Housing is, by principle, the main urban function, but a set of housing units does not imply, in itself, an urban reality; so, in the next articles the focus will be on municipalities with populations over 20.000, a size beyond which realities of a certain complexity should become apparent and you get rid of some extreme cases as 100% rural communities or municipalities that are just a ski resort.
According to the 2011 census the Spanish municipality with the smallest vacant housing units ratio was Moguer (close to 21.000 residents), with 8.068 overall homes, of which only 105 vacant (1,3%) and 199 seasonal homes. It is a municipality which is linked to the metropolitan dynamics of Huelva, the provincial capital; the municipal economy depends partially on the coastal tourism village of Mazagón (shared with the neighboring municipality of Palos, which owns the littoral strip), but also in a large part on an irrigation agriculture (strawberries, among other crops) which attracts a relevant number of foreign seasonal workers, which have had in the past specific housing problems. Attention, the google image only shows the central urban zone.
Just to compare, in France the commune with the lowest vacant housing ratio in 2012 was Colomiers (pop. 34.300), with an overall stock of 14.813 housing units, including 174 seasonal homes and 148 vacant units (1%). Located west of Toulouse, in its metropolitan area, its growth began in the 1960s and its economy is linked to aerospace (Airbus) and office spaces.
On both cases, the communities are integrated in wider metropolitan scale dynamics.