The renovation of the Dutch parliamentary complex will not be finished until 2031 and the total bill is expected to be six times the original estimate, planning minister Mona Keijzer has admitted.

It is the latest setback for a project that was originally due to take five and a half years and cost €475 million. Keijzer said the latest schedule was “realistic” now that the surveying phase has been completed.

The latest estimate of €2.7 billion reflects the fact that the project has proved to be much more complex than originally thought. MPs and their staff moved out in 2021 in the expectation that they would be able to return before the end of 2026.

Many areas of the building were damaged by mould and leaks, specialist workers were in short supply and several archaeological discoveries on the site held up the process.

Earlier this year archaeologists found evidence that the Binnenhof was the site of an extensive ruler’s palace in the late 13th century, 30 years earlier than previously thought.

Excavations also uncovered a 13th-century quay wall that would have held back the water from the canal that used to run alongside the Binnenhof and a cellar used to dump refuse, which contained evidence that the complex was patronised by wealthy people.

Keijzer said the complexity of the building, with a total area of 90,000 m2, 4,000 spaces and various architectural styles, made the project a huge challenge.

“Restoring the foundations, contaminated land and archaeological discoveries are all issues that have caused delays,” she said. “And the wage bill has risen because it’s taking us two years longer.”