A mistaken calculation in the design of a new state psychiatric hospital is pushing back the project’s completion date until late 2027, and will potentially leave New Hampshire taxpayers on the hook for an additional $5 million.

The error in the building’s foundation was found earlier this year, leading to a months-long work stoppage while engineers determined how to correct the problem.

A recently negotiated settlement calls for an insurance company for one of the project’s building contractors to pay the state $9.9 million to account for the foundation fix. But during Wednesday’s Executive Council meeting, officials said the delay — combined with the costs to redesign the hospital — could run closer to $15 million, or $5 million less than the insurance payment.

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