What you described (strict building requirements) is, in fact, a sociopolitical problem, not a technical one. We used to build houses for much less. They were worse houses. Many people would not like them today. Others would. They are not allowed to have them, though.

In the US it's even worse - we have those older houses, but they're almost always in older parts of the city (duh) and their valuation is sky-high.

That's a real problem, but will correct itself eventually. In the oldest parts of the oldest cities in the US, mostly the nicer old buildings survive for the long haul unless they have historical significance. (e.g. Paul Revere's house in Boston.) Eventually the value of building a nice building on that land will surpass the value of keeping a shitty old one there. I'm more concerned with the explosion of shitty quality "luxury" 5/2 buildings being blighted and falling apart right when they stop inflating market value in their neighborhoods and pushing people out.

In Europe we have stricter building codes on average but it's still very much a non-technical problem. Prices of land are insane in Spain, and I bet in other countries too.

And taxes, and a myriad of other things that have nothing to do with actually building the thing.

I'm not sure by what measure Spain has high land prices. It effectively has the lowest population density of any sizeable (under 11 million) country in Europe [1]. It also has one of the highest rates of second property ownership in Europe [2]. I can't find figures but I suspect it also has one of highest rates of home ownership by non domeciles too.

Lots of factors contribute to all this and like anywhere it's always 'location, location, location', but outside a few major cities and popular holiday locations I think it's probably pretty cheap - in relative terms.

[1] https://en.wikipedia.org/wiki/Area_and_population_of_Europea... [2] https://www.researchgate.net/figure/Secondary-property-owner...

If it's pretty cheap, it won't be allowed to build anything residential.