The late payment fee for 400,000 mortgages overdue for one day is stipulated by the CBRC, which is generally five thousandths of the outstanding amount every day. The following is a detailed explanation of this problem:

1. According to the regulations, the late payment fee for one day of mortgage payment is five thousandths of the unpaid amount per day.

2. The calculation method is as follows:

-First determine the amount of overdue payment, that is, 400,000 yuan.

-The overdue amount is multiplied by 0.5 ‰ per day to get the daily overdue fine: 40,000× 0.005 = 200 yuan.

-So, the one-day late fee for the 400,000 mortgage is 200 yuan.

3. Summary: 200 yuan will be fined for overdue mortgage of 400,000 yuan.

4. Extended information:

The calculation standard of late payment fee may be different for different banks or regions. It is suggested that the borrower should know the relevant terms in detail when signing the contract, and consult the bank or professionals to obtain accurate information.