Andreas F Vögeli
Niederer Kraft Frey, Zürich
andreas.f.voegeli@nkf.ch

Annina Fey
Niederer Kraft Frey, Zürich
annina.fey@nkf.ch

Introduction

Switzerland is a small country with limited construction land reserves; hence, conflict has increasingly developed between urban development and heritage protection. As cities and developers seek to accommodate growing populations through densification strategies, they often encounter rigid constraints arising from heritage preservation requirements. A central instrument in this respect is the Federal Inventory of Swiss Heritage Sites (Bundesinventar der schützenswerten Ortsbilder der Schweiz von nationaler Bedeutung, or ISOS), which aims to protect the most valuable architectural and landscape assets of the nation. More than 1,200 sites are registered in ISOS and cover a significant amount of space (eg, in the canton of Zürich, 75 per cent of building area (Siedlungsgebiet) falls within the scope of ISOS). The application of ISOS has evolved in ways that significantly affect land use, construction rights and urban planning processes, leading to a complex balancing act between competing societal goals.

This article examines how ISOS influences construction law today; how its scope has expanded; and why legal controversies are emerging around the interplay of heritage conservation, private ownership rights and the urgent need for sustainable urban development.

Legal foundations

ISOS was established based on Article 78 of the Federal Constitution (BV) and the Federal Act on the Protection of Nature and Cultural Heritage (NHG). Article 78, paragraph 1 BV assigns the primary responsibility for nature and cultural heritage protection to the cantons. However, Article 78, paragraph 2 BV obliges the Confederation to preserve natural and cultural values during the fulfilment of federal tasks. This led to the Federal Act on the Protection of Nature and Cultural Heritage (NHG), according to which objects of national interest must be inventoried.

ISOS itself is not a regulatory instrument but an inventory, established by the Federal Council, that identifies landscapes and urban ensembles deemed worthy of protection. However, the inventory becomes legally binding when a project qualifies as a ‘federal task’ under Article 2, NHG. In such cases, federal authorities are obliged to consider ISOS entries as part of a comprehensive or qualified interests’-balancing process. In case the protection objectives of ISOS could be significantly impacted, authorities are required to request an expertise from the Federal Commission for the Protection of Nature and Cultural Heritage (FCNC) and in the interest-balancing process for significant impacts only other national interests may be considered. The relevant implementing ordinance – the Ordinance on the Federal Inventory of Swiss Heritage Sites (NHV) – further specifies the procedural requirements. Additionally, the VISOS (Ordinance on Federal Inventory of Swiss Heritage Sites) governs the creation, structure and revision of the inventory itself, including the criteria used for inclusion and the roles of the federal and cantonal authorities.

Evolution and legal role of ISOS

ISOS was created to identify and document sites of significant cultural, historical and architectural importance across Switzerland. Although originally designed as a descriptive inventory without direct regulatory effect, its practical influence has increased substantially.

One critical reason for this change is the interpretation of ‘federal tasks’ under Swiss law, which leads to direct applicability of the NHG. Whereas originally only key federal tasks would trigger its applicability, recent case law has extended the notion of ‘federal task’ significantly, triggering ISOS’s full weight in a broader range of activities than initially intended. Whenever a project requires a federal permit, subsidy, or falls within federal jurisdiction – such as transport infrastructure, groundwater regulation, or even certain energy facilities – ISOS protections become binding. For example, the federal task concept currently also includes, among other things:

• construction projects located in groundwater;
• solar energy projects requiring federal permits;
• installation of mobile telecommunications infrastructure;
• authorisation of a secondary dwelling; and
• construction of a civil defence building.

This expansion means that many projects, even those seemingly local in nature, must now undergo rigorous reviews for compliance with ISOS objectives, and ISOS-related obligations now apply to a wide range of development activities, further entrenching the inventory’s practical relevance. In particular, relatively minor projects – such as installing solar panels on historically sensitive roofs – can now trigger comprehensive ISOS reviews.

Tension between densification and heritage protection

Urban densification is a strategic priority in Swiss spatial planning, aiming to curb urban sprawl and make efficient use of existing infrastructure. However, many of the areas best suited for densification – central, historically rich neighbourhoods – are also protected under ISOS.

