Bafin has 15 days to rule on the bid, and Hochtief shareholders then have a further four weeks to decide whether to accept the offer, a company spokesman said.
ACS, which already holds just under 30 percent of Hochtief, Germany's largest public works group, wants to take a controlling stake through an all-share share swap to create what would be one of the world's leading building firms.
However, the Spanish company offered a price below Hochtief's current market value, which Hochtief says does not represent fair value for the group. The German firm is trying to force the Spanish group to make a bid for Hochtief's Australian subsidiary Leighton, thereby increasing the price and possibly discouraging ACS.
ACS could instead carry out a capital increase of €78.5 million ($107.6 million), which is to be approved during an extraordinary meeting of shareholders on November 19.
Hochtief reported on Thursday a 17-percent third-quarter net profit gain and a reorganisation in Europe as it tries to fight off the takeover.
The company, anxious to prevent ACS from seizing control at a diminished price, said it was considering acquisitions in India and Canada and wanted to boost the installation of sea-based wind turbines, a market it described as "booming."
In Europe it plans to merge several divisions into a single unit in hopes of saving €40 million a year.
The company reported net earnings in the July-September period of €143 million ($196 million). Profit before taxes edged up 1.0 percent from third quarter 2009 to €430 million.