Hello Mags, welcome to the forum.
Firstly, I'm pretty certain that you would have needed permission from the council for the conservatory. Did you get it?
Secondly, the Horizontal Division of property, as I understand it, comes into effect the moment the builder / developer splits the property into it's individual parts, i.e. once the building was completed and before you signed your escritura (deed). You will have to study the Horizontal Law to see exactly what it says about adding such structures, but again, I would imagine that you will find permission is required from the community.
You may also need to get hold of a copy of the estatutos, from the developer, which is like the master deed for the overall property, and may contain certain specific rules that the developer put in place prior to the formation of the official community.