Published in 3/2023 - Block


How to Plan Unplanned Encounters? A Formula Explains What to Take into Account When Designing Urban Blocks

Martino De Rossi

The diagrams show the connections from stairwells to the yard (orange) and to the street (red), and from the street to the yard (blue) in the case study blocks. Diagram and analysis: Martino De Rossi

In recent decades, the perimeter block has become the go-to urban typology in Finnish urban planning and a symbol of urban life. If you take a closer look at the form, however, it does not create life around it as such. The potential lies in more subtle design choices. 

During the design phase of buildings or building blocks, architects and planners rarely take into consideration unplanned encounters between the residents, even though they are a basic ingredient of urban life. The need for unplanned encounters is ever greater when digitalization takes over society and more and more issues are taken care of without leaving home: from ordering food online to remote work.

Unplanned encounters are one of the most valuable gifts that buildings developed around a courtyard can provide. Nowadays, this most often means the urban typology of a perimeter block. It can offer various benefits, such as safety, protection from strong winds, and the prevention of loneliness and segregation. Basing his argument on much more ancient perimeter blocks in Pompeii – an argument that still applies today – archaeologist Mark Grahame suggests that, in buildings with a central courtyard, people demonstrate a clear sense of belonging to a particular household and a definite sense of their own place within it, while this does not happen in houses without a courtyard.1 Courtyards can provide safe spaces for children to play, improving their emotional and cognitive development. They also help to reduce residents’ feelings of isolation and fear of public spaces by providing transitional areas between public and private spaces. There seems to be no doubt that this basic, unstructured awareness of others is powerfully influenced by the architectural form and that this must be a major factor in design.2 This is a direct invitation to architects.

However, the courtyard itself is not the only ingredient to ensure encounters: every design decision, such as where to place the access, can have a significant impact on the creation of communities. The good news is that architects and planners can do something to this effect as early as in the first design phases – for example, by carefully planning perimeter blocks – and, eventually, the encounters within a block can be examined through calculations.

The courtyard itself is not the only ingredient to ensure encounters.

Does the Perimeter Block Typology Fit the Nordic Climate?

The perimeter block became popular in the Nordic countries at the beginning of the 20th century. The model for the new urban plan of Vallila, Helsinki, according to Riitta Nikula3, came from Göteborg, Sweden, from the exemplar urban plan for the Kungsladugård area that was drawn up in 1916 by Albert Lilienberg. The plan was based on huge, regular, rectangular perimeter blocks for social housing. At the same time, large perimeter blocks started to appear in Copenhagen, such as the Hornbækhus (200 x 80 m) designed by Kay Fisker in 1923.

The city of Helsinki has a long tradition of housing organized around courtyards. In the beginning, it happened spontaneously. According to Juha Ilonen4, before the 1860s, the city consisted of wooden houses built along the streets, with access only from the backyards forming courtyards in the remaining spaces inside the blocks. Later, multi-storey buildings started to be built in the backyards of the wooden houses, and, finally, the wooden houses along the streets were replaced with multi-storey apartment buildings. As the yards remained the same size or became smaller, they became dark, now surrounded by much taller buildings. 

Building ordinances were made to fight this issue. In 1875, it was stated that the minimum size of a courtyard must be 159 square metres, while in 1895 it was added that the building should leave space for fresh air and light, placing restrictions on the height of the buildings, until the courtyard block basically disappeared in the 1930s due to changing ideals. 

Current Helsinki city planning shows a return of perimeter blocks in new urban areas, such as Kalasatama and Jätkäsaari, probably because the perimeter block is the densest block type for urban living. In these new areas, the perimeter block is the basic unit that shapes the urban plan, giving space to pedestrian streets and intimate little squares. 

The debate about the suitability of perimeter blocks in the Nordic countries is often characterized by the question of whether the architectural typology is suited for a cold climate with a low solar altitude. To this, we add the cultural aspect: people do not necessarily enjoy meeting and talking with others by chance, and shared spaces are often considered to be contrary to privacy.

