From iconic London landmarks to groundbreaking railways, big money is being spent on the UK’s infrastructure. But if the past 50 years have shown us anything, it's that megaprojects are vulnerable to escalating costs and dragging delays...

Discover the most expensive megaprojects in the UK and find out how much they would cost to build in today's money.

Originally built to house the government-backed 'Millennium Experience', the Millennium Dome opened on 31 December 1999 and closed on 31 December 2000.

Planning for the project began in 1996 under the government of then-Prime Minister John Major. The design was selected that same year – basically an oversized marquee 365m in diameter (one metre for each day of the year) supported on 12 yellow towers (one tower for each month). Construction started in 1997 and even though a change in government put the project’s funding in jeopardy, it still finished in summer 1999, well ahead of the New Year deadline.

However, the exhibition soared past its £350 million budget, ultimately costing a grand total of £789 million, which is £1.7 billion today. This includes works to make the dome accessible by tube, decontaminate the ground and more. The structure itself (pictured here in 2006) cost a relatively mere £43 million.

Making matters worse, the year-long exhibition only attracted 6.5 million attendees, despite organisers predicting 12 million visitors. The venture lost money and, sitting empty on London’s Greenwich Peninsula, the Millennium Dome cost taxpayers about £3 million per year to maintain.

By 2004 it was being used as a homeless shelter before telecoms company O2 bought the naming rights in May 2005 for £6 million. Construction followed to redevelop the site into the entertainment centre now known as The O2 Arena. These works are thought to have cost another £600 million, which is nearly £1.1 billion today.

Thames Water’s 'super sewer' project necessitated the construction of a tunnel that was 15 miles (24km) long and 24 feet (7m) wide, snaking below the River Thames from Acton in West London to Abbey Mills in East London.

With the city’s outdated sewer network dating back to the Victorian era, the Thames Tideway project aimed to divert and collect sewage that would have otherwise entered the river. Construction started on this massive drainage pipe in 2016, after securing planning approval in 2014.

At that point, the project had been priced at £3.52 billion and was scheduled for completion within 10 years’ time. However, the COVID-19 pandemic added nine months to the estimated deadline due to lockdown-related delays, pushing the sewer’s start date from 2024 to early 2025. By September 2020, these delays had added a massive £233 million to the project’s budget.

While the super sewer project has since become fully operational, the final bill has ended up costing closer to £5 billion, according to the BBC. Thames Water’s customers are footing the bill for these overruns, with consumer prices increasing by £25 a year.

As airports across Europe expanded in the later part of the 20th century, the British Airports Authority (BAA), now known as Heathrow Airport Holdings Limited, had its eye on boosting its status as one of the world’s hubs for air travel. To do this, it needed to expand its terminal capacity to meet its runway capacity. It submitted a planning application to build Terminal 5 in 1993.

Infamously, the public inquiry into the project – which started in 1995 – became the longest in British history, lasting 525 days over three years and 10 months before it ended in 1999. Construction on the terminal started in July 2002.

Building a new terminal at the UK’s busiest airport required more than just a flashy glazed façade and new luxury shops in the departure lounge. Thousands of workers diverted two historic rivers, built a new spur road from the M25, extended the Heathrow Express and the Piccadilly Line, and mined an 8-mile- (13.5km-) long tunnel system for the terminal – below live airport operations including flights. All of this took place within just six years.

The new terminal opened to passengers in March 2008 on time and within budget at a cost of £4.3 billion, which is £7.3 billion today.

Heathrow was ranked the fourth busiest airport in the world in 2024 and says it’s running at 99%. It will submit a proposal this summer for building a third runway.

Returning to the Greenwich Peninsula: while the Millennium Dome rose above ground in the late 1990s, a second major megaproject was being constructed underground.

Extending the Jubilee line had been in discussion ever since it opened in 1979, but the project stalled for decades until redevelopment started at Canary Wharf. The massive regeneration scheme at London’s former docks spurred the extension project and in 1993, construction started to bring the line 10 miles (16km) underground from central London to the Millennium site at the Greenwich Peninsula.

