The Garden Museum has increased its redevelopment fundraising appeal by £300,000 after the coffins of four Archbishops of Canterbury were discovered in its grounds.
The museum, which reopened in 2017 and celebrates British gardens, successfully raised £7.9m for its redevelopment.
But the cost of the programme rose unexpectedly when workers lifting concrete slabs to create access for wheelchair users exposed a staircase to a lost vault containing 34 coffins.
Four belonged to Archbishops of Canterbury, including Archbishop Bancroft, who died in 1610 after overseeing the production of the King James Bible.
The vault was discovered in a former parish church that houses the museum adjacent to Lambeth Palace, the Archbishop of Canterbury's residence.
Christopher Woodward, director of the museum, last week appealed for an additional £300,000.
In an open letter on the museum's website, he said most of the sum had been met by the National Lottery Heritage Fund, formerly the Heritage Lottery Fund, and the Charities Aid Foundation.
But he said £58,000 was required from individual donors to pay for structural work on the vaults and sewer problems.
"We are one of very few British museums to operate without public funding, a founder’s endowment or being part of a university or bigger institution," Woodward wrote.
"But if we are to continue to grow we do need to pay the final instalments due to our builder, an excellent (and patient) family firm."