Elizabeth Passmore ruled that parents' right to a place for their child at the school of their choice was more important than Cheshire council's desire to limit the number of surplus places.
Little Leigh primary appealed to the adjudicator after the council rejected its request to increase its planned intake for September 2005 from 18 to 21 pupils.
Defeat for the primary would have thrown Tony Blair's promise to increase parental choice into disarray. This year it had to turn away 11 prospective pupils, although three later won places on appeal.
Cheshire council said that it would accept the adjudicator's decision.