Nominal interest accrues monthly on the current loan balance. With equal instalments most of the first payment goes on interest, less on principal; with declining instalments the reverse — you repay principal at a fixed rate and interest falls with the balance. The rate itself is set by the bank, bounded by the Polish civil code (art. 359 §2¹): the ceiling is 2 × NBP reference rate + 3.5 pp, giving around 14–15% a year in 2026.
Why does bank advertising feature this figure? Because it reads the lowest. A bank can honestly write „rate from 4.9%" and add 12% commission plus 2 000 PLN of insurance on the same loan, with RRSO landing at 22%. The law does not forbid quoting the nominal rate — it just requires the RRSO and a representative example next to it.
The third trick is a variable rate. The bank shows 4.9% because that is where the NBP reference rate sits today. When NBP hikes, your contract rate follows automatically. On a 25–30 year mortgage that usually means several years of higher payments in every decade.
Frequently asked questions
No. The civil code caps it at 2 × NBP reference rate + 3.5 pp. Non-bank lenders sidestep this with a non-interest commission (not treated as interest), which is separately capped by the anti-usury act.
Because the interest rate reads lower. The law makes them show RRSO next to it, but advertising anchors on the smaller of the two. In the contract itself, RRSO must come first.
Variable is usually 0.5–1 pp cheaper, but risk rises with contract length. For loans up to 5 years variable usually wins. For 10–30 years (mortgage) fixed gives peace of mind and lets you plan payments on paper.