15 mortgage mistakes and what each one costs in koruna

The costliest mortgage mistakes have less to do with the rate than with timing and small print: a reservation signed before the bank says yes, a valuation below the purchase price, a missed window before the fixed period ends. Each one costs tens to hundreds of thousands of koruna, far more than the 0.1–0.2 pp people usually haggle over with the bank.

Below are the 15 mistakes we see most often, grouped by when they bite: before the purchase, when choosing an offer, in the contract and during repayment.

All 15 mistakes at a glance

The figures marked “example” are worked out in the sections below. Unless stated otherwise, every calculation assumes a CZK 4,000,000 mortgage over 25 years at the average rate of 5.00 % (ČBA, August 2026), which means a payment of CZK 23,384.

Which mistakes do buyers make before they buy?

1. Reserving before pre-approval. A reservation agreement (rezervační smlouva) with a developer or estate agent usually means a deposit of tens to hundreds of thousands of koruna and a deadline for paying the price. If the bank turns the mortgage down, or approves less, you can lose the deposit. Pre-approval (předschválení) takes days, not weeks, so get it first.

2. Counting on exactly 20 % own funds. Your own funds cover the gap between the loan and the price, not the costs around it: a valuation at CZK 3,000–6,000, the application for registration in the land registry (návrh na vklad) at CZK 2,000, attorney escrow, insurance, moving and furniture. If you have saved exactly 20 %, the first unplanned expense puts you in trouble.

3. Paying more than the valuation. The bank lends from the valuation (odhad), not the purchase price. A flat bought for CZK 5,000,000 but valued at CZK 4,700,000 gets a loan of CZK 3,760,000 at LTV 80 % instead of CZK 4,000,000. That is CZK 240,000 more of your own money, often needed a week before you sign the purchase agreement. With older flats up 14 % year on year (ČBA, Q2 2026), valuations lag behind prices more often.

4. Changing jobs mid-process. A new contract with a probation period can stop an approved mortgage, because most banks do not accept income during probation. Change jobs only after drawdown, or check in advance whether your bank accepts income after 3 months with the employer's confirmation. What else the bank checks before approval is set out in our first-time buyer guide.

Mistakes 5–9, when choosing an offer

5. Comparing the rate instead of the total cost. A lower rate with fees can cost more than a higher rate without them, which is what the APRC (RPSN) is for. The example below puts figures on it.

6. Insurance for a discount that does not cover it. A 0.2 pp discount saves roughly CZK 470 a month on CZK 4 million. If the required insurance costs more than that, the “discount” is really a surcharge.

7. Choosing the fixation by today's price. In October 2026 the one-year fixation is the cheapest, because banks are raising mainly the long ones. But a year later you negotiate again at an unknown rate, and every extra percentage point costs CZK 2,300–2,400 a month. A short fixation is a bet, not a saving.

8. Borrowing to the limit with no buffer. A net income of CZK 60,000 and a payment of CZK 27,000 is 45 %, exactly the usual internal DSTI threshold, and corresponds to a mortgage of about CZK 5.03 million over 30 years at 5.00 %. If the rate rises by just 1 pp after a five-year fixation, the payment on the balance (about CZK 4,620,000 over the remaining 25 years) goes up by CZK 2,758 to CZK 29,758, which is almost 50 % of income.

9. A 30-year term without a plan. Compared with 25 years it lowers the payment from CZK 23,384 to CZK 21,473, but total interest rises from CZK 3.02 to 3.73 million, by roughly CZK 715,000. A longer term makes sense only if you plan extra payments.

A lower rate that costs more

Our article APRC vs. interest rate explains the difference. When comparing, always calculate over the fixed period, not the whole term, because the conditions change after the fixation anyway.

What should you watch for in the mortgage contract?

10. Not reading the discount conditions. A rate discount (bonifikace) usually depends on your salary going into an account, a number of card payments or insurance. Stop meeting the condition, by moving your salary to another bank or cancelling the insurance, and the rate jumps. A 0.5 pp surcharge on CZK 4 million means a payment of CZK 24,563 instead of CZK 23,384, or CZK 1,179 more a month.

11. Missing the drawdown deadline and conditions. The loan agreement sets out what you must deliver before drawdown (insurance assigned to the bank, known as vinkulace; the application for registration; the purchase agreement) and by when. Miss the deadline and you need a new approval, possibly at a different rate.

12. Not knowing the early repayment rules. Outside the statutory exceptions, the bank may charge at most 0.25 % of the amount repaid for each started year until the end of the fixation, max. 1 % in total. Send an extra CZK 1,000,000 three years before the fixation ends and you may pay up to CZK 7,500, when you could have waited for the month before the contract anniversary and repaid 25 % of the original loan amount free.

Before signing the loan agreement, go through our contract signing checklist. One careful reading catches most of mistakes 10–12 before they cost you anything.

Mistakes 13–15, while you repay

13. Quietly accepting the new rate. The bank must tell you the new rate at least 3 months before the fixation ends, and if you do nothing, you move to the rate offered. With a balance of CZK 3,543,000 (after 5 years of the mortgage above) and 20 years to go, 5.80 % instead of a negotiated 5.00 % means a payment of CZK 24,976 instead of CZK 23,382: CZK 1,594 a month, over CZK 95,000 across the next five-year fixation. The window for switching banks without a penalty lasts 3 months from the announcement of the new rate; our article End of a mortgage fixed-rate period explains how to use it.

14. Leaving free extra payments unused. Once a year, in the month before the contract anniversary, you may repay up to 25 % of the original loan amount without a fee. An extra CZK 300,000 after 5 years of repayment saves about CZK 460,000 in interest and ends the mortgage 2 years and 8 months sooner.

15. Not claiming the interest deduction. On a mortgage for your own home taken out from 2021, up to CZK 150,000 of interest a year can be deducted from your tax base, worth up to CZK 22,500 a year at the 15 % rate. All you need is the bank's confirmation, submitted with your employer's annual tax reconciliation (roční zúčtování) or your own tax return.