The Henry George Theorem (HGT), or the golden rule of local public finance, states that, in first-best economies, the fiscal surplus, defined as aggregate land rents minus aggregate losses from increasing returns to scale activities, is zero at optimal city sizes. We derive a general second-best HGT in which the fiscal surplus equals the excess burden, expressed as an extended Harberger formula. We then apply our theorem to various settings encompassing urban eco- nomics, the new economic geography and local public finance to investigate whether or not a single tax on land rents can raise enough revenue to cover aggregate losses from increasing returns to scale activities.