owners and planners must address critical points

According to the Drees & Sommer expert, it’s also necessary to examine the legal requirements, especially the issue of whether planning law permits the height of a building to be raised. In the case of heritage-listed buildings, another question is whether major changes to the property are allowed. Special status doesn’t need to be a barrier, though, as demonstrated by Union Investment in the case of the former Unilever headquarters in Hamburg, which is a heritage-listed building. Constructed between 1961 and 1964, what is now the Emporio building grew by an additional two storeys as part of an extensive refurbishment, bringing the total to 24 storeys. “Vertical extension enabled us to generate additional space and thereby increase the value of the property,” says Cyril Hübner, Senior Project Manager at Union Investment Real Estate GmbH, who oversaw the refurbishment and add-on project through to completion in 2011.

Vertical extensions are not always obvious

Hübner recalls that the standpoint taken by the then director of Hamburg’s City Building Department proved helpful in discussions with the heritage conservation authorities, pointing out that numerous buildings in Hamburg had been vertically extended so the additional storeys added to Emporio would not affect the urban panorama. Visually, the new levels are indistinguishable from the storeys below. 

Structural strength was likewise a key consideration at Emporio. “The structural survey showed that the foundations were able to bear the additional storeys,” says Hübner. “That was important for the project because work on the foundations would have been a major task.” As it was, all that had to be reinforced were the vertical steel supports on the top floor of the existing building. Fire safety requirements posed no problem because the existing three escape routes were deemed adequate for the additional space.

Rooftop developments can significantly enhance a building’s architecture

Emporio and the Tour Montparnasse are not the only examples of vertical extension as a viable option for office properties and residential buildings alike. In Berlin, real estate group Signa is currently refurbishing the Schicklerhaus building in the city’s historic heart and adding a further three storeys using lightweight construction techniques. There have been a few surprises along the way, reports project manager Georg Kölbl. Those included supports that turned out to lack the necessary load-bearing capacity and beams that were too short. Many other properties are also suitable for vertical extension, such as multi-storey car parks, as shown by another project in Berlin. In the Lichtenberg district, a niu-branded hotel was created above the multi-storey car park of the ECE-operated Ring-Center 2 shopping centre. HQ Real Estate used timber modules to build on the roof of the car park.

High-rise bunker putting on a growth spurt in the heart of Hamburg

One particularly spectacular project is located in the Hamburg district of St. Pauli, where a company owned by investor Thomas Matzen is remodelling an anti-aircraft bunker dating from 1942. Five additional storeys are being added in a pyramid shape to accommodate a hotel and restaurant as well as a sports and events venue. 

The load-bearing structure for the vertical extension largely consists of reinforced concrete, with the bunker’s immense strength proving to be an advantage. The added load is transmitted to the floor slab via the walls, which are up to two and a half metres thick. 

Such vertical extensions are not cheap, however. Especially if structural reinforcement is required, the costs can be significant, according to Simon Dietzfelbinger of Drees & Sommer. Other factors that can stymie a vertical extension include fire safety requirements proving to be too complex, or inadequate gaps to neighbouring buildings.

German housing association organisation GdW is therefore calling for greater flexibility with regard to the minimum required distance between buildings, the provision of parking spaces and fire safety. This will help ensure that “building owners are encouraged to create living space through the addition of extra storeys and rooftop development.”

By Christian Hunziker