I would assume one can challenge that valuation with the tax authority or through the courts. A building isn't actually worth €300k if it can't be sold for €300k. If I buy something from a third party for €100k today, it's worth exactly that.
Tax lawyers are expensive. If people can't even pay the tax, how would they afford a tax lawyer? I have achieved beneficial settlements with unreasonable tax collectors with a good lawyer, so often it is that easy.