The goal of sustainability is becoming increasingly apparent in the Netherlands in our desire to deal more carefully with the scarce and finite means at our disposal. And we can consider space, too, as a scarce and finite resource in our densely populated country. That is why Dutch government policy focuses on more intensive land use and more compact cities. It is therefore remarkable that countless sites remain unused for long periods in spite of this government policy.

In Amsterdam, for instance, there are numerous site where nothing happens for long periods of time. Along the Zuidas development corridor, development sites cleared and prepared for construction lie empty owing to the economic crisis. In the Houthavens area, construction projects are delayed because of tighter European regulations. In the city centre streets remain blocked off because of technical complications resulting from the construction of the new subway line. A site next to the Kostverloren waterway remains undeveloped owing to a keenly contested legal dispute between a housing association and a Turkish religious organisation. Taken together, these examples illustrate a growing problem of more and more land lying idle because of postponement or delays.

Faltering Area Development
Area Development Plans — the result of collaboration between clients, banks, designers, contractors, controllers and occupants — has become a complicated and vulnerable mechanism in the Netherlands. This has a negative effect on the optimal use and exploitation of available land. More stringent (European) regulations, obscure public inquiry procedures, ground pollution, declining consumer confidence, elections and archaeological finds are all phenomena that can lead to a faltering mechanism. And a faltering mechanism can perform its tasks less rapidly and accurately. That results in the inefficient use of land.

We do not need to explain why vacant sites are socially undesirable. But it is more difficult to trace the whole combination of causes and keep them in check. A side effect of the increasingly stringent regulations to improve air quality, for example, or of the cautious policies of banks because of the crisis, is their negative influence on area development. But in themselves, they are justified and unavoidable developments. They all amount to uncertain factors, and the result is that area development is increasingly accompanied by uncertainty about planning and, consequently, costs. When can we start building and how long will it then take? We have to learn to deal with that uncertainty and learn to improvise better so that we can make use of land more rapidly and effectively.

Improvisation
Since our physical planning system is largely based on the elimination of uncertainties and the establishment of guarantees, the possibilities for improvisation seem limited at first glance. Nonetheless, a brief inventory leads to an number of interesting discoveries.
For example, our building regulations include the notion of temporary building permits. By means of a shortened permit procedure, it is possible to deviate from the prevailing development plan for a limited period. In financial terms, the use of space for a shorter period means a shorter exploitation period and, hence, a shorter period in which to recuperate the investment. It goes without saying that we should therefore look for cheaper solutions than we are accustomed to. In term of programme, improvising in how we use space leads to exceptional possibilities for the most varied programmes. In technical terms, there already are many construction methods that enable us to build faster and more cheaply than we are accustomed to.
Therefore the tools for the improvisational use of space already exist in current practice. What we now need most of all is a change of mentality and, when it comes to vacant sites, a willingness to turn to less obvious tools than we have been accustomed to until recently in terms of area development.

Programme
There are locations that have a negative influence on their immediate surroundings because they are empty. For the good functioning of the city it is important to activate these places with temporary programmes. This works best when the programme relates to the immediate surroundings. An empty site in a residential area, for example can be turned temporarily into a playground, while commercial programmes such as a temporary supermarket are a welcome addition to a retail street. Temporary functions not only give rise to exploitation possibilities but also have a beneficial effect on the exploitation possibilities of programmes in the surrounding area.
In addition, there are empty sites that are not necessarily a problem for their surroundings. Such sites are usually in less densely populated areas, for example on the fringes of cities. These locations offer an opportunity for programmes and activities that are impossible or prohibited elsewhere because they cause inconvenience. Examples include motocross circuits and shooting ranges. Such sites can act as temporary valves where 'tensions' that accumulate elsewhere in the city can be released.
Both types of sites offer possibilities for programmes that are difficult to accommodate on permanently available sites because their realisation is too expensive or because the need for accommodation is temporary by nature. Sports facilities, for example, are located further outside the city because it is simply too expensive to keep such facilities in centrally located parts of the city. Temporary peaks in demand for amenities such as schools, childcare facilities and housing for the elderly are often so brief that the construction of temporary accommodation is not undertaken. Programmes such as housing for the homeless are sometimes accepted for a short period but meet with opposition from the immediate surroundings when permanent.

Construction methods
Accompanying the improvisational use of space are construction methods that allow amenities to be completed quickly and cheaply. It is therefore obvious to use lightweight materials that can be transported and erected using relatively little energy. Examples of this are pneumatic (inflatable) building systems and structures of weather-resistant fabric. In addition, prefabricated building systems such as scaffolding systems and prefabricated building containers reduce the time required for construction. These building systems are available for hire for a short period, and that helps keep construction costs relatively low. Temporary spatial structures have a greater chance of being built if the ground beneath the available sites remains untouched. The absence of foundations reduces the duration of the construction period, saves money, and prevents difficulties arising from possible ground pollution. What’s more, it’s easier to leave behind a ‘clean’ site when vacating it again.

Temporary building as a task for architects
Carnivals, circuses, markets and festivals have been erected as temporary structures for centuries. Recently, advance building components have enabled us to create temporary structures for other purposes such as student housing units, temporary extensions to schools and temporary office units on building sites. These are usually solutions quickly worked out by engineers and managers to deal with acute problems in which no consideration is given to the careful integration into the urban fabric. The standard building systems are ‘parked’ in a very pragmatic manner on the available sites, resulting in unwanted residual spaces and an absence of the desired urban continuity. A careful integration of temporary structures is not only a task for engineers and managers but also, and above all, an architectural challenge. Just like musicians who can improvise well, are familiar with a broad musical repertoire and can recognise structures quickly, good architects possess similar qualities in the area of spatial planning. In our opinion, we must find a good balance between the deployment of quick and cheap standard prefabricated components and tailor-made unique adaptations in finding suitable solutions for sites. A new balance must be found between ‘be quick’ and ‘adapt to fit’.