Before landing in northern Europe, the perimeter block had existed for centuries in Central and Southern Europe, as a natural continuation of the Roman settlements, kept alive by climatic needs but, above all, by the culture of sharing. At least in Italy, this housing typology has largely disappeared in the new settlements, where separate buildings are preferred, each with its own garden with no common spaces. The resulting paradox undermines all the clichés about the social nature of Central and Southern Europe.

 The courtyard block basically disappeared in the 1930s due to changing ideals. 

The Perimeter Block and Unplanned Encounters

In order to explain how to prepare a fruitful field for unplanned encounters, it is easier to look at a selection of case studies. Two of the presented perimeter blocks are from the early 1900s. The third case study is an architecturally outstanding contemporary residential perimeter block in Sompasaari, Helsinki. The fourth case – the only one not in Helsinki – is not a real perimeter block, yet shows the challenge of planning a closed perimeter block from scratch today. Comparing contemporary and historical examples shows that the social potential does not lie in the form itself.

Kone ja Silta, Helsinki

The Kone ja Silta block was built between 1917 and 1929 in the Vallila district of Helsinki, initiated by the company bearing the same name. The block, designed by architects Armas Lindgren and Bertel Liljeqvist, was initially planned as a perimeter block with four entrances symmetrically distributed to each side of the block, but only a quarter of the block was built according to the original plan due to changes in the company’s needs in the post-war context. The rest of the block was completed as social housing by the City of Helsinki, with a different logic and two small entrances on each side instead on one large gate. The block now has six entrance gates and 25 stairways only accessible from the courtyard, which is the size of a football field.

The large courtyard of the Kone ja Silta block has six entrance gates. Photo: Martino De Rossi

Nelikulma, Helsinki

Nelikulma (Quadrangle) is a large perimeter block also located in the Vallila district. It was designed by Jussi and Toivo Paatela and completed in 1923. Nelikulma has 138 apartments built around a courtyard. It is the first completed example of a large perimeter block (suurpihakortteli) in Finland. The courtyard has two gates facing each other, creating a visual connection across the entire courtyard from the street. It offers privacy from both the apartments and streets. The emphasis on residential use has given the yard the stamp of a functional everyday environment, with various activities taking place. Nowadays, with direct accesses from the courtyard, there are also two common saunas, a laundry, a wood workshop, as well as areas for children to play and for outdoor cooking and dining. 

The fact that the courtyard is in frequent use makes it possible to have a higher level of security and social control than if staircases were accessible directly from the street. Every apartment has at least one window facing the courtyard, which ensures a continuous visual connection between what is happening both inside and outside.

Nelikulma in Vallila from 1923 is the first completed large perimeter block in Finland. Photo: Martino De Rossi

Sompasaari block, Helsinki

The first perimeter block on the island of Sompasaari, designed by Anttinen Oiva Architects and completed in 2021, is a 4–5-storey building on a parking deck. The Sompasaari perimeter block is one of the finest contemporary Finnish examples of a dense building developed around a central courtyard. The courtyard is very bright and of intimate proportions. It is accessible through a small opening in one of the secondary corners of the block. In the opposite corner, there is a restaurant that overlooks a small square formed by the shaping of the block. 

All seven stairwells have access to the courtyard. This would, potentially, make the courtyard a place of enjoyment and encounters, were it not for the fact that the urban plan forces all the stairs to also have an entrance from the street. So, in total, the stairs are accessible from three different places: the street, the parking garage under the deck, and the courtyard. This creates a dispersion of flows and makes the encounters in places other than the stairwell more infrequent. A smaller issue here – which can be easily solved, though – is that the courtyard is not closed by a gate. This undermines the equilibrium in the level of openness of the household: the constant crossing by strangers will tend to undermine household identity.5

In the Sompasaari Block, designed by Anttinen Oiva Architects, there is a parking garage under the courtyard. Photo: Tuomas Uusheimo

Kuokkalan Kalon, Jyväskylä

The Kuokkalan Kalon project, still under construction, is the result of the 2018 Housing Reform competition, won by the architecture studio Collaboratorio, where I am a partner. The project includes five buildings positioned to form an internal courtyard, and a total of 165 dwelling units. The unit types range from supported housing and housing for people with memory disorders to privately owned flats. The stairwells are mainly accessible through the courtyard, and the common spaces are adjacent to the stairwells, on the ground floor, accessible by all inhabitants of the block.

