How Sunlight is added in Bidirectional Path tracing?
This can be done in two ways - either use a sky with a sun in it as an cubemap-type light source outside your building, or use the windows as light sources. With BDPT, you could sort of do both, and consider the windows area light sources where the light differs depending upon the angle, using a sun and sky image/model, and then you could implement importance sampling so that you usually get light rays going from the sun instead of the darker sky. This way you get all the accuracy of a full sun and sky model, and it'll still look good at low sample rates.