Before you start any model, I think it's important to make sure you plan all the details of what you want to model first. In order to deliver it is wise to draw a sketch or take some reference photographs. I gathered a lot of reference photographs before I started work on this piece (Fig.01).
When I model something I usually prefer methods that allow me to return to the starting point if possible so I have the chance to change my idea. This is a serious advantage not only in one's personal work but also commercially.
I started by drawing the wall of the room, using the AEC/Wall command for it (Fig.02). If you are not sure about the measurements when using this command then you can alter these after you enter sub-object mode whenever you wish. After you finish the drawing, it's important to remember to align the object to the center of the scene. If you change the dimension of the wall afterwards, then center the pivot again and the align operations will be correct. When the snap commands are used correctly, it's possible to work in a precise manner.
Next I activated the Edge/Segment Snap command so that I could start to create the Windows. I create the first window with the Windows/Sliding command (Fig.03). After you create a window, you can change measurements easily. Select and move with Shift button and make copy beside it.
The objects that are created from under the Windows menu will automatically interact with walls. I created the door with the Door/Pivot command (Fig.04).
It's important to make sure that Edge/Segment Snap has opened. If the doors and windows that you create don't intersect with the wall automatically, you can make them intersect after linking them to each other. When you create the objects, don't forget that in order to make them intersect automatically, the first click has to be done on the wall.
After activating the Endpoint Snap, I created the floor plane. Thanks to Endpoint Snap you will see it is easily aligned to the corners. Next I copied the floor object, then rotated it 180 degrees along the X axis and aligned it to the top part of the wall. This resulted in the ceiling (Fig.05).
I created the skirting board using the Line command. After creating the object, I converted it into a solid mesh in the Viewport and Render. Then I selected the section type as Rectangular and set up suitable dimensions in accordance to the proportions of the room (Fig.06).