Coming soon to a city near you? The organic skyscraper that grows with demand as new tenants move in.
A reports shows the huge bill is "technically possible", but unprecedented in this country.
Transport bosses in have launched a bid to plug a £5.5m-a-year-black hole in London's flagship "Boris Bikes" scheme.
More doubt has been cast on the prospect of the so-called "Boris Island" Thames Estuary airport plan going ahead.
Favoured by London Mayor Boris Johnson, the estuary scheme, involving a new four-runway airport, is being studied by the Whitehall-appointed Airports Commission.
One of three reports prepared for the commission published today has said of the estuary plan: "Overall, the challenges to transition are considerable and amount to a significant cost and risk to the taxpayer in terms of commercial negotiations, infrastructure development and potential failure."
- London, 18.7 million
- Bangkok, 18.37 million
- Paris, 15.57 million visitors
- Singapore, 12.47 million visitors
- Dubai, 11.95 million visitors
London mayor, Boris Johnson said: "With nearly nineteen million visitors this year London is the world's favourite place to visit. Our city perfectly combines history, heritage, arts and culture, not to mention vast amounts of green space and major events that are the envy of the planet."
London has beaten Bangkok to become the most popular tourist destination in the world. The Global Cities Index says 18.7m visitors are expected to visit London during 2014. The Thai capital was the top destination in 2013.
Local Government minister Brandon Lewis appeared to dismiss the idea.
There is no public appetite for a barrage of new stealth taxes on hard-working people and local firms, which would force up the cost of living and destroy jobs.
A Treasury spokesman said stamp duty land tax and business rates were important sources of Government revenue, raising several billion pounds each year to pay for the essential services.
Boris Johnson said: "There is a now growing and welcome cross-party consensus that recognises the economic benefits of giving greater control of taxes raised in English cities back to those cities, which is very difficult for the Treasury to ignore.
London's mayor added: "The modest reforms that I and others have been campaigning for and that are endorsed in this report, would provide England's cities with the means, incentives and crucially the stability of funding to deliver much needed jobs, growth and infrastructure."
– Clive Betts, Chair of the Communities and Local Government Committee
If the citizens of New York, Frankfurt and Tokyo can be trusted with tax-raising powers, why not the people of London, Greater Manchester or the North East?
Local areas know best how to stimulate their economies. With a wider range of revenue streams at their disposal, they would be able to invest in infrastructure and projects that mattered locally - without having to rely on or wait for handouts from central government.
London mayor Boris Johnson has won the backing of an influential group of MPs in his bid for more tax-raising powers.
The Communities and Local Government Committee is believes cities like London and Manchester should have similar powers to the Scottish Parliament and Welsh Assembly. The committee wants ministers to hand over property taxes like stamp duty.
- The survey comes in the wake of an official report from the government's Airports Commission into the environmental impact of a four-runway hub in the Thames
- The feasibility study claimed the creation of new habitats away from a so-called "Boris Island" airport could cost £2bn