Creating an interior has been made very simple. By default, BuildR has interiors disabled.
To begin creating an interior, click on the interior tab to open the interior editor. If interiors are disabled, you will see a warning and the option to quickly enable them. Interiors are split into floors and are edited on a floor by floor basis. To edit a floor you will need to select a volume and then a floor. The editable space will be highlighted for you in the scene.
Below the floor selection are some options to add rooms, doors and other interior elements.