Unmarried couples sharing an address sometimes discover that the tax office takes a view of their relationship they never signed up for. Fiscal partnership follows a specific list of conditions, and it cuts both ways: allowances and deductions can be pooled on the return, while toeslagen shift onto combined household income.
Possibly, yes — marriage isn't required. The Belastingdienst (tax office) treats unmarried cohabitants as automatic fiscal partners if you're both registered at the same address in the municipal population register (BRP) and at least one of these also applies: you have a notarial cohabitation contract (samenlevingscontract), you have a child together or one of you has legally recognized the other's child, you jointly own the home you live in, you're both registered as each other's pension partner with a pension fund, or you were already fiscal partners in the previous tax year. If none of those apply, sharing an address alone doesn't make you partners for tax purposes — you'd each file separately. Being fiscal partners lets you pool your Box 3 tax-free allowance and split deductible items (like mortgage interest) between returns in whatever ratio saves the most tax, and an unused general tax credit can shift to the partner who can actually use it. The catch: income-tested benefits like zorgtoeslag (healthcare allowance) and huurtoeslag (rent allowance) then get assessed on your combined household income, which can shrink or wipe out what either of you was getting separately.