BLACKPRINT: Mr Haas, with the visitor centre for Rapunzel in Legau you have created a place that organically combines architecture, brand identity, visitor circulation and sustainability. What idea lies behind the architecture, and how does it reflect the Rapunzel philosophy?

Martin Haas: The company generates enormous interest amongst its customers. Many people want to experience on site how sustainable, ecological farming works. In close collaboration with the client, we therefore wanted to create a house full of discovery. From the outside, the large, hovering roof welcomes visitors like a ribbon spanning everything. Inside, they are greeted by a varied mix of uses: an organic market, a bakery, a café, a coffee roastery, an exhibition, a wine cellar, a planted roof terrace, and training and yoga rooms. All functions are connected by a 15-metre-high spiral staircase in timber – our "Rapunzel braid" – which links all floors up to the roof terrace and opens up a panoramic view over the surrounding landscape.

BLACKPRINT: You speak of four key principles in relation to the building: regionality, a sense of enchantment, landscape and long-term use. Could you explain what lies behind each of these?

Martin Haas: The first point was primarily about transport and the involvement of local craftspeople. From experience with earlier projects, we knew that in sustainable construction transport becomes a decisive factor. We therefore wanted to engage firms and craftspeople for timber construction, the façade and furniture making predominantly from within a 20-kilometre radius, and to involve them in the development of details. It was a fascinating process, reminiscent of earlier, more craft-oriented ways of working. We now try to transfer this approach to all our projects: regardless of location, we want to bring craft knowledge into the architectural development process at an early stage. It has proved extremely valuable.

BLACKPRINT: The themes of enchantment and landscape are also central to the visitor centre in Legau. How do these manifest themselves in the building?

Martin Haas: We wanted to convey the sense of enchantment very subtly – through the window shapes, the sweeping roof form and the central spiral staircase. Drawing on associations with the company name "Rapunzel", a poetic, almost fairy-tale quality has emerged that runs throughout the entire building. Equally important to us was the integration into the landscape context. The building form developed as a triad of three wings. The roof floats above the ground, provides shade and draws on archetypal architectural imagery. It was particularly important to us to make the roof publicly accessible: with planted roof covering, a flowering roof garden and wide views over the surroundings. This surface forms part of the educational trail and supports visitor circulation.

BLACKPRINT: Als vierten Punkt hatten Sie die dauerhafte Nutzung angesprochen...

Martin Haas: Yes, our goal was to create a building that is not only used between 9 and 5. Buildings that are only active during the day stand empty for the greater part of the time. We therefore deliberately extended the hours of use: through the bakery with its early production start, a café open throughout the day, and the Rapunzel products market. Added to this are educational offerings such as seminars, workshops and training courses on ecological farming, as well as a club and a wine cellar. The result is a kind of "all-round provision" – a public building that brings together functions once found in a village square: food and drink, a meeting place, education and leisure. This layering of uses forms the conceptual backbone of the project.

BLACKPRINT: What role does the building services play?

Martin Haas: The technical sustainability concept was almost a by-product, since the company was already generating a great deal of green energy and waste heat. We were able to connect to this and operate the building in a climate-neutral way.

BLACKPRINT: The distinctive roof cladding is also striking.

Martin Haas: In the Allgäu region, timber shingles are typical of the area – we wanted to draw on this in the design. Classical timber shingles would, however, have been too costly to maintain at this roof scale. An alternative in Canadian cedar would have worked, but was similarly problematic from an ecological standpoint due to its limited lifespan. We therefore looked for a durable, craft-based solution. Initially we found a manufacturer in Denmark, but the transport would have been too involved. In the end we found our solution in Switzerland. In Rapperswil near Berne there was still a kiln from 1790 in which tiles were fired. Together with the manufacturer we developed special clay shingles with individual engobes. The project very nearly failed because the operating licence for the kiln was expiring. In a last-minute effort, the shingles were still completed in a more modern kiln, as one of the final products of this traditional manufacturing line.

BLACKPRINT: The solid timber spiral staircase inside must also have been a considerable challenge?

Martin Haas: Indeed. The staircase was realised entirely without steel, which very few contractors were willing to undertake. Looking back, I would evaluate this approach more critically today – in parts it was too ideologically driven. A balanced mix of materials is often more sensible: each material should be used where it can demonstrate its strengths. In this case, the scale and complexity of the staircase pushed timber construction to its limits.

BLACKPRINT: Accordingly, the main structure is also built in the conventional manner as reinforced concrete...

Martin Haas: Yes, exactly. But there were special aspects here too. We managed to find a local manufacturer who works with recycled concrete and wanted to realise the project as a pioneering achievement.

The large solid timber spiral staircase was realised entirely without steel.

BLACKPRINT: Closely connected to the Rapunzel building in Legau is the new Alnatura headquarters, which you completed a few years earlier in Darmstadt...

