Westleigh is an established partnerships housebuilder, covering the Midlands and South Yorkshire.
In the year to March 31, it delivered 1,159 homes in partnership with local authorities and housing associations of which 92 per cent were affordable.
It has a land bank of 4,981 plots and an overall new business pipeline of 13,000 plots.
The business was founded in 1985 by the current chairman, Chris Beighton, who will step down from his role on completion. The management team and the company workforce of 456 will be retained to continue the growth of the business.
The acquisition of Westleigh is in line with Countryside’s strategy to expand its Partnerships business. Countryside said Westleigh will broaden its geographical reach into regions that have strong underlying demand.
Ian Sutcliffe, chief executive of Countryside, said: "Westleigh is a well run business with a strong strategic, geographic and cultural fit to Countryside and provides us with an excellent platform for further Partnerships growth.
"We see huge opportunity in combining Westleigh’s regional expertise with the Countryside mixed tenure business model."
Countryside said the enhanced buying power from being part of a larger group will improve underlying returns in the Westleigh business.