Interstitial spaces are the city’s unsung heroes—they slip in between buildings, under bridges, along railroad tracks, and into gaps that no one ever planned for. What many consider wild no-man’s-land turns out, upon closer inspection, to be a laboratory for urban innovation, social movement, and ecological diversity. Anyone who views “unplanned” spaces merely as leftover areas misses out on the most exciting stories of urban development and the greatest opportunities for transformation.

• Definition and Significance of Urban Interstitial Spaces in the Context of Contemporary Urban Planning
• Historical development and cultural perception of the “unplanned” in the city
• Mechanisms and potential for transformation: From brownfields to creative uses
• Strategies and challenges in integrating interstitial spaces into sustainable urban development
• Case studies and best-practice examples from Germany, Austria, and Switzerland
• The Role of Participation, Interim Use, and Temporary Architecture
• Ecological, social, and economic values of unplanned spaces
• Risks of Commercialization and the Displacement of Informal Uses
• Perspectives for Future Planning Approaches and the Understanding of Urbanity

What Are Interstitial Spaces? The Unplanned as a Resource of the City

In classical urban planning, interstitial spaces were long regarded as “blank spots” on the map, as remnants of supposedly flawed planning, or as evidence of stagnation and disinterest. Yet this perspective falls far short of the mark. Interstitial spaces are those areas that elude direct designation for a specific purpose: railroad embankments, overgrown lots, forgotten backyards, brownfields, residual areas along transportation corridors, and unused building complexes. They are often the result of economic shifts, structural change, or simply planning errors. Yet it is precisely in their indeterminacy and openness that their greatest potential lies.

Interstitial spaces are spaces of possibility. They offer room for experimentation, for temporary uses, for spontaneous interventions, and for social and ecological innovations. While planned parks and squares structure everyday urban life, interstitial spaces invite people to experiment, linger, and transform. They serve as a stage for processes that elude control: unplanned growth, graffiti, guerrilla gardening, neighborhood festivals, and urban art often emerge where urban planning turns a blind eye—or chooses not to look.

However, it would be a mistake to view interstitial spaces as mere “empty spaces.” They are not simply what remains once the city is “finished.” Rather, they are fundamental components of the urban fabric—dynamic zones where urban society and nature converge. They allow us to experiment with new forms of appropriation and coexistence, beyond the logic of investors and the pressure to exploit space. Those who understand these interstitial spaces understand the city as an organism, not as a machine.

The concept of the “unplanned” is by no means negative. Rather, it refers to a quality that is often lost in the rigid framework of classical urban planning: flexibility, openness, and unpredictability. It is precisely these unplanned places that lend the city the vibrancy and diversity that make it so fascinating. They challenge planners to think in terms of processes rather than products—and to embrace the adventure that is the city.

The appreciation of in-between spaces is by no means a marginal phenomenon. Among international experts, they are discussed as key resources for resilient, sustainable, and inclusive urban development. Interstitial spaces are not the opposite of planning, but rather its necessary complement. They serve as buffer zones against monoculture, overdevelopment, and displacement—and provide a stage for the “unexpected.”

Historical and Cultural Perspectives: Interstitial Spaces Through the Ages

The history of in-between spaces is as old as the city itself—and at least as fascinating. Even in ancient Rome, there were vacant lots between the insulae that served as meeting places, markets, or gardens. In the Middle Ages, these were the “commons” and “village greens,” communal open spaces on the outskirts of settlements. But with modernity and the functionalist urban planning of the 20th century, interstitial spaces were increasingly viewed as flaws, as disruptive elements in the grid of the perfect city.

The Athens Charter and the ideas of the car-friendly city emphasized clear zoning and the separation of living, working, and recreation. Anything that did not fit the scheme was eliminated or ignored as “disorder.” It was only with the critique of this functionalist logic—by authors such as Jane Jacobs and Jan Gehl—that the “spaces in between” returned to the forefront of urban research. Jacobs’ famous statement, “Cities have the capability of providing something for everybody, only because, and only when, they are created by everybody,” sums it up: The quality of a city often lies in the unplanned, in the interplay of many small open spaces.

