A specific example. The very same building developer completed these two buildings and offered the flats for sale more or less simultaneously, in the second half of last year IIRC.
Building 1. Renovated early 20th century building (originally built in 1923). Google Maps coordinates (the renovation is not yet visible in Google Maps): https://www.google.com/maps/@43.3621801,-8.4070271,11a,75y,2... Photos of the renovated building: https://ammapromocion.com/plaza-de-vigo/ Flats have 117 useful sqm and the middle flats were being offered at between 1.1M-1.2M €, and sold fast (you can still buy the 1st floor flat for 950K. But it's a 1st floor with a bar directly below, which reduces its value rather sharply. This is Spain, bars are noisy and open until late).
Building 2. Newly built. Google Maps coordinates (again, finished building not visible yet in Google Maps): https://www.google.com/maps/@43.3599669,-8.4061383,23a,75y,2... Photos of the finished building: https://ammapromocion.com/promocion-juan-florez/ Flats are different sizes, but some are only slightly smaller than in the other building - around 110 useful sqm. They were being offered for around 700K. They also sold fast.
The distance between both buildings according to Google Maps is 290 m, a 4-minute walk, so they are basically in the same neighborhood. You can argue that the new one is in a worse location because it has more traffic and the street is uglier, and it's true (although the other is in an area with many bars, so I think not everyone would prefer its location). Anyway, that kind of difference could be worth at most 50K in this market, being generous - and most likely more than fully offset by fact that the newly built flats include garage while the renovated ones don't have it! Anyhow, it definitely doesn't explain a 400K+ price difference.
It's just one concrete example, but I've seen many (I was recently looking for flats... finally I settled from one from the 1940s, building not renovated :)). Of course I don't know the trends outside my city, but at least here, fully renovated old buildings are the apex of the housing market.