HS2: How a £37.5bn National Railway Became a £100bn Stub
HS2 is the clearest live example of the Iron Law of Megaprojects in Britain. It was sold in 2009 as a £37.5bn national network reaching Manchester and Leeds. The northern legs were cancelled, yet the surviving London-Birmingham section alone is now estimated at £87.7bn to £102.7bn, and first trains have slipped from 2026 to somewhere between 2036 and 2039.
How much has HS2 gone over budget?
The full HS2 network was estimated at £37.5bn (2009 prices). After the Manchester and Leeds legs were cancelled, only London to Birmingham remains, and in May 2026 the Department for Transport estimated that single section at £87.7bn to £102.7bn (2025 mixed price base). That is roughly double its own 2020 funding envelope of £44.6bn, and up to about 2.7 times the original full-network figure for a fraction of the route.
The cost history, step by step
HS2's numbers are easy to misread because the price base and the scope both keep changing. The honest way to read them is to hold the two comparisons separate: what the whole network was meant to cost, and what the surviving Phase 1 was meant to cost on its own.
| Date | Scope | Estimate (price base) |
|---|---|---|
| 2009 | Full Y-network (London-Birmingham-Manchester-Leeds) | £37.5bn (2009) |
| 2013 | Full network | £50.1bn (2011) |
| 2015 | Full network (funding envelope) | £55.7bn (2015) |
| 2019 | Full network (Oakervee / Cook stocktake) | £72bn-£98bn (2015) |
| Apr 2020 | Phase 1 (London-Birmingham) Notice to Proceed | £40.3bn target, £44.6bn envelope (2019) |
| Oct 2023 | Phase 2 (Birmingham-Manchester) cancelled; London-Birmingham only | - |
| May 2026 | Phase 1 (London-Birmingham) only, post-reset | £87.7bn-£102.7bn (2025) |
Two things stand out. The route got shorter, losing the entire northern half, while the cost of what is left more than doubled its 2020 funding envelope. And the single London-Birmingham line now costs more than the whole national network was ever supposed to.
How much has HS2 cost so far?
The figures above are forecasts of the final bill. The money actually spent to date is smaller. In its May 2026 six-monthly report to Parliament, the Department for Transport confirmed that £44.2bn had been spent on the HS2 programme in nominal (cash) prices as at 31 March 2026. Set against the £87.7bn to £102.7bn now forecast to complete the London-Birmingham line, that puts the programme roughly halfway through its expected spend with first services still a decade away.
One caveat matters when quoting these two numbers together: the £44.2bn is a cash total that mixes prices from every year since work began, while the £87.7bn to £102.7bn forecast is stated on a fixed 2025 price base. They are not a clean subtraction, so the honest reading is "about half the forecast has been spent", not a precise remaining balance.
Spend to date: HS2 six-monthly report to Parliament, May 2026 (Department for Transport). Figure is nominal (cash) prices as at 31 March 2026.
Why the cost rose
The National Audit Office and successive government reviews point to a consistent set of causes, most of them textbook optimism bias and strategic misrepresentation of the kind reference class forecasting is meant to correct:
- Politically low initial estimates. The early figures were set to secure approval, not to survive contact with the engineering. This is the same pattern the NAO identified on Crossrail and the Big Dig.
- Underestimated works. Tunnelling, viaducts such as the Colne Valley (Britain's longest railway bridge), and the scale of environmental and planning mitigation all cost far more than the business case assumed.
- Delivery inefficiency. The May 2026 reset attributed a large share of the increase to inefficiency within HS2 Ltd and its supply chain, alongside weak cost control.
- Inflation not captured. Earlier forecasts did not consistently reflect construction inflation, so real cost growth was masked until it was re-based upward.
A fundamental reset of the programme began in June 2025 under a new chief executive; the May 2026 Accounting Officer assessment is the first full re-baselining to come out of it.
What HS2 teaches about budget overruns
HS2 is a live illustration of Bent Flyvbjerg's Iron Law of Megaprojects: over budget, over time, over and over again. It also shows why the choice of baseline matters so much. Cutting scope did not cut cost, because the fixed early works, the tunnels and the reset overheads all stayed. Anyone forecasting a project of this class should have started from the reference class of comparable rail megaprojects, where a 40%-plus real cost overrun is the base rate, not the worst case.
Sources
- DfT (May 2026). HS2 Phase 1 Accounting Officer Assessment (£87.7bn-£102.7bn; opening 2036-2039).
- DfT (May 2026). HS2 six-monthly report to Parliament (£44.2bn spent to 31 March 2026, nominal prices).
- UK NAO (2024). HS2: update following cancellation of Phase 2.
- House of Commons Library (CBP-8601). HS2: the business case, costs and spending.
- Institute for Government. HS2: costs and controversies (cost timeline).
Related
Frequently Asked Questions
How much has HS2 gone over budget?
HS2 was first estimated at £37.5bn (2009 prices) for the full Y-network linking London, Birmingham, Manchester and Leeds. The northern legs were cancelled in stages, and since October 2023 only London to Birmingham survives. In May 2026 the Department for Transport put the cost of that single remaining section at £87.7bn to £102.7bn (a mixed 2025 price base). That is roughly double its own 2020 funding envelope of £44.6bn (2019 prices), and up to about 2.7 times the original full-network estimate for a fraction of the railway.
How much has HS2 cost so far?
To 31 March 2026, £44.2bn had been spent on the HS2 programme in nominal (cash) prices, according to the Department for Transport's May 2026 six-monthly report to Parliament. That is set against a forecast of £87.7bn to £102.7bn (Q3 2025 price base) to finish the surviving London to Birmingham line, so the programme has spent roughly half of its expected total with first services still years away. The spend-to-date figure is a cash number and the forecast is stated in a fixed price base, so the two are not a like-for-like subtraction.
Why did HS2 cost so much more than planned?
The National Audit Office and successive government reviews attribute the growth to optimistically low initial estimates set for political approval, underestimation of the engineering work (tunnelling, viaducts and environmental mitigation), inefficiency within HS2 Ltd and its supply chain, scope and specification changes, and inflation that earlier forecasts did not consistently capture. A June 2025 reset under a new chief executive re-baselined the programme upward again.
When will HS2 open?
The May 2026 Accounting Officer assessment expects first services between Old Oak Common in west London and Birmingham Curzon Street to begin between May 2036 and October 2039. Services into the London terminus at Euston, via Handsacre Junction to the north, are expected between May 2040 and December 2043. The original target was 2026.