The cultural perception of unplanned spaces fluctuates between fascination and rejection. While temporarily repurposed brownfields in Berlin, Vienna, or Zurich are celebrated as hotspots of the creative scene, elsewhere they are regarded as eyesores or places that instill fear. The media and pop culture also play a role: these in-between spaces are sometimes romanticized as “lost places” and other times stigmatized as “problem zones.” This ambivalence reflects society’s approach to uncertainty and change.

In the 1990s, “temporary use” emerged as a distinct planning and participatory tool. Projects such as the “Kalkbreite” in Zurich, the “Praterinsel” in Munich, or the “Holzmarkt” in Berlin demonstrate how innovative neighborhoods can emerge from seemingly worthless spaces. The transformation of industrial brownfields into urban gardens, cultural centers, or communal housing projects became the model for a new, open approach to urban development.

Today, in-between spaces are experiencing a renaissance—not as a problem, but as a resource. The debate surrounding the “post-pandemic city” has shown how important flexible, informal, and low-threshold open spaces are for the social and ecological balance of urban areas. Interstitial spaces are an expression of a culture of change, appropriation, and resilience—and pose a challenge to planners, policymakers, and society alike.

Potential for Transformation: From Brownfield to Urban Laboratory

The transformation of unplanned interstitial spaces into productive, livable, and innovative places is one of the greatest challenges and opportunities in contemporary urban development. This is where it becomes clear whether planning can do more than just deal with regulations and property lines—whether it is ready to facilitate processes rather than merely deliver end products. The potential of such transformations is diverse, ranging from ecological enhancement to social inclusion and economic revitalization.

From an ecological perspective, interstitial spaces are often biodiversity hotspots. Where humans withdraw, pioneer plants, insects, and birds settle in. Even small fallow areas can serve as stepping stones in the urban biotope network, as refuges for rare species, or as green corridors between parks and gardens. Examples such as the “Gleiswildnis” in Munich or the “Park am Gleisdreieck” in Berlin show how former railway facilities can be transformed into species-rich natural areas that are surprisingly valuable to city dwellers.

Socially, these in-between spaces offer room for appropriation, encounter, and participation. They are low-threshold places that require no admission ticket, impose no obligation to consume, and dictate no fixed rules. Especially in highly densely populated neighborhoods, they are the last refuges for informal activities: Children use them as adventure playgrounds, young people as places of retreat, and community initiatives as venues for festivals, flea markets, or urban gardening. For marginalized groups, too, such spaces are often vital—they offer visibility, protection, and the opportunity to engage with the city.

Economically, these in-between spaces are increasingly being recognized as sources of innovation. They offer niches for startups, studios, workshops, or pop-up restaurants—all at rents that even young companies can afford. At the same time, they serve as testing grounds for new mobility concepts, sharing models, or sustainable construction methods. Where the traditional development chain reaches its limits, space opens up for experimentation and new business models. The “circular city” often begins precisely where the market fails.

However, transforming these in-between spaces requires a delicate touch. Not every vacant lot needs to be developed immediately, nor does every instance of unplanned growth need to be curbed. It often makes more sense to identify, preserve, and moderate potential rather than hastily “cleaning up” these spaces. Participation here is not a luxury but a prerequisite for sustainable success. Only when residents, users, and owners work together on development can places emerge that function sustainably and gain acceptance.

At the same time, dangers lurk: The commercialization of unplanned spaces can lead to the displacement of informal uses and a restriction of social diversity. Those who view interstitial spaces solely as investment opportunities destroy their unique qualities. That is why we need guidelines, innovative participatory formats, and a planning culture that can tolerate the unfinished—and views it as a resource.

Strategies and Challenges: Integrating the Unplanned into Urban Development

Integrating interstitial spaces into formal urban development is a balancing act. On the one hand, there is enormous potential—for climate adaptation, social integration, the creative economy, and urban resilience. On the other hand, planners face the task of maintaining the fragile balance between openness and control, between appropriation and order. Integrating the unplanned requires new tools, processes, and, above all, attitudes.