In the competition proposal, we designed the block with two courtyards connected by a “loggia” structure. The accesses to the courtyards were controlled through gates. This preserved the pedestrian connection between the existing church and the square on opposite sides of the block. During the development phase, the courtyards were unified to form one larger courtyard. 

Kuokkalan Kalon in Jyväskylä was the winning proposal of the Housing Reform 2018 competition. An intimate courtyard will be created in the middle of the block that is currently under construction. Visualisation: Collaboratorio
Table 1: Basic information of the case studies

* The numbers of housing units in Kone ja Silta and Somparaari are calculated based of floor plans available in online archives of Lupapiste Kauppa. For Nelikulma, the information comes from the building management company, and for Kalon, from architecture studio Collaboratorio
** In 2020, according to the Official Statistics of Finland (OSF), the average number of inhabitants per housing unit is 1.96.

Courtyards are a challenge for contemporary design for many reasons. Firstly, the staircase must be economically efficient: an elevator must serve as many apartments as possible, which causes long central corridors and deep apartments. Secondly, there are the fire regulations: if an apartment only faces the courtyard (which happens when the apartments are on both sides of the central corridor), the fire engines must be able to access the courtyard, which means not only that the courtyard cannot be closed along its entire perimeter but also that the yard often becomes an asphalt field. The third issue is the property management and the investors’ usual request for building in phases.

Kalon is not a true perimeter block, but the buildings are very close to each other, giving the courtyard intimacy and an almost medieval atmosphere of narrow streets, squares and interstitial spaces. One thing that has remained strong, starting from the competition proposal, are the accesses to the staircases, which, like in Nelikulma and Kone ja Silta, are mainly offered from the courtyard itself, to promote encounters and make the courtyard alive. Since there is no parking under the courtyard, there is space for large trees and plants, which allows the courtyard to be on the same level as the surrounding streets.

Planning Unplanned Encounters

By using Space Syntax, a method developed by professors Bill Hillier and Julienne Hanson, we are able to subject the case studies to the same level of analysis. It is based on counting encounters. As Sonit Bafna describes, “[t]he aim of space syntax research is to develop strategies of description for configured, inhabited spaces (of buildings, settlements, or built complexes) in such a way that their underlying social logic can be enunciated.”6

The result of space syntax is a simple graphic diagram that helps us to define where unplanned encounters could possibly occur. From the bottom to the top, the diagram shows which spaces the person passes through, from the most public space, the street (black dot), to the most private one, the home (white dot on top). What remains in the middle are all transitional spaces, like the courtyard (orange dot) in our case.

The space syntax diagrams show which spaces the person passes through from the street (black dot), to the home (white dot on top). What remains in the middle are all transitional spaces. Courtyard is marked with an orange dot. Diagrams and analysis: Martino De Rossi

In terms of space syntax, the space of the home is positioned deep within the syntax and away from building entrances and the outside world. This is reflected in relations which happen only in smaller and smaller groups, creating internal segregation, loneliness and depression. As Hillier and Hanson state, “random encounters and awareness of others may be a vital motor of social systems at some, or even all levels.”2

In Kone ja Silta and Nelikulma, the central role of the courtyard is very clear. In Kalon, even if it is not a perimeter block, we see that the courtyard plays a fundamental role in the traffic flow of inhabitants. On the other hand, the space syntax diagram of Sompasaari demonstrates that, hierarchically, the underground parking has equal importance to the courtyard.

