With regard to taxes in spain, a non resident has the same opportunities as the resident/national:
To be able to calculate the amount of taxes
To be able to complete the required declaration for each taxpayer
To present with payment to the tax office
Residents and non residents if they have the knowledge to calcualte, the ability to complete the tax declaration and the time to present and pay can of course choose to look after this matter themselves. Otherwise one can employ the services of a third party to act on their behalf and perform the functions as mentioned above. Saving money by DIY must be balanced against making the correct calculation, filling out the declarations correctly and effecting the correct payment of taxes.