* ACS holds 27.72 percent Hochtief voting rights
* Has another 2.5 percent tendered so crosses key 30 percent line
* Can now buy shares in market; fight for control ahead
* ACS shares up 0.1 percent, Hochtief up 0.3 percent
(Recasts lead, clarifies sourcing in third paragraph)
By Michelle Martin and Jose Rodriguez
FRANKFURT/MADRID, Dec 23 (Reuters) - Top Spanish builder ACS has cleared the way to own 30 percent of German peer Hochtief, a threshold that allows the Spanish predator to build a controlling stake in the open market.
ACS, saddled with a heavy debt load of more than 9 billion euros ($12 billion), will be strengthened if it can consolidate Hochtief's strong balance sheet as it moves to reduce its exposure to Spain and gain size to win big international deals.
ACS said on Thursday it had built up its stake to 27.72 percent of the voting rights in Germany's biggest builder.
A source close to the deal said that figure did not include a 2.5 percent stake in Hochtief that Southeastern Asset Management has pledged to tender to ACS's takeover bid.
Under German regulations, a 30 percent stake gives ACS the right to buy shares in the market without renewing its bid and without making further disclosures on its holding in Hochtief as it tries to build a controlling stake of more than 50 percent.
Neither ACS, headed by Real Madrid soccer club president Florentino Perez and with projects from a dam in Puerto Rico to a Spain-France high-speed train, nor Hochtief would comment further on the matter.
Hochtief's real estate, facilities and infrastructure projects stretch across Asia, Australia, the United States and the Gulf region, an opportunity to diversify geographically for the Spanish builder whose portfolio is centred on Europe.
All eyes now turn to when and how ACS might purchase the shares needed to take it to a controlling 50 percent stake, a steep hill to climb and one other companies trying to take over German companies in the past have failed to achieve.
"(The) 30 percent does not mean ACS has reached its goal yet," said Marc Gabriel, analyst at Bankhaus Lampe. "In any case, we need to wait until the end of the year to see what will happen -- it is still too early to crown a victor."
ACS shares were up 0.1 percent at 1550 GMT, with Hochtief up 0.3 percent.
Hochtief had fought the takeover by increasing its capital -- selling a 9.1 percent stake to Qatar. That diluted ACS's stake in Hochtief to around 27 percent from 29.9 percent, making it harder to reach 30 percent.
ACS responded by sweetening its bid, offering nine ACS shares for every five Hochtief shares, up from its previous eight-for-five offer. The offer expires Dec. 29.
While Hochtief can still employ other defensive moves, chief executive Herbert Luetkestratkoetter has signalled he would avoid destroying shareholder value further to fend off ACS.
ACS signed a deal with the only union in Hochtief, IG BAU, on Wednesday, committing to maintain working conditions until 2013 once its full acquisition of the company is completed.
Analysts saw that deal as significant.
"This is very good news for ACS, as it shows the fierce opposition from the employees against ACS has disappeared and they are looking for solutions in the future," BPI analyst Daniel Gandoy said.
ACS said its exchange bid had been accepted by holders of 357,535 Hochtief shares, or approximately 0.46 percent of share capital, as of Dec. 22.
ACS wants control of Hochtief without having to buy all its shares, so it kept its original offer low.
ACS will want to use the least capital possible to buy the remaining shares, LBBW analyst Alexander Groschke said.
"It may wait until Hochtief's share price falls, so we do not know when or even if the takeover will take place." (Additional reporting by Tom Kaeckenhoff and Paul Day; Writing by Sonya Dowsett; Editing by Dan Lalor and Jon Loades-Carter) ($1 = 0.7613 euro)