Nexus, which owns concrete contractor Tamdown, said the deal would take the business into key civil engineering sectors of water, rail, highways and marine.
Established in 2000 by Barry Coleman, the business turned over £8.3m last year.
Charles Sweeney, chief executive of Nexus, said: “The acquisition of Coleman presents an exciting opportunity for Nexus, diversifying into other critical sectors of UK infrastructure, enhancing our service offerings and driving future growth.
“This move enables the group to benefit from long-term opportunities that are less vulnerable to short-term economic fluctuations. Coleman’s established reputation and expertise in water, rail, highways and rivers & marine sectors further strengthens our portfolio and we look forward to welcoming the Coleman team to Nexus.”
In a trading update, Nexus also revealed revenue had risen 23% to £52m in the year September.
Nexus CEO Charles Sweeney said: “Diversification is a principal element of Nexus’ strategy, and the acquisition of Coleman provides future growth opportunities outside of the Group’s current core sector of residential housebuilding. Coleman offers services in sectors which are critical to the UK’s national infrastructure, driven by climate change, environment protection, shifts in societal needs, and improvements to energy security.”