[7] Joseph had five sisters, four of whom became governesses, including two – first Frederica and then Margaret (Peggy) – to the Royal Household of King George III and Queen Charlotte.
On the death of Andreas Planta, Joseph returned home to London, and immediately succeeded his father as assistant librarian at the British Museum.
[3][10] Soon afterwards Planta was appointed as an assistant at the British Museum, he published a paper on the Romansh language of the area of Switzerland in which he was born in the Philosophical Transactions of the Royal Society of London.
Planta married Elizabeth Atwood (1744/5–1821) of the parish of St George's, Hanover Square, Westminster.
They had a daughter, Sophia (born April 1779) and a son, Sir Joseph Planta (1787–1847), MP for Hastings.