There are many tools available in Germany that support you doing your tax filings. They have endless questionnaires in (tax law free) easy language that guide you through all the taxing niches. They not only respect all rules regarding the laws but also the latest jurisprudence.
Like: If you have a lockable, separate office and work (almost) exclusively from home, you can basically deduct the entire room for tax purposes (rent + electricity + heating + insurance + etc).
....but wait until Finanzamt comes along and measures the size and checks with a controller if this room is really ONLY for work - if there is the slightest sign that this room maybe used for other things, your plan is gone.
Even an additional single sofa/couch can crush this plan.
And: If you say the room is worth 500€, you dont get back this 500€ with yearly tax declaration - you only get this amount deducted from total income, rising your after-tax income a little bit. In fact, with this solution you loose a room PLUS some money - rather rent out the room 1 week per AirBnB and pocket this in cash and you are fine.
