You already know where your windows are or where you want to place the mirror in the hallway? You can set the exact position measurements for any object, just follow the instructions below.
To begin with, when you place items on the wall, it is much easier if you use the Snap Furniture function - in this way the objects automatically 'stick' to a wall.
When you move an object along the wall (or anywhere in the room), you can see 4 arrows that show its distance from the four walls in the room.
However, to specify an item's position on the vertical axis, you need to click on the i-button and open the Property Inspector.
The X / Y / Z axis determine the object's exact position in your plan. To specify on what distance from the floor you want to place an item, enter a value for Z. If Z=0, this basically means that the object is on the floor. You can also use the values for Z to place one object on top of another or for different compositions.