12k is upgrades is both enough to potentially have the landlord owe additional taxes if they are assessed and not enough to be able to increase amenities enough to meaningfully raise rent.

The real issue here though is that you don’t go altering someone’s property without their consent. I don’t know how that isn’t the obvious answer here. The amount spent doesn’t even really matter (although I’d argue more spent is even worse, considering it implies greater alterations without consent).

Landlords can be and very often are terrible. But on a base level if I own a piece of property for which I am ultimately responsible, I see no justification for being ok with someone else making thousands of dollars of changes to that property without getting my ok first. It seems incredibly basic that I as owner should have a say in it.