Property taxes are based on several things--size of property being the main one, but the features of or improvements to it are another. If there improvements made prior to buying the property which had not been factored into the tax assessment for some reason prevously, if there is a reassessment or the county somehow else becomes aware of them, it can then raise your taxes to reflect them *regardless* of who did them or when. Whenever the county becomes aware of improvements, they can go into the tax calculus.