dimensions, built in corner desk, stair landing, etc.

[don3211]

Mail slots can be created with Cabinet Partitions.

For the landing, I believe you want to create two landings at different levels. The two landings will break the stairs into two sections that will allow you to place the railing independantly.

Try re-sizing the existing landing to the desired size. Then copy and paste it. Open the 2nd landing dbx and adjust it to the desired height. Then construct the upper stair section.