You are able to sell your home at any time but you must inform the housing association in writing. If you own a share of your home, the housing association has the right to find a buyer for it. However, if you own 100% of your home, you can sell it yourself. When you do put the home up for sale, the housing association has the right to buy the property back first. This is known as the 'right for first refusal'. The housing association has this right for 21 years after you fully own the home.
When an existing part buy part rent home becomes available for resale you can buy the share from the current owner. The homes are available through different housing associations. You can buy shares worth between 25% to 75% of the property's market value and you pay a subsidised rent on the remaining share. You are then able to buy more shares (staircase) at a later date.