The projected cost of building the World Trade Center memorial complex at ground zero has soared to nearly $1 billion, according to the most authoritative estimate to date.

Rebuilding officials concede that the new price tag is breathtaking — "beyond reason" in the words of one member of the World Trade Center Memorial Foundation board — and it is sure to set off another battle over development at the 16-acre site, with calls to cut costs, scale back the design or even start over.

The foundation, which had planned to start construction in March, has already quietly broached the possibility with some victims' families of moving important parts of the memorial out of the twin towers' footprints to ground level.

Only two or three years ago, the problems faced by the memorial, the spiritual centerpiece of the site, would have been unimaginable. The underground complex, with its pools, waterfalls and galleries, was the product of a worldwide design competition that drew 5,201 entries and inspired tremendous public passion.

It was supposed to be immune to the controversies that had engulfed the commercial rebuilding at the site, with its completion assured by an outpouring of good will and open checkbooks. But fund-raising has lagged, with just $130 million raised from private contributions.