The winning scheme at Siggenthal starts by refusing to tear something down. The timber roof of the old material shed at Holcim’s cement plant stays where it is and becomes the covered arrival for visitors. Behind it, between shed and silos, a compact new volume stands on a short-span grid of carbon-prestressed concrete. The project is called «Holcim Lab». Its authors are Allen + Crippa, an eight-person office with desks in Buchs SG and Zürich-Altstetten, founded in 2022 by Timothy Allen and Ronan Crippa, both ETH-trained and from Grabs.

A word on timing. espazium’s review by Clementine Hegner-van Rooden ran on 21 October 2026 and is our hook. The win is older: Holcim announced it on 24 June, and the office’s Prix SIA student prize for Gässli 5 dates from 11 June.

The procedure, in numbers

Holcim ran an open SIA 142 competition, organised by raumplan wirz in Basel and published on 16 July 2025. It drew 32 entries. After the first jury day on 16 January 2026, the jury chaired by Barbara Holzer sent three projects back for anonymous rework. The teams resubmitted on 18 May, and the decision was unanimous on the third day. The prize sum was CHF 120,000, with two first prizes awarded ex aequo at CHF 24,000 each. Construction is planned for 2027, subject to permits.

Christine Dietrich, a client juror for Holcim, told espazium that the second round let the jury judge the projects “by what the teams actually wanted to develop” (our translation). From an office’s side of the desk, an anonymous rework round cuts both ways. It is a rare chance to argue the idea instead of the render, and it is a second block of hours on an entry that may still not win.

Keep, stack, expose

Floors and roof use CPC elements prestressed with carbon strands instead of steel. Holcim’s release says this saves around 75 % of the material. The walls are stacked re-used slabs, made load-bearing again with added reinforcement and an overlay of new concrete. Energy comes from the plant’s own waste heat, and the structure stays visible as part of the exhibition. Holzer, in the same release: the construction, the choice of materials and the energy concept “fit together perfectly”.

The jury report calls «Holcim Lab» “a project carried by a strong idea”, an architecture “that stands apart from the generic”, where “the sparing use of new concrete strikes a convincing balance between Holcim’s identity and ecological sustainability.” Re-use has its own cost: the old slabs need new steel and new concrete before they carry load again.

The plant has run since 1912 and produces more than 900,000 t of cement a year. The brief describes the visitor workshop as a portal and “a place of dialogue” that makes “critical engagement” possible. Plant manager Thomas Brühlmann sees the project strengthening the plant’s century-long roots in the region and “the existing dialogue”.

←TODAY: In 2026 a four-year-old Swiss office won an open competition whose brief invites critical dialogue, and its first move was to keep a roof. →3012: Zurich 3012 starts every brief by reading what already stands, before anything new is drawn. Fulcrum: The inventory of the existing site is the design itself, not the step before it.

A method older than the office

The habit came before the practice. With Gässli 5 in Grabs, the founders dismantled a 17th-century log house, moved it 700 m through the village and rebuilt it beside a prefabricated newbuild of about 200 rammed-earth elements, completed in 2025 with Merz Kley Partner, Lehm Ton Erde and Egga Holzbau. The Prix SIA student jury wrote that Gässli 5 “illustrates beautifully the preservation of historic heritage and of the social fabric in rural space.” At the Fribourg ceremony Allen said: “You have to be on site and talk with people.”

The authorship is shared. «Holcim Lab» is an ARGE with Julian Meier Architektur, with Léna Grossenbacher in the core team; scenography by Aroma, structure by Fabrice Meylan at co-struct, and an outdoor space of demolition material, limestone and pioneer vegetation by Eva Bärlocher at Atelier für Landschaft. Vonder Mühll Consulting, Wirkungsgrad Ingenieure and Durable worked on building physics and energy. Without the engineers, re-used slabs would stay an idea on a drawing.

Atelier: In most offices a competition starts with massing, and the existing site is drawn too late, in grey. This entry starts the other way round, with an inventory of what stands, what can carry load and what can be stacked. Monday move: add a fixed half-day to your competition kick-off for a site inventory of photos plus a kept / reused / removed list, before anyone opens a massing file.

Hack: Count what the survey model already holds before you design over it. If a site comes with an IFC survey or a Bestand model, list the elements you might keep by type, so your inventory sheet starts from numbers instead of memory. These five lines use IfcOpenShell:

import ifcopenshell
m = ifcopenshell.open("bestand.ifc")
for t in ("IfcRoof", "IfcSlab", "IfcColumn", "IfcWall"):
    n = len(m.by_type(t))
    print(f"{t:10} {n:4}  keep? reuse? remove?")

Paste the output into the first sheet and give each row a decision.

PAZ Takeaway: PAZ can set up an office’s Archicad competition template so that the first layout is a site inventory: the renovation filter (Existing / To be demolished / New) wired into the template, with a schedule that lists kept and reused elements automatically. The inventory then exists in every entry, not just in the good ones.

Dietrich ended her espazium interview on this line: sustainability “also begins with the culture of discussion within one’s own ranks.” Start your next entry the same way. Go to the site, talk to the people there, and write down what stays before you draw anything new.