BLACKPRINT: Mr. Stenzel, the Z8 is regarded as the first five-storey residential and commercial building in solid timber construction in Saxony. How did the decision for the project come about?

Dirk Stenzel: I have now been working as an independent architectin Leipzig for 27 years and founded the practice ASUNA – Atelier für strategische und nachhaltige Architektur [Studio for Strategic and Sustainable Architecture] – in 2015. The reason was that I had encountered many questions in the construction industry and often found no satisfactory answers. I wanted to focus more strongly on sustainable architecture: ecological building, social responsibility – all topics that are taken for granted today, but were hardly discussed back then. The Z8 was our first self-initiated project. We bought the site and formed a client group, with a mix of uses comprising 40% commercial and 60% residential. The first two floors are commercial spaces, with four flats above them, which are occupied by members of the group. From the outset, it was important to me to introduce timber construction, as larger timber buildings were almost unknown in Leipzig and Saxony at that time. In 2016, neither a timber construction directive nor established regulations existed, timber construction on this scale was virtually uncharted territory. Ultimately, the project helped to strengthen the topic here in the region and make larger solid timber buildings possible in the first place.

BLACKPRINT: So you developed the project together as a client group from the very beginning?

Dirk Stenzel: Yes, exactly. Once the initial idea was established, we gradually sought collaborators who were willing to get involved. Everyone contributed their ideas and requirementsfor the flats or commercial spaces, together we discussed floor plans, choice of materials, façade design and ecological aspects intensively – always with the aim of balancing private use, communal areas, energy efficiency, costs and long-term maintenance.

BLACKPRINT: Which sustainability aspects were paramount in the design of the building?

Dirk Stenzel: With the Z8, we consistently implemented a holistic concept: the floor plans are designed to be flexible and barrier-free, with column-beam constructions that also enable subsequent changes of use. The green roof creates usable open spaces and reduces inner-city heating. Energy supply is provided by two heat pump systems, geothermal energy, roof-mounted collectors and fireplaces connected to the heating system. As part of the timber construction method, we used cross-laminated timber external walls and load-bearing columns to keep the flats flexible in their use. The living areas range from 120 to 180 square metres. The commercial spaces are also used by the group themselves, including a furniture and office furniture shop. There were no external investors; only the KfW 55 standard was subsidised.

On the three upper levels, four airy and bright flats were integrated.

BLACKPRINT: You are yourself part of the client group...

Dirk Stenzel: Yes, and that always requires intensive dialogue, close coordination and many compromises, sometimes even mediation. Most of our projects are created in such client groups, in cooperatives, housing syndicate projects or property owners' associations. The groups contribute their life plans; in this way, buildings are shaped by the users, not by investors.

BLACKPRINT: How did the iconic form of the Z8 come about?

Dirk Stenzel: The triangular site was quite challenging and not very attractive to investors. As a result, the special building form actually arose quite naturally. The design is oriented to the site boundary, takes up the rounded corner edge and adapts in height to the neighbourhood. We used the raised corner above a public water pipe creatively as an entrance area. And in other respects too, we developed many details in dialogue with the client group and tested, discussed and adapted proposals. We also made the decisions on interior fittings, such as timber species, clay plaster or window profiles, jointly in several workshops.

BLACKPRINT: Looking back today: what was the greatest challenge in the project?

Dirk Stenzel: Surprisingly, implementation proceeded comparatively smoothly, as we agreed deviations from the building regulations early on and closely involved fire protection, the checking engineer and the building control authority. More difficult was the site itself, which was heavily contaminated as a former petrol station. During excavation work, we encountered heavily contaminated soil down to a depth of 3.50 metres. That was technically and organisationally demanding, but solvable. Above all, it was exciting to see how timber construction and urban densification can be combined, particularly on such an unusual site.

BLACKPRINT: Most recently you realised the K10 project, using recycled concrete formwork facing and timber elements from the former timber floor. What opportunities and practical challenges do you see in upcycling in building construction?

Dirk Stenzel: The K10 was created as part of a concept-based land allocation by the City of Leipzig, in which sites are awarded exclusively to building groups via heritable building rights in order to specifically promote such projects. It was one of the first two allocations of this kind. The group that subsequently formed is part of the Mietshäuser Syndikat – a model that very deliberately bypasses the property market, as the building cannot be sold and remains permanently under collective administration. The group has a fairly diverse composition and comprises 16 adults and 12 children. Based on this, we developed a cluster housing model: each floor comprises around 200 square metres with a central communal area and adjoining private rooms. One of the biggest challenges was the tight budget. Originally, a basic rent of €7.50 per square metre was planned, after completion it is around €8. For Leipzig new builds, however, that is still very affordable, the current market rent is €14 to €16. Nevertheless, we were able to implement central sustainability aspects.

BLACKPRINT: Which aspects are these?

