I have read on your guide to buying a house in Spain section that if a neighbour has altered the façade, by enclosing their balcony for example, or raising the level of their balcony fence, then other neighbours in the same building can do the same. In my case I have a first floor flat with a terrace and in the same block of flats there is an attic flat with an enclosed terrace, I would like to do something similar using an aluminium structure the same colour as my windows. Have I got the right to do this?
In general, under the Shared Property Law (la Ley de Propiedad Horizontal) such alterations are not allowed (article. 7 and 12). But it is also true that the law has evolved and been reinterpreted since it was originally written, and is often open to different interpretations.
In your situation you should not have any problems justifying your alterations if other neighbours have done the same, although this goes against article 3 and 7 of the Civil Code and in theory if other members of the community of property owners complain, you (and your neighbour) could be forced to undo the new alterations.
If you want to go make completely sure that this does not happen, write a formal letter to the President of your community informing him of your intentions and seeking approval.