53 Pa. C.S. § 8575
(a) Amount.--An annual real estate tax deferral granted under this subchapter shall equal the increase in real property taxes upon the homestead of the claimant.
(b) Prohibition.--No tax deferrals shall be granted if the total amount of deferred taxes plus the total amount of all other unsatisfied liens on the homestead of the claimant plus the outstanding principal on any and all mortgages on the homestead exceeds 85% of the market value of the homestead or if the outstanding principal on any and all mortgages on the homestead exceeds 70% of the market value of the homestead. Market value shall equal assessed value divided by the common level ratio as most recently determined by the State Tax Equalization Board for the county in which the property is located.
53 Pa.C.S. § 8575