A modular fenestration system includes a collection of standardized functional units such as double hung window units, casement window units, picture window units, transom units, and the like. The functional units all share common sightlines, common glass set-back, common interfaces, common design language, and are sized according to a unique modular or incremental sizing grid. The sizing grid specifies individual window units in terms of the rough opening size in which they are designed to be installed. Individual functional units are easily combinable in a vast array of combinations to create a large number of possible composite window units. The system also includes a collection of interior and exterior trim units, grilles, and hardware in various architectural styles that can be attached to installed standardized functional units via the common interfaces to match the architecture of a particular home or building.