Cranes crowd the skyline of the City of London. The oldest part of the capital seems to be an eternal work in progress as buildings are demolished and rebuilt. Concrete panels replace brick and stone, and glass façades replace concrete panels. When a building begins to feel antiquated, occupiers move on and developers demolish and rebuild. This constant churn of construction and demolition comes at an environmental cost – and pits investors in a race against heritage lobbyists seeking to list buildings – but commercial developers and occupiers are unsentimental. Over half of the City’s office space has been built or refurbished in the last 20 years. ²⁹

A few miles away, the streets of Mayfair and Belgravia seem more stable, mixing grand mansions with polite 20th-century infill in timeless squares. But behind the stately façades, they, too, are providing for change. The expansive houses built for aristocrats in the 18th and 19th centuries have been repurposed for the use of hedge funds, embassies and family offices. Wood-panelled dining rooms now host board meetings and client discussions, and grand salons become front offices; support staff huddle under the eaves in former servants’ quarters. These generously proportioned houses, with their extensive space for entertaining and receiving visitors, have adapted to the changing demands of the city. Similarly, in places like Notting Hill, large Victorian houses were subdivided into flats or bedsits in the 20th century, then reintegrated over the last 30 years as the property market boomed.

Successful buildings in London are those that can be adapted or replaced easily, as economic circumstances change and needs dictate. Talk of transformational change and disruption fills the air, but we do not know how we will live in the future, any more than the speculative developers who built Georgian London anticipated home cinemas and open-plan kitchens. As the American writer and ecological activist Stewart Brand put it in How Buildings Learn: “All buildings are predictions. All predictions are wrong.” ³⁰

Will we see tighter living spaces as Londoners dispense with “stuff”? Or a revival in intergenerational living as we live longer and need to find new ways of providing care for older people? Will living and working space converge or diverge across the capital? Will more of us be renting, moving more frequently, or changing homes as regularly as we change jobs? Or will buying and settling in a home remain the aspiration?

How will the homes being built today cope with these uncertain futures? The portents don’t look good: many new homes are capable neither of demolition nor adaptation. Single-aspect flats with optimised internal layouts will never become anything else: they have been designed to precise space standards and to optimise profitability. Any slack space or indeterminacy has been eliminated through value engineering, the relentless search for the cheapest way to deliver a tight specification. Micro-flats, designed for theoretically unencumbered young people, are the tightest of all.

This inflexibility might not be such a problem if poor-quality or obsolete residential buildings could be demolished and rebuilt as easily as commercial buildings. But the very idea seems alien: we invest a lot more than financial capital in the idea of “home” (though many tenants would be envious of the five-year leases commonplace for commercial buildings). The slum clearances of the post-war period were traumatic both for individuals and communities.

In any case, patterns of tenure make demolition and rebuilding extremely difficult. Privately owned flats are held on long leases or shared freeholds. Agreeing to repaint a hallway can take months of negotiation; jointly agreeing a communal demolition and rebuild would be virtually impossible. Right-to-buy has made demolition and replacement almost as hard in social housing developments, which, being largely in public ownership, are more frequently targeted for redevelopment. Even 1980s suburban office buildings, once the last word in disposable architecture, are now being filled with tiny flatlets under permitted development rights.

Clad in green-tinged glass and clip-on brick panels, many of London’s new housing developments risk being frozen in time, unable to move forwards or be replaced. Rather than risking this unplanned obsolescence, how can we build more adaptable and resilient homes? As space becomes tighter, housebuilders and policymakers have been exploring ways to enhance flexibility, so many flats now include technology that can enable reconfiguration from day to night: tables, beds, chairs and screens that slide in and out at different times. The Lifetime Homes standard also requires homes to make provision for adaptation as people’s mobility changes.

But these are examples of what Jeremy Till, architect and Head of Central St Martins, calls “hard space” flexibility. ³¹ These homes may be capable of adapting at different times of day or different times of life, but they will forever remain what they were built to be: their flexibility is carefully specified and therefore constrained. “Soft space” flexibility goes further: it doesn’t try to prescribe what takes place within a room or a building, or anticipate future needs. It leaves slack space to allow immediate and future occupiers to experiment and change it.

In some cases, this means leaving parts of a building unbuilt. Alejandro Aravena, the Chilean winner of this year’s Pritzker Prize for Architecture, talks of “Half a Good House”, the low-cost housing that his practice Elemental has designed and built in several South American locations, leaving space for residents to complete the building. In London, Naked House is developing plans for 22 units in Enfield consisting of a basic shell construction that residents can customise over time.

A more fundamental flexibility would not merely leave open the question of how rooms are arranged, but also step back from determining what a building is – home, office, workshop, school. Who is to say that the building used as a house today will have the same use in the future? Buildings that last and adjust, like the Mayfair homes that have become offices and the Spitalfields workshops that have become homes, prove to have an inherent adaptability.

This raises questions for planners as well as designers. At the moment, planning policy allocates land according to purpose – housing, offices, light industrial or other use – fixing the price of that land in the process. Buildings are then designed to optimise the specified use: the “hard space” flexibility systems are one way of doing so, amongst others. With sky-high land prices and constrained building heights, there is no incentive to build looser-fit structures.

Perhaps extensive planning deregulation could change the rules of the game. One approach would be to say that planners would no longer seek to perfect the city’s mix by allocating land for different uses, focusing instead on creating the right links between density, design and transport. Zoning regulations could determine density, scale and architectural features, but planners wouldn’t dictate what a building was for.

The incentive structure would change (particularly once land values had adapted to the brave new world) and developers would have a reason to build for resilience and change, rather than for fixed function – arranging services so that kitchens, bathrooms and bedrooms could move, and creating internal partitions that could be easily removed and replaced.

City governments wouldn’t have to abandon the management of the mix and segregation of different land uses (from separating unneighbourly activities to enabling the economic boost provided by agglomeration), but they would not manage land use using the blunt instrument of planning policy. If planning deregulation were accompanied by a reform of property taxes, different tools could be used. City governments could set and adjust tax rates for different uses in different locations, responding to technological change and the shifting needs of the city. If more offices were needed in an area, the tax rate could be lowered to the point where landlords and tenants adjusted. Such wholesale reform of the planning and taxation system would simplify the tangled pile-up of policy that planners implement today. Tax levels could be adjusted to promote efficient and flexible land uses, or promote the provision of social goods like affordable housing.

There would be significant issues of equity and practicality, of course, not least in getting the balance right between providing flexibility for occupants and certainty for developers and investors. And the idea that cities are perfectible entities, which just require fine-tuning by technocrats, is as spurious as it is seductive. All the same, we do need to find a way, in an urbanising world, of thinking about how our buildings, districts and cities can respond to the unknown unknowns of the future. We build as if tomorrow is fixed, when we can already see how much has changed since yesterday.