How do I get to sewers

Quick question about map traversal. It says I unlocked sewers and the other map and I was just wondering how to get there? Do I have to make it all the way through the alley first every run? Like do I have to beat the Rat with the bombs before I can go to the sewers every time or is there a way to change maps I start the next adventure in from the house? Thank you! Pretty new to rogue lites so this is mostly new to me.