© Project Kastanienallee am Teich GmbH & Co. KG

BLACKPRINT: Mr. Müller what exactly is being planned at “Kastanienallee am Teich”? What uses and building-structures does the project envisage?

Thomas Müller: The plan is for an ecologically oriented residential quarter comprising 47 houses in total. This development includes 20 terraced houses, 22 semi-detached houses and five detached single-family homes. The detached homes and semi-detached houses together comprise a total of 27 “in-law” flats that may, depending on residents’ life situations, serve as independent housing units or augment the main living space. All houses are arranged around a lake and are accompanied by generous new planting. Thanks to their loose arrangement, most of the houses enjoy a direct view of the water. The continuous row of terraced houses to the west also serves as passive sound insulation from a beer garden adjacent on the western side.

Thanks to the loose arrangement, the majority of the houses receive a direct view of the water.

BLACKPRINT: The planning of the development stems from the experienced Darmstadt-based architecture firm planquadrat Architekten, which places special emphasis on a modern, environmentally conscious construction approach…

Thomas Müller: Yes, the residential buildings are designed as timber-hybrid structures, featuring two full storeys plus a recessed (set-back) top storey. The extensively greened roofs not only improve the micro-climate but also serve for rainwater retention. In addition to the use of ecological materials, sustainable energy supply forms an integral part of the plan: together with researchers from Technische Universität Darmstadt we developed a regenerative energy concept that uses the lake-water as an energy source. Furthermore, photovoltaic systems on the roofs are planned for electricity supply.

Fully sustainable: The roofs of the timber-hybrid buildings are planted with extensive greenery and equipped with photovoltaic systems for generating renewable energy. The lake water is incorporated as an energy source

Historical aerial view of the site. Until the 1960s, a brickyard stood on the premises.

BLACKPRINT: What urban-planning or landscape-architectural principles guided the design?

Thomas Müller: With our project, we are closing the gap between an existing residential area on one side and the beer garden on the other. Until the 1960s, a brickyard stood on this site; later, a private owner lived here alone for more than 30 years on the roughly 20,000 square-metre property. The area was completely enclosed by a two-metre-high fence and had therefore been cut off from its surroundings for decades. With our concept, we aim to carefully open up this secluded area and reintegrate it into the existing green landscape. To that end, we have planned additional plantings with freely growing, site-appropriate hedges. Around 200 new trees will be added, while roughly a dozen characteristic old trees will be preserved.

For access to the houses, we have designed two private residential paths on the side facing away from the lake. The quarter itself will be walkable for the public – a path between the beer garden and the row houses will be designated with a public right of way. For safety reasons, however, the lake itself will not be accessible to the general public.

BLACKPRINT: What kind of energy supply have you planned for the quarter?

Thomas Müller: For this project, we intend to use the lake water as a heat source for all houses – implementing a concept that is, so far, unique in Germany. Through a pump line, the water is directed via a heat exchanger to a central technical building, where it releases part of its thermal energy. This energy is then distributed to the residential buildings via a cold local heating network, with heat pumps in each unit providing decentralised heating.

The technical components themselves are nothing new – heat exchangers, heat pumps, and geothermal systems already exist. What’s new, however, is the consistent combination of all these elements, which we developed together with the Department of Hydraulic Engineering at Technische Universität Darmstadt. To ensure supply security, the system is supplemented by a geothermal collector and, at peak loads, by an air-brine heat exchanger.

Decentralised buffer storage units ensure that each dwelling is reliably supplied with heat for space heating and hot water. In addition, photovoltaic systems on the green roofs generate electricity for residents’ own use. Together, these measures form a carefully designed, nearly CO₂-neutral energy concept – a true lighthouse project for Hesse and the City of Science, Darmstadt.

BLACKPRINT: You acquired the site in 2015 and have faced a number of challenges since then. What happened?

Thomas Müller: We are not a traditional project developer – we’ve always focused on unconventional tasks, on projects that others either didn’t want or couldn’t handle. Together with my partner Claus Wisser, we took over the property in 2015, which already had a history of its own. From the very beginning, it was clear to us that we didn’t want to pursue dense infill development, but rather a balanced residential quarter of single-family homes that integrates harmoniously into the urban fabric.

To this end, we commissioned external specialists and expert reports – covering noise, soil, water, climate and traffic. In total, nine expert assessments were produced. A key issue was noise protection in relation to the neighbouring beer garden: the houses were planned so that all living spaces face the lake, while ancillary rooms such as kitchens or hallways face the beer garden. This approach allowed us to achieve passive sound insulation without constructing a high noise barrier.

All of this is reflected in the dedicated, project-specific development plan. Along the way, we also had to counter some rather absurd accusations – for example, that the quarter would supposedly block the flow of fresh air to the adjacent residential area. Climate studies clearly disproved that. Darmstadt has only a few remaining sites suitable for new housing, which made it all the more important for us to develop a concept that treats the location sensitively while still creating urgently needed living space.

