[KellyCriterion]

....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.

Source: I was once hit by them with these rules.