Don't use tax return App, warns tax office

This means they could miss out on deductions they are eligible for if the details are not included on the draft, says the independent in-house technical service, GESTHA.

The App 'encourages the taxpayer to validate the draft declaration without reviewing it in depth', GESTHA says, meaning many users 'may forget to include relevant fiscal information that is to their advantage'.

Mortgages, personal pension contributions, personal loan repayments, charity donations and a host of other elements can reduce individuals' and sole traders' income tax bills.

The self-employed can offset their accountants' fees, membership of professional organisations, tools required to carry out their jobs, travelling costs, union membership and relevant training or studying courses.

Individuals can offset their main residence in some circumstances, and reductions may be available for those who have dependent children or other relatives.

Civil status, births, deaths and name changes may not have been updated and not appear in the draft on the App, explains GESTHA.

The technical team says the App is only really suitable for the simplest of declarations, where nothing is likely to need to be altered.

These may include workers employed full-time in the same job on a permanent contract that began before the start of the last tax year, and who do not have any other income, assets or regular payments.

“Technology can go from being an ally to becoming the enemy, since in this case, taxpayers who decide to file their declarations via their SmartPhones may forget to include details that could benefit them,” concludes GESTHA personal taxation expert Antonio Paredes.

Photograph by Hacienda, Spain's tax ministry