Answer this with comparable sales, and skip the generic percentage rules of thumb. Reach out to the agent who helped you buy the home, or get a referral to a strong local agent, and have them pull comps of similar homes in your tract or neighborhood, ideally the same model, where one has a finished basement and one does not. Comparing those on price per square foot, in similar condition, tells you far more than any national return figure, because the value depends heavily on how buyers in your specific market treat below-grade space. A few things to keep in mind: - Finished basement square footage often does not get valued like above-grade living space, and appraisers may treat it separately, so the dollar-for-dollar return can be less than you expect. - The payoff depends on the quality of the finish and whether it adds something buyers actually want, like a bedroom, bathroom, or usable living area. - If the main goal is your own enjoyment for the years you will live there, the calculus shifts and the resale return matters less. If this is mostly a value play, get the comps first so you are deciding with real local numbers instead of a guess.