If it is just the looks of the bathroom that you would like to change, you will not be able to get the deduction in your tax return. On the other hand, there is the 20% lump sum (20% from the Eigenmietwert -imputed rental value) that can be deducted anyway (if you do not want to list all the costs separately and, of course, if the deductible costs in total make less that this 20% lump sum).

But you can try anyway - if the tax office does not accept it, so be it. Maybe they will accept a partly deduction (it also sometimes depends on the tax officer who will process your tax return). In any case, you should keep all the receipts and invoices for later - once you would like to sell your house, depending on the case, you may need to pay a tax on the profit - then you can can deduct all the costs / expenses you incurred in "adding value" to the house.