I will explain how speculation works here: they bought million of houses during the 2008 crisis, they keep some closed to reduce the offer and increase the pricing. meanwhile they sell housing for vacational rent or other investors and the price continue increasing.
is not just a lack of "offer" or "constrained supply". they bought the houses and keep a lot of them closed to increase their earnings artificially.
and the right wing setup rules to "free" social houses in a lot of regions and allow them to be sold to speculators after that time finished (Madrid is an example). plus a lot of the houses were sold to private investors by the right wing during the crisis to the cheapest pricing possible.
a lot of people living in those houses are at the edge of eviction because those investors want to resell them or increase the rent as rich foreigners offer them more, or just resell them to other investors.