[hibikir]

Spain's multi-story buildings are often independently owned anyway: the "law of horizontal property", dealing with condominiums, is there to mostly deal with shared disputes among owners. Buildings where the ground floor is stores, the next three have small professional offices, all owning the space, and the rest is a collection of apartments occupied by their owners is very common where there's sufficient demand for said offices.

Barcelona's problems come from a very complicated regulatory regime that hasn't just lowered construction, but ultimately led to fewer people living in the same space. Regulations that favor the renter enough to make putting housing up for rent pretty risky in many ways. Minimal property taxes that let having an apartment unused be pretty attractive. Big tax advantages for owning an expensive apartment vs living in a smaller one and putting your savings into stocks. Prices that don't come down. A location so interesting, and yet at prices that are globally affordable, so it is more profitable to have an apartment dedicated to low-occupancy rate foreign tourism than to have it occupied all year long by a long term tenant. Cheap enough for an American in tech to buy an apartment downtown, and use it 3 weeks a year.

The market-centric changes that wouldn't just allow more building, but make the existing buildings be used more efficiently are just not in any local regulator's radar, because lowering prices might be nice for renters, but it'd be awful for owners, and there's a lot of those, and they vote.