Keeping records and documentation
You will have to keep careful records for income tax and VAT – not only for submitting your quarterly tax records but also in case you are ever audited or want to claim any tax reductions. Keep copies of all your invoices (facturas) and all your receipts.
Spanish invoices should include the following information:
Your business name
Address and contact details (phone, email, website)
Your business ID number (NIE or DNI number)
Description of your product or service
Invoice amount, including VAT (IVA) if applicable
Terms of payment and how you would like to be paid (eg. bank transfer, with account number etc.).
Being in English does not invalidate an invoice. But being in Spanish will make things much easier if you should receive a tax office enquiry. Many British businesses in Spain issue dual-language invoices.