Into the 21st century, it has worked on numerous complex civil engineering and commercial construction projects and has been involved in various Private Finance Initiative (PFI) schemes.

[3] Costain gradually expanded through Lancashire; by the outbreak of the First World War, it was building its presence in South Wales, where it built numerous houses for munitions workers.

[4] Following the First World War, Costain began to develop housing estates in Liverpool on its own account, primarily to offer continuity of employment to its workforce.

Under him, Richard Costain & Sons expanded its housing building large estates all around London, the largest being a site for 7,500 homes in South Hornchurch, started in 1934.

[8] Losses on the railway, on Beckton sewage works and the costs of Dolphin Square caused financial problems, and Costain had to look for alternative funds when Barclays withdrew its overdraft facilities.

[12][13] In the immediate post-war years, only small estate development was undertaken by the firm; it was not until the acquisition of Nottingham-based Rostance Group in 1962 that private housebuilding resumed on any scale.

[15][16] During the 1980s, recognising that exceptional Middle East profits could not continue, Costain sought to redeploy its extensive cash balances into coal mining, international housing and commercial property.

However, over expansion in the end of the 1980s led to high gearing just as international markets were turning down; this circumstance was exacerbated by a disastrous explosion which killed ten people in 1989 at a Costain owned coal mine in the United States, for which the firm was fined $3.75m in February 1993.

[17] As a consequence of the early 1990s recession, the company incurred substantial losses that not only rapidly depleted its reserves but also compelled the sale of several key assets along with various cutbacks, particularly in its underperforming housebuilding division, that left Costain as a predominantly construction-oriented business.

[25] In the early years of the 21st century, Costain worked on the Channel Tunnel Rail Link, which included the modernisation of London St Pancras station to accept Eurostar trains,[26] and on the Thameslink,[27] and Crossrail projects in central London; on Crossrail, Costain's contracts included the Paddington [28] and Bond Street stations (both with Skanska),[29] and the north east network upgrade.

[35] In December 2019, a court ruling increasing Costain's liabilities on a Welsh road project caused the firm to cut its full year profit forecasts; its share price fell 19% in early trading.

As a result, the board and senior leadership team agreed a 30% reduction in salaries and fees for up to three months, while also making other short term economies.

[43] Costain's activities are organised into two operating divisions: Natural Resources (water, nuclear process and oil & gas) and Infrastructure (highways, rail and power).

[75][76] Subsequently, it was also one of the eight businesses that were involved in the launch in 2013 of the Construction Workers Compensation Scheme,[77] a move that was condemned as a "PR stunt" by the GMB union, and described by the Scottish Affairs Select Committee as "an act of bad faith".