Prior to 6 April 2017 housing associations needed to obtain approval from the Homes and Communities Agency before they could dispose of any housing stock.
From that date onwards though the provisions of the Housing and Planning Act 2016 have been in place, meaning that housing associations no longer need to seek such approval. (They only have to notify the HCA after the sale of a property has been completed).
The changes have generally been welcomed, as housing associations now find it easier to manage their stock to meet the needs of potential tenants. For example, if they've got several 3-bedroom houses standing empty but a big waiting list for one-bedroom properties, it's easier for them to sell off the larger houses and replace them with a greater number of smaller ones.