Heathrow’s third runway: £23.4bn, by the government’s own count
The Department for Transport’s own model puts the net cost of Heathrow expansion at up to £62.5bn.
By The Ledger · · 4 min read
A third runway at Heathrow would cost the country between £23.4bn and £62.5bn, net of any economic gain. That range comes from the Department for Transport, not from campaigners. The department that is meant to build the thing has also modelled the case for not building it.
The number worth sitting with is the GDP gain ministers have cited for years: 0.43%. Rachel Reeves used that figure as chancellor to argue expansion would pay for itself in growth. It came from a report Heathrow itself commissioned. The Department for Transport's own analysis, run independently, put the real gain at 0.03% to 0.05% of GDP — between eight and fourteen times smaller than the number used to sell the project.
The gap between 0.43% and 0.05% is the whole argument
Scale that down to money and the gap stops looking like a rounding error. UK GDP sits near £2.5tn. The airport's claimed 0.43% implies roughly £10.8bn a year in added output. The department's own 0.03% to 0.05% range implies £750m to £1.25bn. One estimate comes from the project's promoter. The other comes from the ministry weighing whether to approve it. They are not close enough to call a disagreement about method. They describe two different projects.
Set against that shrunken gain, the department counted the costs: construction, noise, air pollution, and — the line that rarely makes it into airport press materials — harm to the health and wellbeing of up to 3 million people living near the flight paths. Net it all out and the department's own modelling produces a loss measured in tens of billions, not a gain measured in growth.
Even £62.5bn is the UK's bill alone
The department's figure covers Britain. It does not cover the climate cost the runway exports to everyone else. A new runway means more flights, which means more carbon, which does not stay inside UK borders once it is in the atmosphere. The government's own Climate Change Committee has said that if current trends hold, almost all UK CO2 emissions by 2050 will come from aviation, and that Heathrow's expansion alone would add more carbon by 2054 than any other sector of the economy.
Carbon dioxide is also not the whole story of what a plane does to the atmosphere. A peer-reviewed analysis in the journal Atmospheric Environment found that nitrogen oxides, soot, water vapour and other compounds released at cruising altitude raise the heating effect of flying by 115% over a 100-year horizon — and by 517% over 20 years. Neither the UK's Climate Change Act nor international treaties count these non-CO2 effects. The 20-year number matters because it is the window in which melting permafrost and drying wetlands can start releasing greenhouse gases of their own, independent of anything humans do next. Official accounting misses most of what the project actually warms.
"Sustainable aviation fuel" solves a different problem
The industry's standard answer to the carbon problem is sustainable aviation fuel. The Royal Society modelled what it would take to run existing UK aviation demand — not future growth, just what already flies — on fuel made from energy crops. The answer: roughly half of all UK agricultural land. Synthetic fuel made from captured CO2 and green hydrogen avoids the land question but creates an energy one; producing it at the scale current demand requires would need vastly more renewable electricity than the grid plans to build in the relevant timeframe. Neither path scales to match a third runway's added traffic without either starving farmland or outrunning the power supply.
The strongest case for the runway is still the growth argument, and it deserves to be stated at full strength rather than knocked down early. Airports create jobs during construction and after. Heathrow's position as a long-haul hub matters to trade and to UK firms that need direct flights to reach distant markets quickly. A hub that loses capacity to Paris or Amsterdam can lose business that never comes back. These are real costs of inaction, and the department's model does not pretend otherwise — it weighs them. It just weighs them against the 0.03% to 0.05% GDP gain, not the 0.43% headline, and against a local health toll affecting 3 million people. On that weighing, the government's own number says the costs win.
The clock, not the arithmetic, may decide this
Heathrow told the public last year its plans were "shovel-ready." It has since admitted the runway would take four years longer than claimed, pushing completion to 2039. Expansion remains official UK government policy on paper. But Andy Burnham, asked about it last weekend, called the runway "contested" and said it was "principally a matter for London and London MPs" — a notable retreat from a cabinet minister on a project that is, officially, national infrastructure policy.
If the runway is built on the current timeline, it opens into a climate regime that by 2039 may no longer tolerate the emissions it was designed to carry. If a future government takes the Climate Change Committee's warning seriously by then, the runway would need to close not long after it opens — a sunk cost with no operating life. Either the arithmetic is wrong, or the timeline makes the arithmetic irrelevant. Someone pays either way: the 3 million people near the flight paths pay in health, the country pays in the £23.4bn to £62.5bn the department itself calculated, and if the runway proceeds regardless, everyone downwind of the plane's final, uncounted 517% pays last and longest.