The two partners have signed a framework agreement targeting the delivery of a further 5,000 private rented sector (PRS) homes over the next three years.
Sigma's relationship with Countryside began in December 2014 with an initial agreement to build 927 homes, which was extended to include a further 900 homes in February 2016. During the year to 30 September 2017, Countryside delivered 721 PRS homes for Sigma across the northwest of England and the West Midlands.
Sigma chief executive Graham Barnet said: “This major new agreement targets the delivery of 5,000 high-quality family homes across a number of regions over the next three years - over and above those already built or in process.
"There is a structural shortage of housing in the UK, across all tenures, and our unrivalled delivery platform brings together local authorities, home builders and funders with the common aim of creating new, professionally-managed rental homes for families, a largely neglected and growing part of the overall rental market."
Countryside group chief executive Ian Sutcliffe said: "This new agreement supports the continued expansion of Countryside's Partnerships division and our mixed tenure delivery model. It will allow us to speed up the delivery of homes more quickly on larger Partnerships sites and meet the strong demand we are seeing for our product."