Building on Air

Giles Martin

Combining innovation with collaboration to make difficult urban sites economically viable and home to high-quality development.

With development land in limited supply in every mature city, demand is driving up values to the point where even the most challenging sites are becoming financially viable. One key to unlocking potential is over-site development. Recent examples of building above existing infrastructure have shown that innovative design approaches can successfully unlock swathes of city-centre land by building on air.

Major cities worldwide are seizing the opportunities of creating developments over roads, parking lots, rail and stations. In Australia for example, proposals include a new sports stadium over Sydney’s Central Station, while among ideas in the US there is an elevated tree-lined park for Atlanta above rail tracks and a busy roadway. In the UK there has been discussion about constructing much-needed new housing over rail networks and Crossrail has integrated designs for a dozen major property developments over and around its key central London stations. 

For over-station developments in particular, locations provide attractive sustainable development opportunities by integrating homes and workplaces with ideal transportation connections.

The potential is clearly enormous, but so are the obstacles. With every urban site constrained by its own set of specific issues, collaboration and great teamwork are vital to finding innovative solutions and getting a project underway. Here below we outline three challenges common to many over-site developments, and the collaborative solutions that are making them viable.

Piling and design potential

Key to unlocking the site is the ability to position piles in useful locations while also avoiding disruption to existing infrastructure, such as live rail tracks. Piles also need the right ground conditions to support development at a scale and quality that warrants the enhanced costs of working on a difficult site. In finding a solution, piling is a crucial starting point. From these constraints it becomes possible to create the structure diagram to indicate development scale and potential, and therefore move the project forward. 

Prices and profitability

Unsurprisingly, the financial cost of developing constrained, complex and unpredictable sites are higher than anything more straightforward. The numerous challenges make it difficult to create accurate forecasts for what it will take the make the site viable. Along with the piling work, costings therefore need to consider the extra spend required for building over live rails – insurance is one part of equation, and there are also likely to be very limited working hours. All of these considerations must be factored in to the financial plan and balanced with the value of the space being created. One positive here is that as more projects are completed, the more we learn and costs becomes firmer, and hopefully go down.

People and place

By their very nature, these dense urban sites have high levels of pedestrian traffic in and around them and the potential for disruption is high. Any development must take care to be considerate to local communities and other stakeholders; from the outset, good design will enhance this type of inner-city development. Excellent permeability will help weave a new scheme into the existing streetscape, and there are likely to be opportunities to incorporate public space. Meanwhile, through the construction phase, issues including air quality and construction noise must be high on the agenda, along with reducing delivery traffic by incorporating elements of modular construction.