It’s a custom-designed Hamptons home — and it may never be completed if its architect has her way.

A Hamptons architect claims a rival hijacked her designs for a $1.3 million Water Mill house, according to a federal lawsuit.

Architect Kathryn Fee says she designed the gracious, shingled estate home for Diana Cochran in 2007, but pulled out of the 8,000-square-foot project when Cochran failed to fully pay her, according to the suit claims.

To complete the manse, Cochran then illegally handed Fee’s copyrighted plans over to architect Bruce Siska, who passed off the almost mirror-image designs as his own, the suit says.

Fee is suing both Siska and Cochran, demanding more than $1 million in damages for copyright and intellectual-property violations.

Siska’s work on the house appeared near completion yesterday — but Fee is asking a federal judge to order Cochran to stop construction, destroy the plans and pay the money owed.

Siska declined to comment and Cochran could not be reached for comment.