Unplanned encounters, by definition, cannot be planned. However, something can be done to stimulate encounters. To do this, inspired by the theory of space syntax, I developed a tool to analyse different design solutions: the Coefficient of Unplanned Encounter, or CUE. The CUE indicates the theoretical number of people exiting the courtyard to the street from the same gate in one day. The coefficient can be calculated with the following formula: 

IN the number of inhabitants of the building (estimation can be derived from the total number of apartments and their rooms)
ET an estimated average of how many times each person leaves and enters their home during a day
EX public accesses to the staircases
EXC accesses to the common outdoor area or courtyard from the staircases
G accesses to the common outdoor area or courtyard from the street

For example, if we try the formula with the assumed information that every person exits or enters the building twice a day between 7 am and 10 pm – for example, to go to the supermarket, the laundry, work, or to take a walk – we would arrive at 18 people per hour passing the courtyard or the same street gate in Nelikulma, and only three in Sompasaari.

It All Starts with the Urban Plan

CUE is a theoretical formula based on architectural data and does not take into account the social behaviour of people. In its current state of development, it gives an idea about how much the quality and quantity of accesses and circulation spaces affect the potential number of encounters.

In order to have a high probability of unplanned encounters, it is not enough to just plan a building as a perimeter block. Accesses, circulation and the placing of common spaces must also be carefully studied. As can be seen from Table 2, even if Kone & Silta and Nelikulma have the same layout – the staircases open only to the courtyard – the presence of four more gates in Kone & Silta decreases the CUE by more than 50%. However, the tool does not take into consideration the bigger scale of the block, which also diminishes the possibility of encounters.

Table 2. The the four case studies analysed using the CUE

Furthermore, there are more aspects that can be considered but are not yet fully calculated with the tool. The physical proximity to neighbours and interaction nodes such as mailboxes can affect social interaction patterns. Outdoor spaces such as porches, balconies, and gardens increase social contacts with neighbours. The physical aspect is also important: there should be little distance between spaces, which results in frequent interaction and intimacy and also destroys social distances. The distinction between interior and exterior space should be minimized. However, an undefined border between the inside and outside encourages strangers to cross it, while vestibules, corridors and passageways protect the building from access by strangers.

In contemporary housing blocks, building codes play an important role in the definitions of functions and accesses. The Sompasaari block was already defined as a perimeter block in the urban plan, where, for example, accesses from the staircases to the courtyard and the street were already defined. Kuokkalan Kalon has been modified from the competition proposal with two courtyards to a single-courtyard solution in collaboration with the contractor, in order to be feasible. However, the idea of entering the building from the courtyard has been maintained. 

The aim of the CUE is to help architects to take unplanned encounters into consideration, starting from the very beginning of the design task, possibly from the concept phase and urban plan. If something is already resolved and written down in the urban plan, it will be present in the design until the realization of the building. If an architect wants to do something in order to support the possibility of encounters, one can start by considering these four aspects: 
1) Place the accesses to staircases in the courtyard.
2) Minimize accesses to/from the street.
3) Prioritize private use of the courtyard.
4) Place shared functions on the ground floor, preferably with a direct access to the courtyard. ↙

Architect. Graduated in 2013 from the IUAV University in Venice. Moved to Helsinki next year and founded, with Kristiina Kuusiluoma, the architectural practice Collaboratorio and began PhD studies in the Oulu School of Architecture.

1 Mark Grahame: Reading Space: Social Interaction and Identity in the Houses of Roman Pompeii. British Archaeological Reports 2000.

2 Bill Hillier & Julienne Hanson: The Social Logic of Space. Cambridge University Press 1984.

3 Riitta Nikula: Yhtenäinen kaupunkikuva 1900–1930: suomalaisen kaupunkirakentamisen ihanteista ja päämääristä, esimerkkeinä Helsingin Etu-Töölö ja Uusi Vallila. Societas scientiarum Fennica 1981.

4 Juha Ilonen: Toinen Helsinki – Kortteleiden kääntöpuolen arkkitehtuuri. The Other Helsinki – The Reverse Face of Architecture in the City. AtlasArt 2014. 

5 Roman houses show that this was possible even with very short distances (Grahame 2000).

6 Sonit Bafna: Space Syntax, A Brief Introduction to Its Logic and Analytical Techniques. Teoksessa Environment and Behaviour 35(1) 2003, 17–29.

7 In fact, based on a survey on site during the daytime, there are almost no moments in which the courtyard is empty.