The floor plan and elevation adapt well to the High Middle Ages Asturian basilica model. It is made up three naves with three sections, separated by stone arches over rectangular section pillars, single head wall rectangular triple header, and an open room in the first section of the southerly nave. It is certain there were no rooms symmetrical to this on the northerly façade. Likewise, it having a three-part western front part, with portico and two lateral quarters is doubtful.
In spite of its current purpose as the rural parish centre, we should not forget that we are before a church founded directly by Alfonso III el Magno and his wife Jimena, consecrated in 891 and destined to house a monastic community. In 1108, bishop Pelayo de Oviedo once again consecrated the three altars, dedicated respectively to San Adrián and Santa Natalia (in the central chapel), to San Pedro and San Pablo (in the southerly chapel) and to Santiago (in the northerly chapel). According to the date, the temple must have been or was about to be at the service of a rural canonry, probably reformed encouraged by the bishop himself.
The original masonry walls are built out of irregular, rough, predominantly limestone stonework, with ashlar in the corners, chiselled in calcareous tuff, the same as the openings of the aisles and the header beams. It only had one door, open to the west. The lighting is provided by six rectangular windows in the clerestory and four loophole openings in the aisles, to which the gaps in the chapels are added. Except for the barrel-roofed header chapels, the remaining spaces are covered by wooden carpentry.
The inside of the temple fully responds to the rough impression caused by the external part. The sculptural decoration is limited to the couple of columns of the arch of triumph in the central chapel. The capitals are reused pieces.
Santo Adriano de Tuñón contains important samples of wall painting in the central chapel. To be highlighted is a frieze of triangular stepped battlements, with a lobule-shaped interior, located at the level of the vault impost line, with the direct inspiration of Córdoba.
Together with the underlying baseboard, with vegetation stem, this frieze of battlements also runs along the southerly wall of the central chapel, over the credence tables, where a cross over the processional shaft has been preserved, with equal arms and finishing in three widened lobules, from which two spiralling curved antennae sprout towards the inside.
A baseboard with circles in the thread of the arches surrounds the perimeter of the credence tables.
The vegetable frieze with rose buds enclosed in the inner waves of the stem equally proves the application of decorative repertoires with a southerly origin, as this is a prelude to the frequent stem and palmette edgings decorating epitaphs, imposts, abaci and other sculptural elements presents in the architecture and plastic art of the 10th century. Finally, the baseboard of the head wall window stands out, with concentric circles separated by beams with three stems, recalling the arrow and ova friezes, with representations of the Sun and the Moon shining next to them.