- All teams from the four Treta liga groups and the 28 OG may enter the Preliminary Phase.
- Within each oblasten savet (of 28) teams play against each other until a single team remains. This is followed by eliminations between those winners within each of the four zonalni saveta, until a team each is determined, a total of 4.
- The winners of Northeastern Bulgaria and Northwestern Bulgaria meet each other to determine a representative of Northern Bulgaria.
- Winners are determined over one leg with penalties if score is level after regular time.
- The lower level team within each pair is host. If both teams from a pair are from the same level, hosting is determined by draw.
- The three winners of the Preliminary Phase must be determined no later than 10 September 2018, Monday, 17:00 EEST.