The partners have agreed terms to build 375 homes in the Warwickshire town of Leamington Spa.
Once work starts at the Europa Way site a total of almost 1,000 homes will be being delivered through the joint venture partnership.
Sanctuary and Galliford Try are currently on site building 251 homes in Biddenham, Bedfordshire, a further 219 in Brampton, near Cambridge, and 135 in Greenwich.
Stephen Teagle, chief executive of Galliford Try Partnerships, said: “As a partnerships-focused business we look to build and establish long-term relationships that deliver real value and change for our clients.”