The lake will also continue to form the centrepiece of the site.

BLACKPRINT: What is the current status of the project?

Thomas Müller: We now have a legally binding development plan, approved by the city council at the end of 2022. For the first construction phase – the terraced houses and the technical centre – a building permit was granted at the end of 2023. We are currently preparing the implementation planning and intend to start sales and construction of the terraced houses either at the end of this year or early next year. At the same time, we are applying for building permits for the subsequent phases, which include the semi-detached and detached houses.

BLACKPRINT: For “Kastanienallee am Teich”, an environmental report including a life-cycle assessment was prepared early on. What were the key findings?

Thomas Müller: First of all, it’s important to note that we made full use of all landscaping measures available to us in order to minimize the ecological impact of the development as much as possible in terms of the biotope value balance. The environmental report accordingly concluded that a high-quality, richly greened residential quarter will emerge, grouped around the existing lake. It also states: “The former clay extraction area, which was later used for private residential purposes by only a few individuals, will thus be opened up to broader residential use – a positive result in terms of land consumption (protected resource: land).”

Despite all efforts, there remained a deficit of around 43,000 biotope-value points, equivalent to a financial compensation of roughly €15,000 – hardly worth mentioning given the overall investment of more than €50 million. I still remember going to the head of urban planning with the report, feeling a bit disappointed. He just laughed and said other projects had deficits in the millions – ours was almost negligible by comparison.

BLACKPRINT: Many building projects compensate for ecological impact through financial payments. You deliberately chose tangible on-site compensation measures instead …

Thomas Müller: Yes, to offset the deficit we gladly responded to the request of the city’s parks department and are financially supporting the conversion of parts of the neighbouring public park into extensive wildflower meadows.

BLACKPRINT: Ecological measures often take decades to unfold their full impact. How do you ensure that their value is recognised and protected in a short-term-driven environment?

Thomas Müller: The project is based on a project-specific development plan that records all measures in detail. In addition, there is, for example, a usage charter that will be handed over to the new owners’ association, defining how the lake may be used.

Modern architecture meets an innovative energy concept: the lake’s water is designed to serve as the heat source for all houses.

BLACKPRINT: Would you consider your project a “lighthouse project” for sustainable construction?

Thomas Müller: The term was first used in connection with our energy concept – not by us, but by TU Darmstadt. Considering all the project details and the results of the life-cycle assessment, one can indeed call it an exceptional construction project.

BLACKPRINT: Projects like “Kastanienallee am Teich” require patience. What delays have you experienced over the years – and what were the main causes in your view?

Thomas Müller: There were numerous regulations and requirements to comply with. Coordinating the nine expert reports with the various authorities took a lot of time. Our innovative energy concept also caused delays at first, as responsibilities were unclear. But the greatest delays came from two particularly litigious neighbours acting out of self-interest. Although two lawsuits have already been clearly decided in our favour, they continue to file new ones – under different case names but always before the same court. A new ruling is hardly to be expected; rather, it seems the goal is to exploit the overload of the courts to delay proceedings. Despite this, we’re making good progress with the detailed planning. Our goal is for the first homeowners to move in by 2028/2029.

BLACKPRINT: Based on your experience – if you could address one wish to politics and administration: what should change so that sustainable projects like “Kastanienallee am Teich” no longer fail due to avoidable obstacles?

Thomas Müller: Politics needs majorities – and that often leads to opposition parties rejecting projects purely on principle. In our case, parts of the local opposition even refused personal talks but still voted on the project. Yet according to TU Darmstadt, this is a lighthouse project of supra-regional significance.

My first wish to politicians: engage with projects on a professional level and overcome your preconceptions. My second: support the administration. There are experts there who accompany such projects professionally. But when politicians sow doubt for fear of losing votes, it only encourages individual plaintiffs and divides the community. Ultimately, both politics and society need a greater willingness to take responsibility instead of reacting to every housing project with lawsuits and obstruction.

BLACKPRINT: And what is your wish for the administration?

Thomas Müller: I would urge the administration to approach new topics with more openness – and to apply regulations in a way that enables rather than hinders good projects. We need more courage in decision-making and greater trust in responsible developers. Processes must remain predictable and transparent. Ecological quality should never be rated lower than mere compliance with minimum standards. That’s the only way sustainable construction can truly succeed.

BLACKPRINT: Let’s hope your appeal is heard! But honestly – if you had known about all these difficulties in advance, would you still have taken on the project?

Thomas Müller: As crazy as it may sound – yes!

BLACKPRINT: Mr Müller, thank you very much for the open conversation.

The interview is conducted by Robert Uhde.