One of the most senior figures in local government, Ms de Groot revealed she will leave the agency next March as a result of the Local Government Association's group development strategy.
The strategy, announced by LGA chief executive Paul Coen last month, will see the IDeA brought 'in-house' to become part of a single support organisation.
"In light of the LGA's agreement on its group development strategy, now is an appropriate moment for the post of the executive director of the agency to be reviewed," Ms de Groot said, adding she hoped to remain in the local government sector.
LGA chairman Sir Simon Milton paid tribute to Ms de Groot, who ran the IDeA for five years.
"Lucy has been at the heart of local government's rapid improvement in recent years," he said.
"As chairman of the LGA and its improvement board I have worked closely with Lucy over the last four years and came to rely on her cool, calm and collected advice and on her positive, can-do approach to getting things done."