Disused rail sheds and a granary, a contaminated derelict landscape, a stadium long empty; rather than things to erase, to build over, these are opportunities to re-imagine. In three different parts of London, each has provided the chance to transform rather than just build anew. They show that the most successful urban projects can be those that exploit the potential of the pre-existing.

Major cities like London are growing. They are facing increasing pressures to provide more housing, more capacity for work and leisure and the need for new public spaces. These demands suggest extending the city, but the reality is this isn't possible. Sprawl is bad and boundaries get in the way. So cities turn inward to reclaim structures and assets that may have long gone silent. Ironically, these places can provide a chance to mend wounds that had been inflicted by their very introduction on the cityscape.

This is why when embarking on any major city centre regeneration, it's important to attribute significance to what is already there. For reasons that are social and economic as much as architectural or cultural, new buildings and masterplans in these contexts should reinforce or reinterpret the urban structure of that which they form a part. There's also an ethical dimension. If we are to minimise our consumption and energy use, we need to do what we can to extend the life of existing buildings and places.

Many of our cities today owe large swaths of their character to the industrial revolution, which added to Enlightenment-era renewal projects that were built upon Medieval settlements. Building a great city is a process of layering. Many of those industrial insertions and additions are now defunct - whether due to economic or technological shifts - but they left behind beautiful typologies and an important, formative, layer of our shared heritage.

King's Cross was just one of these places. The meeting point of two of London's great rail termini - St Pancras and King's Cross - the site contains important remnants of industrial heritage. It is bisected by the Regent's Canal (built 1812-1820), has four listed late 19th century gasholders, granary and transit sheds. It fulfilled an important role in the Victorian economy, where goods were delivered on train and then off-loaded onto horse-drawn carts; it was a crucial piece of infrastructure, but one that was completely off limits to the public. Now long defunct, redevelopment of the area into a new neighbourhood provided the opportunity to make these parts visible again.

Working for developers Argent, an incremental masterplan saw much potential with what was already on site. The incorporation of industrial heritage means there is a strong identity with the magnetism to attract. The old granary and railway sheds provided an ideal home for Central Saint Martins and framed a great new public space - Granary Square. Having a globally-renowned arts university as an early tenant brought youth and buzz that is now attracting occupants large and small, global and local such as the Aga Khan Foundation, Google and BNP Paribas. Elsewhere, new offices and housing are going up alongside retained pieces of industrial heritage, creating a rich urban quarter. And rather than impose a new order of streets, the geometries left by the site's industrial uses provided the basis for the alignment of streets, walkways and public spaces. The adjacent pattern of streets is retained and new connections enable easy access to two of the city's busiest train stations.

In 2012, London's hosting of the Olympics provided the ideal opportunity to reposition a similar kind of post-industrial void. This area, the Lea Valley, had long acted as London's back yard with bus depots, stabling for trains, sewage treatment works, gasworks, power generation and transmission, railway sidings, warehouses, urban motorways and places for waste storage and treatment. Just south of today's parks is a group of 18th century mill buildings of historic value. The valley, with a contaminated river winding down it, also split this large section of London; eastern suburbs were cut-off by an industrialised wilderness that has become a no-go area.

Road patterns has been severed on either side (or never existed at all), so the masterplan connected them rather than create some new kind of grid or pattern. The result is a sinuous texture of streets that to the untrained eye, doesn't look planned at all. During the Games, many venues were temporary while permanent ones were given post-Games community uses. The legacy of this investment is focussed on a new public park that embraced the river. It provided the setting for new urban neighbourhoods of mixed uses including housing and a new waterfront of cultural and educational institutions following a topography that had long faded in the the collective memory. The park has been the anchor of an eastward shift in London's creative energy, which draws on the area's rich industrial heritage and multiculturalism, to fuel the very inventiveness that fuels its growth.

Re-purposing is possible too at the large scale of an individual building. When the Arsenal Football Club outgrew its art deco home in Highbury, an important piece of cultural memory faced an uncertain future until it was re-imagined as Highbury Square, a new residential neighbourhood that retained key elements of the stadiums architecture and urban imprint. Rather than erase a stadium or even preserve it as a relic, it provided a chance to build a new layer of London over it. The stadium's footprint provided the perfect space for a typical London square and the retention of elements of the stadium's architecture made it distinct from typical new build apartments. This sense of distinction is the elusive ingredient that new urban places regularly need to thrive in the long-term. Often, it can be found by simply embracing what is already there.