Section 118(3) in West Bengal Municipal (Building) Rules, 2007
(3)Bathrooms and water closets shall be provided with natural light and permanent ventilation by one of the following means :(a)windows having an area of not less than 10 per cent of the floor area and located in an exterior wall facing a street alley yard or an airshaft whose dimension in the direction perpendicular to the window is not less than 1/3 of the height of the building on which ti, window is located subject to a maximum and a minimum limit of 6 m (20 ft) and 1 m (3 ft) respectively;(b)skylights, the construction of which shall provide light and ventilation as required in clause (a):(c)ventilation ducts, provided such ducts have 1.30 cm2 of area for each m2 (2 in2 of area for each ft2) of floor area with a minimum total area of 330 m2 (48 in2) and a least dimension of 9 cm (31/2 in). The vent register or grills to the duct shall be located in or next to the ceiling or the ventilated space. A separate duct from each space or room shall run to and above the roof and shall be provided with a lowered vent stack :Provided, however that when an exhaust fan is used, the duct opening may be located in an outer wall.
