Heathrow's Third Runway Just Slipped Four Years to 2039, Here's What Actually Changes for Your Fare

Heathrow has told the UK government that its own promise to open a third runway by 2035 cannot be kept. The airport's new target is 2039, a four-year slip, and the reason is not politics but arithmetic: planning permission alone is not expected before 2029, and Heathrow says building the runway takes roughly a decade after that.

What Heathrow just told the government

Heathrow's answer to ministers is blunt: the 2035 date they backed is not achievable. Airport bosses say securing planning permission is realistically a 2029 job on its own, and that construction of a full third runway, plus the taxiways, terminal capacity and airspace changes that go with it, takes about ten years after approval is granted. Run that forward from 2029 and you land on 2039, not 2035.

Why the 2035 date was already the ambitious one

The 2035 deadline was never Heathrow's idea in the first place. Rachel Reeves set it during a major infrastructure speech in January 2025, when she threw the government's support behind expansion as a growth measure and talked about 100,000 new jobs. Heathrow publicly backed the project and the 2029 planning-permission milestone, but industry watchers were sceptical about 2035 even then, given how long the airport's last expansion fight over Heathrow's original third-runway approval dragged through planning inquiries, legal challenges and a change of government. The construction cost, widely put at around £49 billion, is itself large enough that assembling the financing and the regulatory sign-off in under a decade was always the harder part of the promise.

What this means if you're flying through Heathrow before 2039

Nothing changes at check-in tomorrow, but the underlying problem the runway was meant to fix, Heathrow running close to full capacity on two runways, stays in place for at least four more years than promised. That keeps pressure on long-haul fares and slot availability, especially in peak summer and autumn windows, because the airport cannot simply add more flights to ease demand. One widely reported estimate has suggested the eventual bill could add around £15 to the cost of a single flight if Heathrow recovers construction costs through landing charges, though Heathrow has not confirmed a figure. For context, there is no direct flight between London and Kathmandu today regardless of runway capacity, so British trekkers already connect through Doha, Dubai, Abu Dhabi, Delhi or Istanbul. Heathrow's delay does not change that routing, but it is one more reason connecting fares out of London are unlikely to get cheaper soon.

What happens next

The immediate milestone to watch is the 2029 planning-permission target, which will need to survive the kind of legal and environmental challenges that sank Heathrow's previous expansion attempt in the courts. Local residents' groups and climate campaigners, who opposed the last runway push, are expected to contest the plans again once a formal application is lodged. Until permission is granted, 2039 itself is still an estimate, not a guarantee.

Cover photo: Steve Daniels via Wikimedia Commons (CC BY-SA 2.0). Section photos: Tristan via Pexels (Pexels licence); Salva F. Ayala via Pexels (Pexels licence); AXP Photography via Pexels (Pexels licence); Josh Withers via Pexels (Pexels licence); Drinu Cutajar via Pexels (Pexels licence).