HBG's UK chief executive Brian May, who was told of the deal the night before it was announced to the Spanish stock exchange on Tuesday, said it was good news.
"There is, incredibly, hardly any overlap between the two firms," he said. "It's a change in shareholder, in all honesty. It's fairly difficult to see how and why there would be any changes made."
May said he thought Dragados, which has no operations in the UK, would be looking to expand its PFI activities.
May said: "They are quite used to PFI, especially in roads. I know they are interested in other sectors that we have good experience in."
The deal, which is subject to HBG shareholder approval, would see a new eight-strong board running the enlarged group from Madrid. Members would include former HBG UK boss Adrian Franklin, who would head civil engineering activity.
There is hardly any overlap between the firms. It’s a change in shareholder
Brian May, chief executive, HBG UK
Dragados said the new group, which would employ 70,000 people, was not looking to make job cuts. This was confirmed by May, who said: "It's an opportunity for Dragados to expand rather than contract."
Dragados' swoop follows an uncertain year for HBG – Dutch rival Heijmans launched an unsuccessful bid for it last June.
In a statement, Dragados said it wanted to create a truly European construction group. "The combined group will be a pan-European services and construction group and will be less dependent on single geographic markets or businesses, thus providing more stable earnings."
Analysts described the deal as good for HBG, but questioned Dragados' strategy and claimed it was offering too much. One said HBG had found its "white knight".
John Carnegie, construction analyst at Schroders Salomon Smith Barney, said Dragados had done the deal largely to get hold of HBG's dredging activities.