A stamp duty calculator for a development's buying page: a purchase price field and a first-time buyer checkbox on the left above the total tax as a large figure that springs to each new value with the effective rate, a button and a disclaimer; on the right a configurable band table (threshold and rate) lists the tax per band on hairline rows, the active bands in full colour, and the whole table cross-fades when the checkbox swaps in the first-time buyer bands.