"The purchaser pays a mortgage on the share they own, and pays rent to a housing association on the remaining share. [...] The purchaser has the option to increase their share during their time in the property via a process known as ‘staircasing’, and in most cases can staircase all the way to 100%."
https://www.sharetobuy.com/guides-and-faqs/what-is-shared-ow...
"The purchaser pays a mortgage on the share they own, and pays rent to a housing association on the remaining share. [...] The purchaser has the option to increase their share during their time in the property via a process known as ‘staircasing’, and in most cases can staircase all the way to 100%."