They are available on new build properties and existing properties and you can generally purchase a share of the property between 25% and 75%. A Housing Association owns the rest of the property and in addition to the mortgage payment you also pay the Housing Association a monthly rental sum based on the percentage that they own of the property
The added benefit of purchasing a Shared Ownership property is that the Housing Association charge less than the private rental market, they can charge up to 80% of the open market rental value.