New York ? Construction costs for the World Trade Center memorial have risen to an estimated $1 billion – twice as much as officials had planned to spend.

Last year, rebuilding officials said it would cost $490 million to build the memorial and an adjacent museum, which is slated to take up eight acres of the 16-acre site when it opens in 2009. Preliminary site work began in March.

Contractors this week issued a revised estimate of $1 billion.

The new estimate includes $700 million for memorial and museum construction and $300 million to prepare the site, such as work shoring up a retaining wall and other infrastructure costs.