"Mortgage banks" (the purpose of which is to "grant mortgage loans and the issue of mortgage bonds and/or public mortgage bonds based on the claims of a mortgage bank") may be established only in the form of a joint-stock company.
"Art. 1.
The Act specifies:
1) rules for the issue, sale, purchase, redemption and security of letters mortgage loans and
2) the principles of establishing, organizing, operating and special supervision over mortgage banks.
[...]
Art. 9. 
1. A mortgage bank is established only in the form of a joint-stock company. [...]" (Act on mortgage bonds and mortgage banks (Dz.U. 1997 nr 140 item 940), last amended by the Act of 16 October 2019, Articles 1 and 9 [google translation])
Act on mortgage bonds and mortgage banks (Dz.U. 1997 nr 140 item 940), last amended by the Act of 16 October 2019, Articles 1 and 9
[<a href="http://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=WDU19971400940">Act</a>],[<a href="https://www.knf.gov.pl/en/MARKET/Licensing_and_registration/Banking/Banks/Forms_of_activity/Mortgage_bank">Other</a>]