Each Swiss canton sends its own tax return form, usually in February or March for the previous tax year. The same form covers federal, cantonal and communal income tax, plus the cantonal wealth tax. Filings are increasingly fully digital (e.g. ZHprivateTax, VaudTax, BalTax).
Standard deadline is 31 March; extensions to September or November are routinely granted, sometimes against a small fee. Foreigners taxed at source above the income threshold or with non-salary income must also file. The canton issues a provisional invoice, then a final assessment 6–18 months later.
Common deductions include Pillar 3a contributions, professional expenses, commuting costs, childcare, alimony, mortgage interest, property maintenance, donations and medical expenses above a threshold. Missing the deadline triggers an automatic ex-officio assessment, typically at the taxpayer's disadvantage.
A Zurich employee earning CHF 110,000 with CHF 7,258 in Pillar 3a, CHF 3,000 in commuting costs and CHF 2,400 in donations sees taxable income reduced to about CHF 92,000 — saving roughly CHF 5,000 in combined federal/cantonal/communal tax versus no deductions.