to enter or step into a house/room (often implies stepping inside; can suggest entering without permission)
子どもが母の部屋に上がりこんできた。
The child came into his/her mother's room.
訪問中に犬が勝手に庭から家に上がりこんでしまった。
During the visit, the dog ran into the house from the garden without permission.
住民は、警察が令状なしに家に上がりこんだと訴えた。
The resident complained that the police entered the house without a warrant.