Martin Haas: Yes, indeed. Alnatura and Rapunzel are closely linked as businesses, and accordingly the new building in Darmstadt also served as a reference for the visitor centre in Legau. The background to the project was that Alnatura wanted to consolidate its previously dispersed administrative locations whilst realising a consistently sustainable building – climate-neutral, made from renewable raw materials and architecturally restrained. We first examined around 15 sites before discovering a conversion site in the immediate vicinity of the Westwald forest. The microclimate there offered ideal conditions for natural ventilation and cooling.

BLACKPRINT: How did the further design thinking develop from there?

Martin Haas: We oriented the building on a north–south axis to minimise solar gains and optimise natural ventilation. The aim was a thermally inert building that utilises thermal mass, comparable to the solid masonry of late 19th-century construction that is barely ventilated in winter yet remains pleasantly tempered. Early on, a broad barn with a pitched roof, a central atrium and a sawtooth roof emerged as the building form – a clear, simple design, ecological, functional and harmonious. Inside there are no rigid departmental divisions. Instead, flexible working areas, open-plan kitchen spaces and meeting zones separable by acoustic curtains have been created, enabling interdisciplinary working and spontaneous encounters. Daylight floods all levels and reinforces the open, communicative character of the building.

BLACKPRINT: The Alnatura Campus is the largest office building with a rammed earth façade in Europe. How did this unusual choice of material come about?

Martin Haas: Rammed earth has many natural properties: thermal mass, air purification, crystalline structure. Ideal, therefore, for a sustainable office building. We then developed the final solution together with specialist Martin Rauch. The distinctive feature: we reduced everything in the building to the bare essentials – there are no suspended ceilings, no raised floors. Instead, we were able to use the walls for heating and integrated water pipes heated by geothermal energy and the solar roof. The result is an innovative, ecologically self-contained system.

BLACKPRINT: How has the feedback from users of both buildings in Legau and Darmstadt been?

Martin Haas: Very positive, and also over the long term. Initially, staff had of course to get used to the natural ventilation and the "living" building. They now report an exceptional quality of experience – the light, the spatial feeling. The building is perceived as a place that creates identity. The response at Rapunzel has been similar. Both projects work not only architecturally but also organisationally.

BLACKPRINT: You are currently developing the Leibniz Innovation Farm in Groß Kreutz...

Martin Haas: Yes, the focus there is on agricultural production, regionality and the circular economy. A management wing with a fibre technology centre and a research hall are planned. The ensemble draws on farmyard structures typical of the region – robust, simple building volumes derived from the locality. For maximum sustainability, we are consistently focusing on recycled materials, renewable raw materials and circular construction. The aim is to design both the construction and operation to be as climate-neutral and efficient as possible – from the timber load-bearing structure, through the choice of materials, to the processes within the building.

At the Leibniz Innovation Farm in Groß Kreutz, the architects are consistently focusing on recycled materials, renewable raw materials and circular construction.

BLACKPRINT: Another current project in planning is the energy retrofit of the building at Königstraße 1a in Stuttgart...

Martin Haas: The building is situated opposite the main railway station and is intended to mark the beginning of Königstraße. We are largely retaining the existing fabric and supplementing it with a timber hybrid structure. The façade will be dismantled and reinstated with a few additions, with the windows extended to box windows. The aim is maximum retention of the existing fabric, optimisation of natural light and carbon storage through timber. The greatest challenge is reconciling the existing fabric, modern requirements and the legal obligations of a public client. The project will demonstrate in exemplary fashion how sustainable reuse of inner-city buildings can work in practice.

On Königstraße in Stuttgart, the architects are supplementing an existing building with a timber hybrid structure.

BLACKPRINT: You are currently also dealing with a very different kind of existing building project at the town hall in Schömberg. What challenges does that present?

Martin Haas: The town hall dates from 1790 but is in a very critical condition with numerous structural cracks. As an existing building it is no longer structurally sound. We therefore decided to partially dismantle it, retain the original building elements as far as possible and reconstruct the building in its historical form. The realisation is in timber construction, with many of the old timbers having previously been treated with problematic wood preservatives that must therefore be cleaned and conservatorially prepared – a very labour-intensive process requiring precision and experience. The project demonstrates how complex the sustainable reuse of historic buildings can be. Compared with Königstraße in Stuttgart, the effort involved here is considerably greater.

In Schömberg, the architects are reconstructing the 1790 town hall in its historical form, using original building elements from the existing structure.

BLACKPRINT: Perhaps you could look ahead once more: how do you see the prospects for sustainable architecture, both in general and for your practice?

Martin Haas: For me, sustainability is the only genuine driver of innovation in architecture. It is tangibly advancing the transformation of the construction industry. As a founder and vice-president of the DGNB I have insight into politics and business and see there that sustainability is being increasingly implemented. Not always out of genuine conviction, but ever more frequently because it also makes economic sense in the context of the EU CO₂ taxonomy. For us, this is a clear signal: sustainable construction is developing from a special case into the new norm.

BLACKPRINT: Thank you very much for the conversation!

The interview was conducted by Robert Uhde.