The work was expected to finish by 1998 at a cost of £1.9 billion. But by early 1999 the extension wasn’t nearly ready and costs had soared to £3.3 billion, making it one of the world's most expensive transport projects.

The image shows the Jubilee line under construction in 1973.

An independent expert review of the spiralling project reported a failure at London Underground Limited to deliver a project of this scale. Meanwhile, financial skirmishes between the project organisers and subcontractors took their toll. A series of redesigns ate up much of the budget as well.

To make matters worse, a section of tunnel being excavated for the Heathrow Express collapsed in 1994 and many of the Jubilee Line Extension’s tunnels were using the same technique for excavation. Work was suspended for 18 months on three contracts while the other project was investigated to ensure the construction method was safe.

There was a hard deadline too; on 31 December 1999 the extension needed to be operational to take people to the Millennium Dome. American construction powerhouse Bechtel was brought in to oversee the remainder of the works. The project opened in phases from May to December 1999 and the total cost rose to £3.5 billion, which is £7.75 billion with inflation.

The iconic Battersea Power Station powered post-war London, but the coal burning plant faced rising costs and falling output by the 1970s. In 1983 the brick edifice along the River Thames was shut down and then abandoned for several decades.

A group of investors purchased the station in 2012 with ambitious plans to redevelop not just the former plant, but the entire 42-acre site. This would include new neighbourhoods, retail opportunities and restaurants, and office space built in eight different phases and designed by many of the world’s most prestigious architecture firms.

The centrepiece of this massive regeneration scheme is a complete renovation of the power station to restore many of its Art Deco features as well as transforming the plant into a mixed-used facility that would hosue a shopping mall, luxury flats and tech firm Apple.

By 2017, the first residents moved into parts of the newly-developed neighbourhood, but the Battersea Power Station phase of the project had been plagued by delays that of course had a knock-on effect for costs. The station’s rehabilitation work increased from £750 million to £1.15 billion. It would eventually open to the public in October 2022 with a price tag of £1.3 billion.

While all eight phases of the development are yet to be completed, the total cost is expected to be around £9 billion once the work is finished.

Ideas for building a tunnel beneath the English Channel to connect the UK and France had long been floated, and even attempted, before construction actually moved forward in 1988. The route, between Calais in France and Folkestone in Kent, is 31.5 miles (51km) long, making it the world's longest undersea tunnel.

It's actually three separate tunnels – one for each direction of train travel and a service tunnel – that were mined through bedrock beneath the sea. Construction took place from both sides of the tunnel, with workers celebrating their meeting point on 1 December 1990

Construction finished in 1993 at a cost of £4.65 billion, or £12.1 billion when adjusted for inflation, making the Channel Tunnel the largest privately-financed transport megaproject of the 20th century.

However, Eurostar trains in the UK faced speed restrictions, since they had to use tracks ill-equipped for high-speed rail that they shared with local train services between the tunnel and London’s Waterloo Station. In 1996 Parliament kickstarted the Channel Tunnel Rail Link, also called High Speed 1, which would include a new rail terminus at London St. Pancras as well as extensive viaduct, bridge and tunnel construction for a 70-mile (113km) high-speed rail line.

The project reached completion in 2007 at a cost of £6.2 billion, or £11 billion today, which is nearly as much as the Channel Tunnel itself.

Hosting the Summer Olympics in 2012 gave London the opportunity to regenerate an industrial wasteland and boost the economy of Stratford, East London. Along with all the sports facilities needed for the games, the site required major decontamination works in order transform the derelict land into the 617-acre Queen Elizabeth Olympic Park. There were also 10 new rail lines to enable athletes and spectators to attend the events.

