The initial mistake is one of mindset: treating the draft as if it were a correct proposal that only needs accepting. The AEAT presents it as a service, not a signed return. The signature belongs to the taxpayer, and so does the responsibility for any errors or omissions. As a result, before clicking "submit" the draft should be reviewed with the same care one would bring to building a self-assessment from scratch.
Error 1 · Regional deductions not applied
The draft does not pull in any regional deduction (regional tranche of IRPF) automatically, save for very specific cases where the relevant region has cross-fed the data. Madrid, Catalonia, Valencia, Andalusia, Aragon, Galicia and Navarre maintain primary-residence rent deductions with very different percentages and caps. The most recurrent omission is the Madrid regional rent deduction for under-40s: 30% of amounts paid up to €1,237 in tax liability. The AEAT does not have the lease. The AEAT does not have the IVIMA deposit. It must be entered manually.
Error 2 · Payer data carried over with Form 190 errors
Employment-income withholding data come from the employers' Form 190. When the certificate has an amount or NIF error, or was filed late, the draft data arrive incomplete. The fix: compare the draft against the December payslip and against the withholding certificate every employer is required to issue to the worker. Any discrepancy is corrected before submission.
Error 3 · Pension-plan contributions omitted or incomplete
Contributions to individual pension plans reduce the general taxable base up to the lower of €1,500 per year or 30% of net employment and economic-activity income under Article 51 LIRPF. Contributions to occupational schemes add a further allowance of up to €8,500. The draft typically pulls in the contributions made through the employer's plan, but does not always capture individual contributions made at a different financial institution. Cross-check the annual certificate from the pension-plan provider against the figure shown in Renta WEB.
Error 4 · Foreign-broker transactions unreported
Capital gains and losses from disposals of investment funds or securities held through brokers domiciled outside Spain — Interactive Brokers, DEGIRO, Trade Republic, eToro, Lightyear — generally do not reach the draft. The reporting duty to the taxpayer rests with the broker, but no automatic cross-feed with the AEAT operates. Omission in the draft does not exempt the taxpayer from declaring. Capital losses left out, moreover, will not be available for offset against future gains over the next four tax years.
Error 5 · Transitional regime for housing investment lost through inaction
Taxpayers who acquired their primary residence before 1 January 2013 and had been applying the deduction for housing investment retain entitlement under the transitional regime of Transitional Provision 18 LIRPF: 15% on a maximum base of €9,040 per year, with an effective cap of €1,356 in tax liability. The draft typically carries the deduction over if it was applied the previous year, but cancels it if there was discontinuity. Recovering it years later is possible via rectification within the four-year limitation period of Article 66 LGT, but rebuilding the documentary trail is the work.
Error 6 · Article 20 LIRPF reduction miscalibrated
The reduction for low-bracket employment income — top tier of €7,302 for net employment income at or below €14,852 — applies on net employment income. When that figure is overstated in the draft (unreported union dues, omitted college fees, missing legal-defence costs), the taxpayer can end up sitting in the wrong bracket between the maximum and tapered tiers. The tax-liability difference ranges between €400 and €800 depending on the marginal rate.
Error 7 · Imputation of real-estate income across the entire year despite partial leasing
If a property other than the primary residence was leased part of the year and vacant the rest, Article 85 LIRPF apportions the year: leased days generate real-estate income; vacant days generate imputed real-estate income on the cadastral value (1.1% or 2% depending on the date of the cadastral revision). The draft frequently imputes the full year even when there was partial leasing. The fix is manual and requires uploading the lease agreement and monthly payment evidence.
Error 8 · Family minimums with shared custody or ascendant living with multiple relatives
The minimum for descendants (Article 58 LIRPF) and for ascendants (Article 59 LIRPF) is apportioned among taxpayers entitled to apply it. Shared custody splits the minimum 50/50 per child. An ascendant living with two or more children splits the minimum among all entitled descendants. The AEAT does not always capture the actual family situation when there has been a recent change of cohabitation or court ruling. Verify the family minimums line by line in the "Personal and family minimum" section before submitting.
Common pattern
The eight errors share structure: the AEAT does not have the data, does not request it, and if the taxpayer confirms, takes the proposed figure as correct. Professional review is not about finding spectacular errors; it is about closing those gaps with data the Administration does not have, and bringing the tax liability back to the correct level. In most files, the adjustment lies between €200 and €1,500 of tax. In files with two payers or foreign brokers, the correction can exceed €3,000.
Our position
We work every draft using a single checklist that runs the eight points in order and leaves a documentary record of each verification. Where a regional deduction is available, we enter it. Where the net employment income differs, we correct it. Where there is a foreign broker, we rebuild gains and losses and incorporate them. The resulting return is not necessarily more favourable than the draft in every case — sometimes the taxpayer must pay more because they had omitted real income — but it is always the correct return. And it is the only one that protects against a later inquiry.