I have a friend who is in the same situation - working full-time and wants to do some work on the side but wants to know the legalities of it so they don’t get hit with a big tax or SS bill later. This is what I found on Yahoo Espa?a repuestas (yahoo answers in Spain).
1. You have to declare all the income you receive on la declaracion de la renta.
2. You have to register as a professional or small business with hacienda (tax office) which means that you are obliged to declare tri-monthly.
3. If the annual income you receive from the outside activity is more than the basic wage in Spain, you also have to pay social security as an autonomo.
Does anyone have any experience of this or know if this information is correct? Probably my friend will just work under the table for a while as he’s got the SS already, but it’s good to know where you stand just in case.