A key tool is interim use—the temporary, often experimental use of spaces or buildings before permanent development begins. It makes it possible to test new ideas, involve local stakeholders, and highlight the value of places that would otherwise remain dormant. Successful interim use projects such as “NUN” in Linz, the “Alte Feuerwache” in Cologne, or the “Kulturbahnhof” in St. Gallen demonstrate how flexible timeframes, low barriers to entry, and collaborative processes can spur sustainable urban development.

Participation is another key issue. Interstitial spaces are ideal as testing grounds for new participatory formats—from open-space conferences and citizen juries to digital participatory platforms. Here, classic conflicts of interest can be openly negotiated, unusual alliances forged, and innovative solutions developed. Those who take participation seriously must be willing to relinquish control and facilitate processes rather than dominate them.

A particular challenge lies in ensuring accessibility and diversity. Interstitial spaces are vulnerable—they can quickly be privatized, fenced off, or overused. Planners and city administrations must therefore develop mechanisms that guarantee openness and equality while also preventing abuse and displacement. This calls for legal instruments such as ground leases, urban development contracts, or cooperative management structures that safeguard the common good without restricting creativity.

Ultimately, integrating the unplanned is a matter of attitude. It requires planners to have the courage to let go of control, to be open to uncertainty, and to be willing to question their own routines. The traditional logic of master plans and zoning plans reaches its limits here. Sustainable urban development needs tools that can handle ambiguity, change, and contradictions—and that view the in-between spaces not as a shortcoming, but as a resource.

This also requires a new understanding of professionalism. Today, those working in urban development are less “doers” than facilitators, less technocrats than enablers. The art lies in shaping processes, opening up spaces, and productively negotiating conflicts—while consistently allowing for the surprising and the unexpected.

Interstitial Spaces as Laboratories for the Future: Perspectives for a Transformative Planning Culture

The future of the city is open—and will be decided in the in-between spaces. They are the laboratory for a new planning culture that focuses not only on perfection, but also on process, diversity, and change. Anyone who wants to shape transformative in-between spaces must be willing to question routines, forge alliances, and experiment with new forms of collaboration.

A key approach is the consistent opening up of planning processes. Interstitial spaces should be understood as a resource from the outset and integrated into development strategies. This means involving informal actors early on, establishing cooperative procedures, and enabling flexible uses. Cities such as Zurich, Basel, and Freiburg demonstrate that a proactive policy on in-between spaces promotes not only creativity but also social cohesion and ecological innovation.

Technological developments such as digital city models, participatory mapping tools, and urban sensor technology can help reveal potential and create a transparent basis for decision-making. But technology alone is not enough. What matters most is the mindset: interstitial spaces must be understood as places for learning, experimentation, and negotiation—not as gaps to be filled as quickly as possible.

The danger of commercialization remains. More and more investors are recognizing the image potential of informal spaces and attempting to capitalize on their creativity. Here, it is up to the urban community to formulate clear rules and ensure that the common good remains the priority. Only in this way will in-between spaces remain places of diversity, openness, and innovation—and not become mere backdrops for the next trend.

Ultimately, transformative in-between spaces are not a matter of chance, but of courage. They require planners who are willing to take risks, administrations that facilitate rather than dictate processes, and citizens who get involved and take responsibility. The city of the future will emerge where planning and the unplanned come together productively—and where we give these in-between spaces the stage they deserve.

Conclusion: Interstitial Spaces—the Unplanned Heart of the City

Interstitial spaces are far more than gaps in the urban fabric. They are the places where urbanity constantly reinvents itself, where social and ecological innovations emerge, and where the “unplanned” unleashes its productive power. Anyone who takes urban development seriously must view in-between spaces as a resource—as a laboratory for new ideas, a stage for diversity, and a catalyst for transformation. It takes courage, openness, and professional curiosity to unlock this potential and shape sustainable urban development. The future of the city lies not in a master plan, but in creatively dealing with the unpredictable. Interstitial spaces are at the heart of urban transformation—and the city’s wisest planners have long known this.