None of this came cheap, and it definitely did not come in on budget. Originally priced at £2.4 billion, the total cost of hosting was three times higher at £8.77 billion, or £13.2 billion today. The mayor at the time of the bid, Ken Livingstone, argued that East London would never have seen such a significant investment in regeneration if it weren’t for the 2012 Olympics

The cost of hosting the Olympics is undoubtedly steep, but according to government reports the event actually generated £14 billion of trade and investments for the UK economy between 2012 and 2014, which is £20 billion when adjusted for inflation.

Many of venues built to host London's Olympic Games live on for community and professional sporting events. Meanwhile, the wider neighbourhood has flourished with new cultural and educational institutions as well as retail and business facilities, all of which have increased opportunities in the area.

However, much of the housing developed as part of the regeneration programme has been criticised as it's unaffordable for many of East London’s residents.

Dating back more than 50 years, the concept for an east-to-west rail connection across the heart of London had been long discussed. It wasn’t until 2008, though, that Transport for London secured support for the project and construction could move forward, at an estimated cost of £15.9 billion.

Work started in 2009 for this massive infrastructure undertaking and immediately fell prey to budgetary issues, with the coalition government slashing funding to £14.8 billion in 2010. Digging 13 miles (21km) of tunnels below the city lasted until 2015 and a year later the Queen visited the project to celebrate Crossrail’s progress, which was expected to be on schedule for its 2018 completion. At this time it was announced the project would be named the Elizabeth line in her honour.

While Elizabeth line trains started service on overground tracks outside of London in 2017, the long-awaited opening date didn't happen in 2018. Instead, Crossrail bosses admitted to delays as well as a £600 million budget overspend, promising service in 2019, which became 2021 and finally May 2022 – timed to coincide with the Queen’s Platinum Jubilee.

Delays have been attributed to a whole host of problems, from uncovering plague cemeteries to health and safety concerns for workers on site, and that’s before the pandemic even hit. When all was said and done, the total bill came to £18.8 billion.

EDF Energy is building the UK’s first nuclear power station in 30 years, located in Somerset. Approved by the government in 2016, at that time the £18 billion project comprised two reactors that would generate enough power for six million homes over 60 years when completed in 2025.

Everything was looking good. The concrete base of the first reactor was completed in June 2019 and the base of the second reactor was finished, on time, in the summer of 2020. However, in a statement released in January 2021, EDF announced that it didn't expect Unit 1 to start generating electricity until June 2026.

Many of the plant’s major components have been manufactured in France and need to be shipped to the UK. With disruptions like the pandemic, and supply chain costs spiralling due to current events like the war in Ukraine, the project is now set to be one of the UK’s biggest megaprojects as well as the world's most expensive nuclear plant. Further exacerbating the situation, in 2022 EDF moved the deadline to 2027, citing labour shortages.

Budget revisions over the last five years have skyrocketed, with the most recent estimates reaching £46 billion and power generation expected by "the end of the decade."

Few will be surprised to learn High Speed 2 (HS2) is the UK’s most expensive megaproject ever. In fact, it’s one of the most costly construction projects in the world.

Looking to add capacity to north-south rail service across England, the Department for Transport announced the project 2010. This Y-shaped high-speed rail service would be built in three phrases to connect London to Manchester and Leeds, via Birmingham. An incredibly ambitious project, initial estimates suggested the first 140 miles (225km) from London to Birmingham and four new stations would cost £20.5 billion and open in 2026. Much like its predecessor High Speed 1, this would include substantial bridge and tunnel work.

Construction hadn’t even started before HS2 blew the budget. A review of the project in 2020, the year it broke ground, revealed the railway was expected to cost a jawdropping £106 billion.

The government cancelled the next two phases in 2021 and 2023 in attempts to rein in the costs. However, there’s still no clear indication of what the megaproject will cost. In summer 2024, the HS2 told the Department of Transport to expect a range between £54 billion and £66 billion but added this was only a rough calculation. A report published this February suggested inflation could further drive costs to £80 billion.

The opening date is even harder to nail down, with a timeline of between 2029 and 2033. However, construction continues. The project reports 75% of tunnelling is completed and work has started on most of the viaducts and bridges, so watch this space...

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