When a joint Spanish tax return (declaración conjunta) comes out better, how much it lowers the base, and why most two-income couples should file separately.
In the renta you can file individually or jointly with your family unit. A joint return adds all income into one return and applies a reduction.
How much it reduces
Married couples: €3,400 off the taxable base.
Single-parent families: €2,150.
When it pays off
A joint return usually works better when one partner earns little or nothing. If both have similar salaries, adding them pushes income into higher brackets and the reduction does not make up for it, so filing individually is almost always better.
For single-parent families, the €2,150 reduction often makes it worthwhile.
How to know
The renta software lets you calculate both options before filing. Always compare them: you can switch every year.
On your payslip
Your employer withholds as if you were filing individually, unless you tell them otherwise on form 145. If you file jointly, the difference shows up in the renta.
In the calculator profile you can choose an individual return, a joint married return or a joint single-parent return.
The rules differ in the Basque Country and Navarra: see the foral system. Try the options for your case in the calculator.