This creates a structural conflict. While densification requires vertical and volumetric expansion, ISOS zones impose tight constraints on modifications to the built environment. Authorities and courts are tasked with balancing competing interests, but recent practice shows a clear prioritisation of heritage protection.

In many approval procedures, the preservation of townscape quality outweighs the societal need for additional housing, better mobility infrastructure or upgraded energy systems. This prioritisation often frustrates developers and municipalities aiming to meet contemporary demands for living space, affordability and energy efficiency.

Impact on private rights and legal controversies

Property owners face substantial restrictions as well once their property is included in an ISOS-listed area. Although ISOS is, technically, not a zoning instrument, in practice it operates with similar effects by limiting how properties can be used or modified.

One key criticism is that property owners have no effective legal remedy against ISOS entries themselves. The ISOS designation is not subject to direct appeal and critics point out that ISOS itself was established without a formal interests-balancing process: it is purely based on historic relevance and appearance of landscape and townscape quality, with other aspects not taken into consideration.

In construction approval processes, private interests – such as the economic viability of redevelopment projects – frequently take a back seat to public heritage interests. Courts typically interpret preservation as having a higher priority, unless truly compelling reasons can be shown to justify exceptions. Where a project would significantly impact ISOS objectives, only national public interests could overweigh the protection objectives, which is unlikely to apply in case of a private construction project.

As a result, developers often find themselves in a procedural bind, having to conduct expensive studies, engage heritage consultants, and negotiate intricate approval procedures, without any guarantee that the project will ultimately be authorised.

Renovation instead of demolition: new approaches to preservation

In response to these legal and practical constraints, there has been a shift toward promoting careful renovation and adaptive reuse of existing structures rather than demolition and reconstruction.

In the context of ISOS-protected areas, renovation and adaptive reuse are often the more practical path forward. While ISOS does not legally prohibit demolition per se, the requirements for justifying the removal of historically valuable structures are high, especially when the project qualifies as a federal task. In such cases, the legal obligation to protect cultural heritage demands that less invasive alternatives be thoroughly examined and documented. Although there is no explicit legal preference for renovation over demolition, planning authorities frequently expect developers to prioritise conservation-friendly approaches.

However, these projects tend to be more costly and technically demanding due to the need for specialised expertise, and compliance with both heritage and building regulations.

Future outlook and necessary reforms

The growing tension between heritage protection and urban development suggests that reforms will be necessary to ensure a fair and workable balance.

Several proposals are under discussion:

• more differentiated protection categories within ISOS, reflecting varying degrees of significance and allowing greater flexibility for minor sites;
• systematic integration of social and environmental interests into the weighing process during planning and approval procedures; and
• clear federal guidelines for how ISOS considerations should be applied at cantonal and enhanced protection of private property rights, including the possibility to challenge ISOS listings under administrative law procedures.

Additionally, better dialogue between conservation authorities, developers, municipalities and citizens will be crucial to building trust and finding innovative solutions that satisfy multiple public interests.

Practical recommendations

Given the expanding influence of ISOS and the increasing complexity of approval processes involving federal tasks, practitioners must adapt their strategies early in the project lifecycle. It can be useful for developers and planners to engage heritage specialists and legal advisors at the preliminary planning stage to assess ISOS impacts and navigate potential obstacles. Early coordination with federal, cantonal and municipal authorities can streamline project reviews and pre-empt costly procedural delays. Moreover, when designing projects in or near ISOS-listed sites, prioritising renovation and adaptive reuse over demolition could significantly increase the chances of obtaining permits while aligning with broader policy trends in heritage conservation and climate protection.

Conclusion

Heritage protection under ISOS has become a decisive force in Swiss construction law and urban development policy. While the preservation of historically significant sites remains an important public good, the expansion of ISOS’s legal and practical reach has raised serious challenges for developers, modern urban planning, private property rights, and sustainable development.

To maintain the credibility and effectiveness of heritage conservation, Switzerland must find new ways to reconcile the needs of the past with the demands of the present and future.