The young couple on Tuesday rejected the notion that austere times will force them to pare down the royal festivities. The church itself is free, at least.
William and Kate ended days of suspense by choosing the abbey, a grand venue where members of the House of Windsor have been crowned, married, mourned and buried. The abbey was also the site of the funeral for William's beloved mother, Princess Diana.
Palace officials said the two were on "cloud nine" with their wedding choice and want the nation to share their joy.
"We know that the world will be watching on the 29th of April, and the couple are very, very keen indeed that the spectacle should be a classic example of what Britain does best," said William's private secretary, Jamie Lowther-Pinkerton.
William and Kate chose the centuries-old abbey in central London because despite its size - it holds 2,200 people - it has a sense of intimacy, particularly at the altar, which stands at the head of a smaller, partly enclosed space called the sanctuary.
The date - after Lent and Easter but before the 90th birthday of William's grandfather, Prince Philip - allows the pair to have a spring wedding. It also is the feast day of St. Catherine of Siena, whose name Middleton shares.
Best of all for Brits, the day, a Friday, will be a public holiday.
St. James' Palace, the offices of Princes William and Harry, stressed that the Middletons and the royal family would pick up the bill for the wedding - the flowers, the reception, the honeymoon - everything apart from security and transport costs. The couple say they are mindful of the country's tough times.
Much public money already flows to Britain's royal family. Taxpayers sent the royal household 38 million pounds ($60 million) last year, and the queen also receives 7.9 million pounds ($12.5 million) each year to pay for staff and other costs.
However, the family earns millions a year from its vast land and property portfolios. Prince Charles' large estate, the Duchy of Cornwall, earned him 17 million pounds ($29 million) last year. The royal family pays millions in tax on its earnings.
But while all parties involved in the wedding want to ensure that a balance is struck between an enjoyable day and current budget woes, analysts such as Patrick Jephson, longtime private secretary to the late Princess Diana, say the austerity argument "doesn't cut it."
"This is a future king and a future queen, this is the most famous young royal in the world, and it will indubitably be compared to his mother's wedding. So for all these reasons, the palace won't want to be seen as downgrading it," Jephson said.
He wondered what aspects of the pageantry of a royal wedding could be cut without disappointing masses of people.
"What do you want to lose?" he said. "The number of bridesmaids? The ride in the coach? The mounted escort? The Beefeaters? You can do it, but you'll regret it, and people around the world will be disappointed.
"This is about making people happy, not about wearing a hairshirt," he said.
There's no charge for the venue, but there will be competition for space. Many of the abbey's seats will be allocated for diplomatic and protocol reasons, leaving relatively few for the people William and Kate really want to invite.
People around the world are awaiting the spectacle, including Reza Mohammadkhani, 41, from Los Angeles, who was outside the historic church on Tuesday.
"I love the whole kingdom thing," he said.
The abbey has centuries of royal history. Queen Elizabeth II was married there, as was her mother.
But the newlyweds-to-be also have to confront a host of difficult memories associated with the abbey. William was 15 when he walked behind his mother's coffin at Diana's funeral in the abbey in September 1997.
"It must be hard for William to be in the abbey, but it's just another way to keep his mother with him I suppose," said Jackie Lentendre, 33, a mother from West Brompton in southwest London.
Several other members of William's family have wed there, including the queen's sister, Princess Margaret, in 1960, William's aunt Princess Anne in 1973 and his uncle Prince Andrew in 1986. All three marriages ended in divorce.
No matter what, it beats getting married at St. Paul's, a gorgeous London landmark tarnished by echoes of Prince Charles' and Diana's fairytale wedding in 1981 and their subsequent unhappy marriage.
"I think it's a good choice for her not to go in Diana's footsteps," said Marlene Bache, a 38-year-old Danish tourist outside the abbey. Diana "was an icon, and now she's dead. I think (Kate) should be herself."
The choice - and the date - are William and Kate's first moves to put their own stamp on their big day. The couple, who Lowther-Pinkerton described as being "over the moon," are taking an active role in planning all the details.
"I've never seen two happier people," he said.
Greg Katz, Jill Lawless, Gillian Smith and David Stringer in London contributed to this story.