Laid-Back Adventures in Another World from an Apartment in the Suburbs of Tokyo: The Case of My Room Becoming a Dungeon Rest Stop

Tokyo Kougai no Mansion kara no Mattari Isekai Boukenki - Boku no Heya ga Dungeon no Kyuukeisho ni Natte Shimatta Ken
東京郊外のマンションからのまったり異世界冒険記~僕の部屋がダンジョンの休憩所になってしまった件~

Source

My Room is a Dungeon Rest Stop
Boku no Heya ga Dungeon no Kyuukeijo ni Natteshimatta Ken· 僕の部屋がダンジョンの休憩所になってしまった件
novel

Suzuki Touru has just scored an amazingly cheap apartment–but the reason it’s so cheap is the front door opens up to a classic fantasy dungeon! Luckily, Touru has always wanted to explore a dungeon. But when he stumbles across Ria, a beautiful blonde knight, he may well have gotten more adventure than he bargained for! (Source: Seven Seas Entertainment)

Alternative

My Room is a Dungeon Rest Stop
Boku no Heya ga Dungeon no Kyuukeijo ni Natteshimatta Ken· 僕の部屋がダンジョンの休憩所になってしまった件
manga

There has to be a catch when a deluxe rental property is this cheap–and this one connects to a perilous dungeon in another world! Far from being dissuaded, Touru jumps at the chance to sign the lease, hoping to go from high school dropout to big-time adventurer. He soon encounters a stranded knight: a gorgeous young woman named Ria. Touru rescues her and takes her back to Earth, where she promptly mistakes him for a Great Sage, seeing normal things like running water for evidence of his powerful magic. Touru may dream of heroism, but first he has to figure out how to share an apartment with a cute girl from another world! *Source: Seven Seas Entertainment*

Alternative

About My Room Leads to Dungeon: After School Club Activities of Different World Adventure
Boku no Heya ga Dungeon no Kyuukeijo ni Natteshimatta Ken: Houkago no Isekai Bouken-bu· 僕の部屋がダンジョンの休憩所になってしまった件 放課後の異世界冒険部
manga

After my parents disappeared, I, Tooru Suzuki, bounced around from place to place and ended up as a caretaker of a school dormitory in Tachikawa. The building is in disrepair, the student boarders are all weirdos, and of course, there are rumors about ghosts haunting it too. It turns out that believe it or not, the dorm is connected to another world! Although I don't have a single friend in class, I can always just go to the dungeon to slay monsters and level-up. There, I became friends with a slime, a female knight, and even a female sorcerer! Thus, I begins another day of after-school club activities in another world! (Source: Takeshobo, translated) *Note: Includes chapter 0.*