Barcelona resident here. I think it is a combination of not enough new supply and a stupid legal situation. As mentioned in the article, it is most attractive

Barcelona resident here. I think it is a combination of not enough new supply and a stupid legal situation. As mentioned in the article, it is most attractive to rent out your apartment temporary from 1 to 11 months. Longer renting contracts are quite risky from a landlord's perspective. E.g. in Spain if your tenant goes bankrupt and does not pay the rent anymore, you cannot kick them out until they have found something else. Good luck on that. The law sounds nice because no one wants people to end up on the street, but it effectively makes small landlords exit the rental market. Protection from squatters is also nonexistent. Local newspapers are full of stories of places being occupied by squatters while empty for renovations. Also there are these stupid local regulations around having to create social housing in every building / renovation project. That also significantly reduces new supply.