The dungeon build screen allows you to add rooms onto your existing dungeon.
The build screen lists all of the dungeon room deeds you have in your inventory that you can use. As you can see from the image above the right side of the deed display lists the level of the deed but you can not use a deed that is higher than your level to expand your dungeon.
When you choose a room deed the bottom of the build screen will show a build button that allows you to build the room in that location.
You will have to exit and re-enter the dungeon for the room to show up on the mini-map.