Normally the civil code is one of the factors that is considered to make a change of use since in the deeds of horizontal property, on many occasions, a description is made of the use to which each of the proprietary elements is to be put.
However, the ruling of the Supreme Court of June 15, 2018, clears many doubts about whether to request the community for express authorization to change the use of a private unit.
In the event that the deeds explicitly state the prohibition of making a change of use, then it will be necessary to respect what is said in the deeds or reach an agreement with the community unanimously.
On the other hand, if there is no express prohibition, it is understood (this is deduced from the sentence) that this change of use can be made without requesting permission from the community.
It should be added that in case the change of use requires interventions in community elements, essential to comply with other regulations (CTE, urban planning, habitability), in this case it will be necessary to request permission from the community (such as to make an expansion of openings on the façade, the placement of chimneys up to the roof, etc.).
A very illuminating comment on this sentence was published on the website THERE IS A LAW . You can read it in this link .