I don't know much about Eberron, but I can tell you a little bit about Sawmills, having done some research on them recently myself for a game.
First, there's going to be a woodshed where the new wood comes in and is stored. This is probably a large Yard type area with multiple piles of lumber under open shed roofs. More valuable stuff may be in an enclosed smaller yard, but it all needs to be fairly open so the lumber can dry properly.
Second, there's going to be a conveyor system to take the logs into the mill where the saws are. This can be just physical labor, or it could be a powered conveyor belt system with ropes and gears, etc...
Then there is the area where the cutting goes on. If you're using water-power, the water is likely below the mill and there will be water-wheels coming up through the floor and gears and pulleys running off them. If wind powered, then the power source will be above, and the gears/pulley system will come down from above. Bark is stripped, logs are trimmed to manageable sizes, and it is sorted into various categories.
There could even be a number of different milling rooms (the rough cut room at first, then stations where veneer is cut, lumber is "finish cut", planks are hewn, and other stuff is made. Depends on how much "finishing" they do here. Also, do they make any actual products, or does everything get sold to other folks who do the finish work?
You should have one floor that is mostly machinery, with a few machinists tending it, and another floor that is machinery and workers. Big holes in the floor or ceiling where the two mesh would be common. Dangerous areas where pulleys run, gears and saws mesh, etc... could be anywhere.
Then there will be offices, tool stores, some sort of room or rooms where the workers rest and eat lunch, etc...