Back in 1994, London Borough of Lambeth planners had to scratch their heads over just that question.
The trouble with the London Eye, as it became known, was that the council had no guidelines to cover 150m Ferris wheels. It pondered various options, including basing the fee on the wheel's projected turnover or the area taken up by the supports, because these were the only parts of the structure that touched the ground.
After much deliberation, designers David Marks and Julia Barfield were charged the minimum fee of £67.50. "That was just as well," Barfield acknowledges. "We could not have afforded any more." This and other anecdotes about the landmark that London never knew it needed are contained in a new book.
Overcoming planning obstacles was one of the wheel's greatest challenges. For a start, the structure just crept into a strategic view of St Paul's Cathedral. "We had this acronym NITVOSP, which stands for 'Not in the view of St Paul's'," recalls wheel supporter Archie Galloway, a committee member for the City of London on the London Planning Advisory Committee.
Those behind the project went to extreme lengths to show that the wheel would not block the sacred view, making calculations that took the curvature of the earth into account. They concluded that St Paul's would be visible beneath the wheel, leaving planners with a conundrum - did the view extend to heaven?
Luckily for London, the planners decided to take a lenient view. However, the wheel's proponents faced one last hurdle. If the scheme was called in for public inquiry it would never be ready in time for the millennium, rendering it unviable.
To their relief, former environment secretary John Gummer waved the scheme through. "There is not much point in being secretary of state if you cannot do one or two things where you think you are right and everybody else is wrong," he commented.
Eye: The Story Behind the London Eye is priced £24.95 and available from Black Dog Publishing.
One minute we were going to be charged for the rubbish that we throw away, then we were not, but now we are to be penalised again. We have all heard of the concept of bouncing ideas around the place, but this is simply ridiculous.
Ideas for a "save as you throw" scheme as councils call it, or "pay as you throw" according to enraged tabloids, were allegedly dumped last week after intervention from Number Ten.
However, this was fiercely denied by a DEFRA press officer, who claimed that the announcement was merely about the consultation results and the government had no idea when such a policy would be announced.
Imagine Diary's surprise when the power for councils to carry out the proposal emerged in the draft climate change bill later the same day.
A DEFRA spokeswoman insisted that it had only been included in case it was needed in the future and that "no decision has been made", before mumbling something about it being a "minor detail".