Most materials arrive on site and stay put. Light doesn't — it moves through a room differently at nine in the morning than at four in the afternoon, and a plan that only considers where a window sits, not what passes through it across a full day, is only half finished.
We treat light as something to be specified, not left to chance: how much of it, from which direction, filtered through what. A narrow opening reads differently than a wide one even at the same total area — one draws a blade of light across a floor, the other floods a room flat. Both are valid. Neither is an accident.
The best rooms we've made are the ones where light does some of the work a wall or a finish would otherwise have to do — marking a threshold, drawing the eye toward a single surface, making a plain material feel considered simply because of how it's lit. Get the light right and much of the rest of the design follows from it.