Figures obtained by JPIMedia show that the 25-year PFI for the service's training facility at Easingwold is due to cost £28.4m overall instead of £26.1m as initially thought, with the majority of increases being because of inflated interest.
Contractors for Semperian PPP, an equity holder in many PFI deals nationwide, carries out services such as catering, cleaning, security, maintenance, mechanics and electrics.
A response to a freedom of information request shows that projected costs for interest charges originally indicated a steady decline from £313,796 in 2002/3 to £158,955 in 2018/19.
In reality, charges have crept up from £335,254 in 2002/3 to £406,331 in 2018/19, with decreases only recorded in three years during that time period.
However, a model forecast for interest rates up to 2026/27 shows exactly the same projected costs as were originally calculated for the next eight years of the deal.
For example, the £140,719 interest charge forecasted for next year is the same as initial documented sums, while the £22,038 expected cost in the last year of the deal matches the revised estimate.
If the rate of interest continues to rise, therefore, the service's projection of spending nearly £2m more than planned in interest alone over the next eight years could prove dramatically off-budget, with an even higher cost likely.
NYFRS and the North Yorkshire Police and Fire Commissioner have been approached for comment.
Semperian said it had no comment to make.