OVERVIEW | Increasing the real estate value of existing buildings to counterbalance an energy retrofit

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by Dimitra Papadaki (NKUA)


Retrofitting the existing building stock plays a key role in the reduction of energy consumption, fossil fuel consumption and CO2 emissions. Currently, in Europe about 35% of the buildings are over 50 years old; retrofitting this old building stock can decrease the energy consumption and CO2 emissions by about 5%.


Volumetric extensions to the roof and façades of a building, also termed "add-ons", are designed according to climatic conditions and programmatic needs. They can offer extra space or entirely new building constructions. When integrated with renewable energy systems, they are referred to as AdoRes. Add-ons can increase buildings' real estate value, which can therefore counterbalance investments for deep renovation. This idea is based on a multi-benefit approach including non-energy related factors, in order to accelerate the deep renovation of the existing building stock and enhance its energy efficiency.


This overview article presents some examples of this innovative concept, the opportunities and challenges associated with their use, and relevant EU-funded projects.


The 50 metre high Bois-le-Prêtre Tower in Paris was built in 1962. The retrofit project consists of a transformation of all 100 residences with prefabricated extensions added to each apartment. The existing exterior wall has been replaced by sliding glass doors, behind which a thermal curtain reduces heat loss, achieving a 50% reduction in heating demand. Brighter living spaces decrease the need for artificial lighting and increase comfort for residents. Multiple façade layers control light, energy and noise. The panels and winter gardens reduce noise and improve thermal insulation, enhancing the safety and comfort of residents.


The Bois-le-Prêtre Tower in Paris (Source : ,être-EN.pdf )


The additions led to an increase of the net living space by a depth of two metres, and adding a further one metre-wide balcony for each apartment. Within one month after the retrofit, the building gained extra tenants, attracted by the extra space and light. The overall surface of 8,900 m2 was increased to 12,460 m2 while the above-mentioned technical improvements reduce the energy consumption by more than 50%, at construction costs as low as 861 EUR/m2.


Another successful case study is the Grand Parc, Bordeaux project which also includes a rooftop extension. The retrofit led to the transformation of more than 500 apartments, with the addition of extra balconies and winter gardens, increasing the existing living space. These changes gave each apartment the opportunity to enjoy more natural light, more flexibility of use and better views. The initial size of the building was 44,210 m² and the extensions added an extra space of 23,500 m². This approach decreased the demand for lighting, while it increased the human comfort.


The Grand Parc, Bordeaux project (Source: © photo: Philippe Ruault )


Similar examples can be found in Germany, Spain, the Netherlands and Sweden, some of which are presented in the photos below.


Case studies of the use of Roof top Add-ons in Barcelona (Source (left):, Source (right):


Rooftop additional volume in a building in Sweden (Source:


The use of Add-ons with deep renovation of existing buildings offers several opportunities, including increased real estate value due to the volumetric addition of the property.  It can also increase occupancy rates as well as aesthetic value. With this approach, it can be possible to renovate without disturbing the inhabitants, if they can remain in their apartments during renovation. Extra spaces created from these Add-ons, can then be offered as extra living space provided as bonus to the inhabitants that contribute themselves to the investments.


Nonetheless, there are several challenges with the use of Add-ons. From a regulatory aspect, building and land use regulations may restrict the possibilities, for example by setting limits on possible volumes, heights, or façade lines. Barriers may result from property rights, condominium law, as well as local and urban planning rules. There is also a risk of exceeding structural limits, especially in seismic areas. Nonetheless, in order to overcome this problem, additions can be conceived with support structures which offer the possibility to use AdoREs also for the purpose of improving the mechanical, static and seismic performance of existing structures. This is explored in the EU-funded Pro-GET-OnE project, which focuses on the distinctive integration between energy deep renovation techniques and seismic retrofitting actions with the creation of extra living space for the occupants. Other EU Projects supporting the use of Add-ons include ABRACADABRA, a Horizon 2020 project which supports the roll-out of building extensions and building Add-ons, possibly integrated with the use of renewable energy systems.



Plans of the existing building (left) and the volumetric addition (on the right) (Source:


Architectural plans before and after the volumetric addition. (Source: )


At the end of 2016 the European Commission proposed a revised Energy Performance of Buildings Directive (EPBD) as part of its Clean Energy package of legislative proposals. One objective of the proposal is to accelerate the cost-effective renovation of existing buildings. An innovative approach to counterbalance the cost of deep renovation could be the use of volumetric extensions to existing buildings. The use of Add-ons for building renovation offers potential economic gains via increased real estate values, and improved quality of life for residents. The examples provided prove that if a property is renovated in such a way it improves the attractiveness of a building otherwise deemed to be both financially and structurally unsustainable. As regards the political and legislative obstacles, work needs to be done to include local authorities to cooperate in a flexible manner in these projects convincing them to change possible bureaucratic obstacles.