Dorothy,
I cannot answer your question but be aware that you will need the permission of the communality to enclose the terrace.
That means that the percentage of enclosed area you will have is different so, in theory at least, all the escrituras would need to amended in the community. That is why people often just go ahead and do it.
You will also need a licence from the town hall and I am fairly confident that they will need to see the approval of the community, which you will not have.
I know, lots of people just do it and there are no problems. I am just letting you know the risks you would be taking.