Improving animal welfare in livestock farming requires changing the behaviour of many stakeholders. Farmers have to take proper actions on their farm to improve animal welfare, retailers have to market animal-friendly products and consumers have to purchase these products. The theory of planned behaviour (TPB) is a widely applied model that can be used to explain behaviour related to conscious choices. The model also gives indications for possible interventions needed to stimulate desired behaviour. In the current study the behaviour of farmers with respect to the change to group housing for pregnant sows was explored. A computer-assisted personal interviewing approach was applied to elicit behavioural beliefs of 105 farmers. Data analysis revealed that TPB was useful for understanding the farmers' choices with regard to the change to group housing, and that it provides indications for possible interventions to support farmers who have not yet changed to group housing. In contrast to farmers who had made the change already, these farmers thought that it was unlikely that group housing was good for animal welfare, and, related to that, that it was likely that it would lead to more tail biting and less work pleasure. They also perceived to a lesser degree that persons relevant to them expected them to change to group housing or that farmers comparable to them had already changed. Additionally, farmers who had not changed yet and farmers who had changed partly had lower scores for questions related to their skills with regard to the managing of group-housed sows and their knowledge about the required building process. So, providing these farmers with information to increase their knowledge with regard to group-housed sows and the required building process might be a fruitful intervention. However, it should be carefully considered how this information is effectively communicated.