Barby Dashwood-Morris failed to secure proper planning permission before making a series of changes to a 14th century cottage in Hellingly near Eastbourne.
At the time, the 70 year old chaired the planning committee at Wealden District Council - the body she needed to gain permission from before making the alterations.
She faces up to two years in prison and an unlimited fine after admitting at Brighton Magistrates to six charges of demolition or alteration of a building affecting its listed character.
Saira Kabir Sheikh QC, prosecuting, was quoted by the Daily Mail as saying: "These works caused substantial harm which cannot be rectified."
Dashwood-Morris also admitted taking away a partition between a staircase and bedroom and removing a storage area in the £850,000 property known as The Priest House.
Stephen Whale, defending, said: "There was certainly no intention to commit offences."
Her husband, 54 year old Alan Proudfoot, admitted four counts of the same charge.
Priest holes which were hiding places was a regular feature of Catholic clergy homes during the reign of Elizabeth I when they faced persecution.
Dashwood-Morris will be sentenced on Wednesday.