Royal officials said Tuesday that the couple chose the venue for its beauty, intimacy and historic royal connections, and the date because they wanted a spring wedding. It also the feast day of St. Catherine of Siena, whose name Middleton shares - though that is a coincidence.
The government said the day, a Friday, would be a public holiday.
Prince William's private secretary, Jamie Lowther-Pinkerton, said the couple chose the 1,000-year-old abbey in central London because despite its size - it holds 2,200 people - it has a sense of intimacy.
"Even at the altar, it seems like a parish church," he said.
It also has centuries of royal history. William's grandmother, Queen Elizabeth II, and her mother both married at the abbey, where British kings and queens are crowned.
William and Kate decided against celebrating their nuptials at St. Paul's Cathedral, where William's parents married in 1981.
The palace said the royal family and the Middletons would cover the costs of the nuptials, apart from security. There have been grumbles about the propriety of holding a lavish royal bash in the midst of economic austerity.
"All parties involved in the wedding, not least Prince William and Miss Middleton, want to ensure that a balance is struck between an enjoyable day and the current economic situation," Lowther-Pinkerton said.
He said the guest list had not been finalized, but that "we will have a full church."
He said William and Kate were playing "a very active role" in planning the day.
"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," he said.
"The couple are completely over the moon," Lowther-Pinkerton added. "They are on cloud nine."
Prime Minister David Cameron confirmed that the wedding day would be a public holiday in England, Wales and Northern Ireland. Scotland's government was also expected to announce a public holiday.
"The wedding of Kate and William will be a happy and momentous occasion. We want to mark the day as one of national celebration, a public holiday will ensure the most people possible will have a chance to celebrate on the day," Cameron said in a statement.