Thank you for the quick response.
It is inside a building not a spell.
And <StrVal2>OnlyTrainedUnits</StrVal2> should deny the bonus to champions.
Your idea with <HasUpgradableValue>1</HasUpgradableValue> was good. I think it must be included.
I changed it to your proposal, but without sucess.
I tried making a spell too, in which I included <PerTurn>1</PerTurn>.
But it worked neighter as improvement nor as spell.