Basically and to avoid an automatic tax assessment, you must fill in a yearly Swiss tax return.
This not an obligation. If you do not complete any tax return, you will be tax at the highest tax rate (35.50%), without considering any potential expenses.
On the opposite, if you complete correctly your tax form, you may :
1. deduct your effective maintenance costs
2. deduct a part of your interest and a part of your mortgage
3. reduce the tax rate applied on your Swiss taxable income.
Using the file below (second page in English) you can easily check if it is worth to complete a Swiss tax return.
Swiss Tax Services SA could help you and give you the best advice in this area of work. The financial issues may be much higher than expected, depending on each situation. Do not hesitate to ask for more information.