Computer simulation allows to study daylight conditions in the past that afforded activities in antique buildings. The Python module phos4dtools implements the efficient computation of zonal daylight metrics that are considered to indicate affordances. It was employed to solve horizontal and vertical illuminance for different orientations and elevations in the House of the Priestesses, a unit of the Hadrianic Garden Houses complex in Ostia. A reconstruction model of the unit was produced by collating an existing, detailed 3D documentation with other sources and our own survey data. The spatially and temporally resolved results of daylight simulation employing phos4dtools were imported into a GIS database. Assuming typical reflectance properties, illuminance thresholds were determined that are required for the perception of contrast detail and colour differences. Integration over temporal periods and spatial zones that are eligible for residential activities was implemented by queries to the database. First, preliminary results indicated different distributions of affordances by daylight, depending on the characteristics of the considered visual tasks. Horizontal illuminance decreases quickly with increasing distance to the aperture, suggesting that activities bound to a horizontal work plane were constraint to the immediate adjacency of windows and potentially open doors. Vertical illuminance, on the other hand, reaches deep into the building when the receiving surface is oriented to a window, particularly in the absence of exterior obstructions. The exemplary application of phos4dtools shows its potential in the interdisciplinary research on daylight and its implications on living practice in antique buildings.