I think I already know the answer to this but just checking if anyone has any direct knowledge.

Plan to build a multi purpose gym in my keller, can the costs be deducted from Tax?

Thanks

If you own the property, there may be an argument that it is "mehrwertend" when you come to sell the place and it could be deducted from the capital gains calculation. You would need to check with your tax office/look at the policies online to see if it counts or not.