Not limited to Spain: here in Somerville MA I see people doing this pretty often. They leave just enough of the building standing (two walls?) that it counts as a remodel and they can keep all the zoning exemptions from the old structure. If they tore it down entirely they wouldn't be able to build anything nearly as large.

Completely legal, but a good illustration of how even our new (and honestly much improved) zoning is too restrictive.