Only one charter school turned down by the city Department of Education for more space lost its appeal to the state — and it was for a pre-K program.

Universal pre-K is Mayor de Blasio’s signature educational initiative.

But the city rejected the request from Bronx Boys Preparatory Charter School to add a pre-K class in the building it shares with PS 107 in the South Bronx.

City officials said they’re not legally obligated to guarantee pre-K classroom space under state law because pre-K is considered an optional “program,” not a mandatory “grade.”

Bronx Prep appealed to Albany, but lost.

City education officials defended their decision. They said the school was offered alternative public space but chose a private location instead.

There are multiple charter pre-K programs opening this fall in DOE buildings — run by the Success, Renaissance and Community Partnership charter school networks, according to city officials.