Dirk Stenzel: The building is a timber frame construction, only the stairwell core is made of concrete. This creates a hybrid solution in which concrete is only used where it is really necessary. The heat pump system was initially not financially viable; however, the building was prepared in such a way that a low-temperature system can easily be retrofitted later. Currently, heating is provided via district heating, with an ecological heat pump envisaged in the longer term.

BLACKPRINT: The façade design is of course also striking...

Dirk Stenzel: That too was created in close exchange with the client group. The starting point was an old brick cellar on the site, whose bricks were reused for the ground floor cladding. For the upper façades, we also looked for reusable materials and came across shuttering panels from concrete formwork, which are replaced after only a few uses, even though they are still functional. Together with cover boards from another project, we mounted the panels as weatherboarding. In this way, a façade was created that is consistent in design, material-conscious and sustainable. In some areas, you can even read the manufacturer's name. To implement this, we conducted several weekend workshops on the façade theme, ran through all eventualities and finally agreed on one variant.

BLACKPRINT: A very bold solution, the building appears like an experimental collage...

Dirk Stenzel: Yes, we deliberately wanted the building to provoke discussion in order to make our themes visible. The Z8 project was primarily about timber construction, and as a result other timber buildings in Leipzig were also stimulated. With the K10, the reuse of building materials takes centre stage. The construction sector consumes around 50% of resources worldwide, so something urgently needs to change there. With every project, we therefore consider how materials can be reused and construction methods designed to be circular. Another theme is living space: the K10 group had decided that each person should have a maximum of 30 square metres of living space, significantly less than the German average of 47 square metres. In the cluster housing model this works well, including for families, as the communal areas can be used flexibly.

BLACKPRINT: Do you currently have similar projects in planning?

Dirk Stenzel: Yes, several. This is something very close to my heart, which is why I founded a non-profit housing cooperative. Until recently, non-profit housing was hardly possible in Germany – there were only two such cooperatives. One of them is now in Leipzig, where we have just implemented an inclusive social housing project. It is a timber construction with a heat pump system and roof garden and was also realised via the City's concept allocation, as a classic inclusive social housing development in which social and ecological aspects come together.

BLACKPRINT: An important focus of your practice is existing buildings. How do you proceed here?

Dirk Stenzel: One example is the GS1, a relatively large corner building of around 1,600 m² for a cooperative, which we refurbished together with a committed house group. As the building was listed, the external façades had to be implemented carefully. We were able to retain and refurbish the box windows. Such measures demonstrate that consistent refurbishment, participation and sustainability are also possible in existing buildings.

BLACKPRINT: Your own office is also located in a refurbished old building...

Dirk Stenzel: Yes, for our office we converted an old garage, exposed the timber beams and painted the walls with clay paint – it fits the philosophy, even if not everything can be implemented in existing buildings.

BLACKPRINT: These are all exciting projects. And it sounds as though Leipzig is a vibrant laboratory for community engagement and experimental housing forms.

Dirk Stenzel: Yes, absolutely. There are currently many individual house projects, but also larger initiatives such as SOWO, a solidarity housing cooperative that owns a total of eight buildings in west Leipzig. Many of these projects are supported by the City of Leipzig, for instance through our network Leipziger Freiheit. We provide advice, for example, on changes of ownership that become possible 25 to 30 years after the initial refurbishment work in the post-reunification period. Tenants then have the opportunity to take over their own building and implement necessary refurbishment measures. As a member of the Leipziger Freiheit network, I advise such groups who wish to purchase and refurbish a building.

BLACKPRINT: On your website you write: "The knowledge accumulated over centuries in construction is disregarded or deliberately negated with these new and 'innovative' building products." Which knowledge are you referring to in particular?

Dirk Stenzel: Here we are primarily concerned with natural building materials such as timber, clay and lime, whose properties cannot be artificially reproduced. Modern products often contain additives that make recycling difficult. Carbon concrete is also problematic: it has very good properties, but in my view, reusability has not yet been fully clarified. It always comes down to avoiding composite materials. We therefore very deliberately focus on proven materials that are ecological and durable. Timber and clay construction have existed for centuries but are not yet sufficiently established in modern practice. I am therefore always critical of new developments and question what they really deliver.

BLACKPRINT: In recent years, have there been assumptions about sustainable building that turned out differently in practice than expected? And what lessons do you draw from this for your current planning and decision-making strategy?

Dirk Stenzel: Our strategy focuses on three core areas of action: structural work, energy supply and social aspects. In structural work, we consistently use timber; for energy supply, we focus on efficient and renewable systems; and for social aspects, we concentrate on communal models such as cooperatives or housing syndicate projects. Where we need to develop further is implementing certain constructions in an ecological way.. Composite systems cannot yet be built entirely sustainably, and ecological building materials are often significantly more expensive than conventional ones. In the long term, however, prices are gradually equalising through CO₂ levies, recycling and scarce resources. In dry interior construction, for example, prices for plasterboard are rising whilst clay building products remain stable. So we are making progress – slowly but steadily.

BLACKPRINT: Thank you very much for the conversation.

The interview was conducted by Robert Uhde.