Minimize Demolitions, Heed Historical Lessons

Zurich is considered one of the most liveable cities in Europe. You have studied the city's development. Can Zurich grow without losing what makes it special?

Tom Avermaete: Zurich is a remarkably liveable city. Yet if its population grows by up to 25 per cent in the coming years, the city will need to act with forethought - otherwise it risks losing the very qualities that define it.

What population growth are you anticipating?

We worked with a scenario in which Zurich could gain around 100,000 residents by 2040. This issue is of shared interest to academia, policymakers, practitioners and society at large, all of whom are engaged in seeking solutions. Our aim was to provide well-founded, urban-development answers to the question of how Zurich can grow while preserving its qualities. To that end, we examined both the historical development and concrete situations across the city where there is genuine scope for densification. In a unique combination of historical research at ETH Zurich and propositional planning at the Accademia di architettura di Mendrisio (USI) we explored what a qualitative future for Zurich can be.

What conclusions did you draw?

Continuing to expand the city outward by building on greenfield sites is no longer a viable strategy. We therefore propose a sustainable approach to inner densification - one that learns from tradition, favours mid-rise buildings, and seeks a balanced relationship between preservation, adaptive reuse, and new construction, with the aim of demolishing as few existing buildings as possible.

What solutions do you see for housing in Zurich as the population grows?

Thinking about densification from an urban development perspective means asking not only how many people a city can accommodate, but how a city reorganises itself as needs and demands change. We examined which historical building types - or typologies, as we call them in the field - are best suited to the future of housing in Zurich. Two stand out: the perimeter block and mid-rise buildings of six to nine storeys, reaching up to 30 metres in height, which we also call short towers.

Why are the perimeter block and the mid-rise tower Zurich's best response to growth?

Both typologies have proved their worth since the late nineteenth century and are well established in Zurich. They integrate readily into the existing city fabric. We have identified further suitable sites in the Old Town, on the Zürichberg and along the lake in Wollishofen. Perimeter blocks are common today in Aussersihl and Wiedikon, and they remain a strong solution for the flat terrain towards the Limmat valley.

What are the advantages of perimeter blocks?

They enclose a shared inner courtyard and combine residential use with ground-floor functions such as shops, studios, or crèches - creating a dense, lively mix of uses. At the same time, as our historical research shows, this building type adapts well to changing needs, for instance through new combinations of private and communal spaces.

Why are mid-rise towers particularly well suited to densification?

Mid-rise towers do not read as skyscrapers in the cityscape - even though in Zurich, all buildings above 25 metres are formally classified as high-rises. Our research shows that historically, their intermediate height has allowed them to accommodate many people while still maintaining a connection to the street. In our prospective scenario, 807 mid-rise towers could in principle absorb the entire population growth. However, since perimeter blocks are preferable in flat terrain, we see a realistic potential of around 40,000 people for mid-rise towers.

You are proposing perimeter blocks and mid-rise towers as preferred residential building types for Zurich's urban development. What impact would these have on rents?

Our approach seems to align well with the City of Zurich's one-third target, which aims to keep around a third of the city's housing affordable. Our analyses show that the city can manage population growth if priority is given to retrofitting and adapting existing buildings. History shows that knock-down rebuilds tend to come with significantly higher rents, whereas retaining and adapting the existing building stock has a moderating effect on costs. Where replacement buildings are nevertheless built, they should be more compact - and ought to accommodate at least 50 per cent more residents than the buildings they replace, while providing around 30 per cent affordable housing. These requirements would make sense for public, cooperative, and private developers alike.

How does more compact construction affect living space per resident?

The amount of floor space per person has risen steadily in recent decades and currently averages around 41 square metres. In perimeter blocks and mid-rise towers, this could be reduced to around 30 square metres without significant loss of residential quality - particularly where larger communal areas or flexibly usable indoor and outdoor spaces are available.

How do you assess the future of high-rises and single-family homes in Zurich?

A city thrives on a variety of housing typologies, and both high-rises and single-family homes have their place. However, when 100,000 additional people move into the city, the single-family home is not an optimal urban development solution. New high-rises would have the capacity to absorb this growth - but they raise questions from an urban development perspective.

What questions?

How well high-rises integrate into a historically grown city depends strongly on their location and surroundings. Our historical research has shown that Zurich has successful examples, but also less convincing ones. As our analyses demonstrate, Zurich can accommodate 100,000 additional residents very well through perimeter blocks and mid-rise towers. This finding surprised us: we had originally assumed that the city would inevitably need a large number of new high-rises.

