Inevitably, they are rich. Extremely rich. Very, very rich. There are the things you can see: the castles, the country estates and the top-of-the-range cars with which to drive between them. There are enough horses to stable an entire stud. There are the exquisite jewels and enormous gems glittering at state banquets.

There are the things you can’t see, too: the sprawling tax-free hereditary property firms and the share portfolio acquired with their annual dividends. There are the works of art in the family’s private collections, rarely, if ever, publicly exhibited. So if we already know all this, why, on the eve of the coronation of King Charles III, investigate the wealth of the British royal family?

The first and simplest answer is that we simply ought to know. From 10 Downing Street to your local district council, the private finances of public servants are fair game for scrutiny where they are derived from public funds. If anything, the need for clarity here is greater: Rishi Sunak and the mayor take money from the public purse only temporarily. The life of the king is funded at taxpayer expense from birth to death.

The Prince and Princess of Wales – then known as the Duke and Duchess of Cambridge – take part in a royal procession to Buckingham Palace in 2012. Photograph: Cpl Lynny Cash RAF/MOD/EPA

The second answer is that, to be blunt, the Windsors act as if they have something to hide. The finances are murky as hell, and structured according to a formula that means their annual handouts can only go up, never down. For several decades their shareholdings were owned through a secret shell company at the Bank of England that was immune from national transparency law. The judiciary has sealed their wills from public scrutiny in secret hearings for the past century. This is not the behaviour of a family relaxed about the prospect of an informed citizenry.

The question is not merely how wealthy he is, the question is: how much of the king’s private wealth is derived from his public role? Only once Britain has an answer to that can it discuss the most important question of all: is this really a good way to spend public money?

 Just how rich is he?

The first problem with valuing King Charles III’s private wealth is that nobody knows precisely how rich he is, probably including him. Centuries of dynastic marriages with the British aristocracy and the royal houses of Europe have produced a family whose personal history is intertwined with Britain’s national story, and whose personal wealth is inextricable from their public position.

The Cullinan III and IV brooch. Photograph: Peter Jordan/PA

Take the Cullinan III and IV diamonds: two gems cleaved from the largest diamond ever discovered and presented as a gift from the South African government to the king’s great-grandmother Queen Mary in 1910. She wore them in her coronation crown and later as a pair in a brooch, which was left to the queen and then, presumably, the king (the family pays no inheritance tax as long as assets are bequeathed monarch-to-monarch). Elizabeth inherited the brooch in 1953 when Mary died, rather than in 1952 when she became queen. That indicates that the jewels are private. But by modern standards, they were clearly official gifts, so shouldn’t they be national heritage?

The second problem is the culture of extreme deference and secrecy that surrounds the royal family. This is partly a result of a media environment that covers the Windsors as dysfunctional celebrity aristocrats, rather than figures of serious political or constitutional significance. It has created a culture in which the royal institution itself is above normal standards of scrutiny, and where any remotely uncomfortable or probing question, no matter how valid, is ignored or dismissed by default.

Palace responses to questions about the king’s wealth ranged from “we’re too busy, perhaps we’ll respond next week” to “that’s really none of your business”. Questions about which jewels were owned by the state went unanswered. At one point the press office announced it was too busy to respond to further questions until after the coronation. Issued the same day as a palace statement about a celebratory quiche, this did not feel especially convincing. When pushed, the king’s spokesperson said: “Your figures are a highly creative mix of speculation, assumption and inaccuracy.”

 What did we find?

The short answer is that, all told, we think Charles is worth at least £1.8bn. But it’s the longer answer, about how the king came to be worth so much, that is more interesting. Excursions to the parliamentary archives to dig out accounts for their hereditary estates unearthed cash payments to the king and his late mother dating back to 1952. That the king and his mother together pocketed more than £1.2bn in annual dividends from the estates (adjusted for inflation) was just as breathtaking as the discovery that they were paying themselves about 10 times more by the end of her reign than they were at the start.

Queen Elizabeth II and the then Prince Charles during the state opening of parliament in 2019. Photograph: Tolga Akmen/AFP/Getty Images

Then you find the gifts. There are the mint-condition stamps from the governments of Cambodia and Laos that have, it appears, been subsumed into the family’s private stamp collection (estimated value: £100m). There are the works of art: an illustrated Bible from the modernist master Marc Chagall, or an etching from Salvador Dalí, both presented to Prince Philip during official visits overseas and both subsequently exhibited as being part of his “personal collection”. The monarchy’s own policy says that gifts from other monarchs “as a general rule” enter the national collection of state heritage, but two diamond necklaces given to the late queen by Saudi kings are mysteriously absent.

You also find lingering traces of the dark moments in Britain’s history. A 100-year-old memo in the British Library records the looting of the city of Lahore in 1849 and the theatrical presentation of plundered diamonds, rubies, pearls and emeralds to an ecstatic Queen Victoria. Once the pride of the British empire, the Koh-i-noor diamond now sits in a vault in some strange, disgraced hinterland: to wear it would be too offensive, to return it to its rightful owners too humiliating.

The historian Brooke Newman discovered a page in a 17^th-century share register documenting the transfer of £1,000 of shares in the Royal African Company from the slaver Edward Colston to William III, the first of 14 monarchs to either cultivate the slave trade or harvest its profits. Perhaps revealingly, this was the one dimension of our inquiries to startle the palace into issuing an extended public statement describing how “profoundly seriously” Charles considered the matter.

Statue of 17^th-century slaver Edward Colston plunges into the water in Bristol in 2020. Photograph: Social Media/Keir Gravil/Reuters

 Does the king need a state grant?

The immediate political question flowing from the king’s wealth is obvious. If the family is this rich, why does it need an annual sovereign grant (currently £86m a year) from parliament? Why are the multimillion-pound payments from the hereditary estates not paid to the Treasury, or at least taken into consideration when setting their annual stipend? Alternatively, if the estates are genuinely private assets, why don’t they pay corporation tax?

After that come much more difficult questions about how a constitutional monarchy behaves in a modern society. Behind the pomp, the wealth and the opulence is a lurking sense of a monarchy designed for the more deferential age of the 19^th century, when its purpose was fuzzy but simple: unite and represent the nation.

This, however, is a 21^st-century coronation. Whether multimillion-pound salaries and disdain for difficult questions can really unite and represent the values of a modern democracy remains to be seen.

David Pegg Investigations correspondent