From Longman Dictionary of Contemporary Englishopen onto/into something phrasal verbENTERif a room, door etc opens onto or into another place, you can enter that other place directly through it The door opens onto a long balcony. → open→ See Verb table
Examples from the Corpus
open onto/into • In addition, the door that opens on to the roof is not marked.• It opens on to open countryside and is easy to get to.• The key opens on to a beige carpet that looks like a water poodle with burns and saddle sores.• The only source of light and air is the doorway at the end of the bed, opening on to a tiny balcony.• An inquest has been opened into his death.• Now we are opening into new areas.• The store will stay open into the evening today to accommodate last-minute shoppers.• And through the archway that opened into the saloon, he saw a flicker of light. 