Should each high-rise therefore be judged on its own merits?

Yes, with a site-specific focus on location, surroundings, and the mix of uses. This reflects the tradition of European cities: growth is possible without high-rises, and densification is approached according to the specific place and situation.

Does that apply to office high-rises as well?

Yes, they too must be assessed on their own merits. The urban development of cities such as Brussels, where I come from, offers valuable lessons: pure office high-rise districts feel deserted in the evenings and at weekends. Such urban monocultures are increasingly being questioned, and considerable investment is now going into bringing a greater mix of housing, work, and leisure into these areas in order to bring them to life.

How do you see the optimal mix of housing and work in Zurich?

The boundary between living and working is increasingly blurring - this is a profound transformation of our cities. In Zurich, the Hunziker Areal shows exemplarily how the two can be combined. The key question is: how can work and housing be combined in the future so that both find their place close to one another - or even within the same building?

Does working from home play a role in this?

Yes. Working from home reflects the shifting boundaries between living and working spaces. This is why Zurich needs more buildings that integrate both functions - for instance with workspaces in the same buildings as dwellings, or with shared office space within a development. The Kalkbreite housing and commercial development in Zurich is one such example. Perimeter blocks and mid-rise towers lend themselves well to this kind of mix. New buildings are not always necessary: existing ones can be retrofitted and upgraded - older high-rises included. Extending and adapting the existing building stock is a central concern of our research.

What is the alternative to single-use districts?

The 10- or 15-minute neighbourhood: housing, work and daily needs all within walking distance. The city will develop in an increasingly polycentric way, forming several secondary centres. These can function as 15-minute neighbourhoods, where children, for instance, would find playgrounds, sports facilities and green spaces close to home.

What role do schools play in urban development?

In Zurich, schools have always served a purpose that extends well beyond education. With their playground areas, sports facilities and infrastructure, they function as small neighbourhood centres - as meeting points that can be used in the evenings and at weekends for local events, sport and culture, by children, families and older residents alike. This centre-forming role of the school is by no means a given in an international context, and it will become even more important as the city develops polycentrically. More broadly, we recommend that public buildings be designed from the outset to accommodate a variety of uses at different times of day.

What does this mean for urban green spaces?

Green spaces have historically been essential for the urban life, climate and biodiversity of Zurich. However, as the city continues to grow, the demands placed on them will become more diverse - green spaces will be used even more by people of different ages and with different lifestyles. This makes it all the more important to design them in a way that allows for a variety of uses and ensures they are well connected to bike paths, walking trails and public transportation.

What approaches do you see for urban mobility?

The urban and transport planning approaches of the 20^th century no longer provide the solutions for the future. As our historical research has shown, Zurich has long devoted a great deal of space to the car - including through massive interventions such as the Sihlhochstrasse elevated road and the Hardbrücke. Walking and cycling routes, public transport, and multimodal hubs where people can switch between modes - from the S-Bahn to an e-bike, for instance - are now becoming increasingly important. Zurich can reduce the number of car parking spaces in the city; sustainable mobility, car sharing, and the 15-minute neighbourhood all make this possible.

What will happen to car parks?

Various studies show that cars are parked more than 90 per cent of the time - often on public land. These surfaces can be put to better use. Those that are already heavily sealed can accommodate new buildings, such as mid-rise towers. Those that can be unsealed could be transformed into green spaces.

How do you see the relationship between sustainability and architectural heritage - between preservation and new construction?

This question has led to a genuine paradigm shift in the field of architecture in general, and at ETH Zurich in particular, over the past 15 years or so. The principle has become: demolish as few buildings as possible, and renovate, transform, and upgrade as many as possible - including those that appear to have little cultural value at first glance. In planning processes, heritage conservation and ecological sustainability might seem like opposing forces. In fact, they complement each other.

In what way?

A respectful approach to the built cultural heritage can be justified both from a conservation and an ecological perspective. We may choose to retain a building because it is beautiful, typical, or defining of a particular era. At the same time, every existing building contains considerable material resources and embodied carbon that must be taken into account when considering its future. Retaining and reusing the existing building stock should take precedence over demolition nowadays- because preserving built fabric is a concrete contribution to reducing CO₂ emissions in the construction sector.

What is your view on the regulation of densification in Zurich?

Planning and building regulations should be developed further to enable high-quality urban growth. General rules must be flexible enough in their application to meet the specific requirements of each individual site. In selected areas, it should be possible to build higher and denser than current regulations allow. With such an adapted regulatory framework, we will be able to densify the city by learning from history and demolishing as little as possible.