City workers will soon see hefty fee hikes for their supplemental retirement funds, The Post has learned.

Starting next year, the Deferred Compensation Plan’s quarterly fee will increase from $12.50 to $20 and the annual asset fee is going to 0.05 percent from 0.03 — or $50 a year instead of $30 per every $100,000 saved.

The increases are to offset the rising cost of administering the plan, which is worth an estimated $4 billion to $5 billion, the Office of Labor Relations said.