N.Y. Const. art. VIII, § 10
Hereafter, in any county, city, village or school district described in this section, the amount to be raised by tax on real estate in any fiscal year, in addition to providing for the interest on and the principal of all indebtedness, shall not exceed an amount equal to the following percentages of the average full valuation of taxable real estate of such county, city, village or school district, less the amount to be raised by tax on real estate in such year for the payment of the interest on and redemption of certificates or other evidence of indebtedness described in paragraphs A and D of section five of this article, or renewals thereof:
(e) Notwithstanding the provisions of sub-paragraphs (a) and (b) of this section, the city of New York and the counties therein, for city and county purposes, a combined total of two and one-half per centum.
The average full valuation of taxable real estate of such county, city, village or school district shall be determined by taking the assessed valuations of taxable real estate on the last completed assessment rolls and the four preceding rolls of such county, city, village or school district, and applying thereto the ratio which such assessed valuation on each of such rolls bears to the full valuation, as determined by the state tax commission or by such other state officer or agency as the legislature shall by law direct. The legislature shall prescribe the manner by which such ratio shall be determined by the state tax commission or by such other state officer or agency.
Nothing contained in this section shall be deemed to restrict the powers granted to the legislature by other provisions of this constitution to further restrict the powers of any county, city, town, village or school district to levy taxes on real estate.