No land? No problem – airspace building could deliver the next 150,000 homes

Imagine walking through Sydney’s CBD and looking up, not at another glass tower emerging from a demolition site, but at new, lightweight apartments quietly perched on the sturdy rooftops of buildings that have been standing for decades. No extra land, no urban sprawl, just clever use of the “fifth facade” that we already own.

What is airspace development?

Airspace development simply means adding new floors on top of existing buildings. Instead of buying land (often the single biggest cost in city projects), we “occupy, buy” the empty sky just above the roof. With advanced modular construction, those extra storeys can arrive on-site as largely complete pods, craned into place within days, and plug into the building’s existing utilities; meanwhile, the offices or apartments below keep operating.

Why now?

• Land cost disappears – in dense CBDs, land can equal 40-60 per cent of a project budget. Using pre-owned rooftops wipes that cost a line item entirely, CBRE calls it Australia’s “untapped multibillion-dollar opportunity”.
• Double the speed –  factory production shortens site programs by up to half; London’s affordable home project went from planning to handover in roughly a year.
• Lower carbon – re-using an existing concrete frame avoids the biggest chunk of embodied emissions, while timber or light-gauge-steel pods weigh only 20 per cent of an equivalent concrete floor.  
• Facelift old and tired buildings – sale or rent of the rooftop homes can finance lifts, solar panels and façade upgrades, a model proven by several London housing associations.

How big is the prize?

NSW Government modelling shows its new low- and mid-rise policy alone is expected to unlock 112,000 additional homes across 171 transport hubs in the next five years. Scaling that uplift to other capitals and assuming only moderate take-up, a conservative national potential of about 150,000 rooftop apartments emerges, roughly one-eighth of the 1.2 million-home, five-year target that housing experts say is likely to fall short by at least 262,000 dwellings.

Success stories to emulate

Table 1 shows several cases where the engineering is solved, the residents can benefit, and the economics stack up when policy clears the path.

Australian proof-of-concept

Australia’s first cross-laminated timber (CLT) vertical extension shows the model works here at home. Recently, at 55 Southbank Boulevard, Melbourne, the existing commercial office building built in 1989, was able to accommodate a future extension of six levels with the use of concrete-framed construction, but the design challenge we faced was how to surpass this in order to deliver a hotel with 220 rooms. The solution involved the use of Cross Laminated Timber (CLT) construction, which means the building will be able to support an additional 10 levels, thereby achieving the target room number.

Due to the quantum of off-site fabrication, the time spent on-site was significantly reduced. Most importantly, sequestered within the timber itself are approximately 4000 tonnes of CO2 emissions, the equivalent of the annual carbon emissions of 130 homes.

This is a scalable and effective new model of development that showcases one of the most effective ways to realise the much-needed urban revitalisation of our cities, which also drives us towards Net Zero.

A practical pathway for Australia

• Map the roofs – councils and state agencies should run GIS scans to locate flat roofs, recent structural drawings and buildings erected after the 1950s building boom (often over-engineered for extra floors)
• Streamline approvals – replicate the UK’s “if-it-fits-it-passes” permitted-development model so compliant rooftop schemes are fast-tracked.
• Publish a national structural guide – a shared checklist will enable engineers to quickly confirm column capacity, vibration limits and wind loads, reducing costly over-testing. Mid-century offices were designed for upward expansion decades ago.
• Kick-start with government sites – using state-owned car parks, health buildings, or depots removes land negotiations and demonstrates proof of concept. NSW’s own planning department is already piloting rooftop housing on public assets.
• Demand lightweight modular construction – set weight limits (e.g., <3 kN/m² dead load) so proponents default to timber or light-gauge steel modules, as in Rotterdam.
• Share the upside – clear strata or commercial leasing rules should spell out how rooftop sale proceeds are to fund common-area upgrades, keeping existing owners on side.
• Design for neighbours – require setbacks, green roofs and solar arrays so additions improve both skyline and street amenity; many Nordic schemes wrap new plant rooms in green terraces to cut glare and runoff.

Upskill the workforce – short courses in adaptive-reuse engineering and modular assembly will ensure that assessors, builders, and certifiers keep pace with demand.

What could success look like?

Success will come when we breathe new life into the city. By 2035 Sydney, Melbourne, Brisbane and Perth could each showcase dozens of rooftop communities: timber-framed studios above CBD car parks; family duplexes on setback steel decks atop mid-century offices; green terraces where HVAC units once baked in the sun. Around 150,000 Australians, key workers, downsizers, and students, could live closer to jobs and services. At the same time, existing owners enjoy upgraded lifts, cooler roofs and fresh façades, all funded by the new homes above.

Call to action

The cheapest block of land is the one we already own. Airspace development flips the housing equation: instead of paying for the earth we don’t have, we invest in the sky we do. With policy certainty, smart engineering, and fair sharing of the rewards, Australia can quickly turn idle rooftops into a powerful, low-carbon housing pipeline.

It is time to raise the roof.

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