A method for replacing a conventional lavatory monument with a modular lavatory system in an aircraft cabin, the method comprising: removing the conventional lavatory monument; positioning a base lavatory module (9, 930) in a region of the aircraft cabin previously used by the conventional lavatory monument, wherein the conventional lavatory monument utilized a footprint having a width Y, and positioning the base lavatory module comprises positioning a lower edge of a first side panel (21, 831) of the base lavatory module of the modular lavatory system near a first end of the width Y of the footprint, wherein the base lavatory module has a lower width (10, 830) less than Y and an upper width greater than the lower width, wherein the first side pane (21, 831) comprises a widening region disposed above a passenger seat headrest height and extending to an upper edge of the first side panel; anchoring the base lavatory module to an upper structure of the aircraft cabin using one of at least two overhead anchors provided for fixing the conventional lavatory monument; positioning an inner side panel of a selected interchangeable module of at least two interchangeable modules (16, 70) proximate the other side panel of the base lavatory module; and anchoring the selected interchangeable module (16, 70, 1120) to the upper structure of the aircraft cabin using another one of the at least two overhead anchors; further comprising relocating a preexisting aisle attendant seat (832, 922) to a location proximate an aisle panel of the base lavatory module such that the preexisting aisle attendant seat does not block a door in the aisle panel of the base lavatory module. removing the conventional lavatory monument; positioning a base lavatory module (9, 930) in a region of the aircraft cabin previously used by the conventional lavatory monument, wherein the conventional lavatory monument utilized a footprint having a width Y, and positioning the base lavatory module comprises positioning a lower edge of a first side panel (21, 831) of the base lavatory module of the modular lavatory system near a first end of the width Y of the footprint, wherein the base lavatory module has a lower width (10, 830) less than Y and an upper width greater than the lower width, wherein the first side pane (21, 831) comprises a widening region disposed above a passenger seat headrest height and extending to an upper edge of the first side panel; anchoring the base lavatory module to an upper structure of the aircraft cabin using one of at least two overhead anchors provided for fixing the conventional lavatory monument; positioning an inner side panel of a selected interchangeable module of at least two interchangeable modules (16, 70) proximate the other side panel of the base lavatory module; and anchoring the selected interchangeable module (16, 70, 1120) to the upper structure of the aircraft cabin using another one of the at least two overhead anchors; further comprising relocating a preexisting aisle attendant seat (832, 922) to a location proximate an aisle panel of the base lavatory module such that the preexisting aisle attendant seat does not block a door in the aisle panel of the base lavatory module.