I have to tell you about one of the best kept free online tools.  It's called Icovia Room Planner-

You can make any size room and put any type of furniture in the room, you can re-size things and move walls/windows in a quick click. There are hundreds of furnishings, lighting, plants etc to add into each room.

Honestly, it makes me giddy. No, I don't work for them.

Here's an office plan I made for a client in 30 mins~Check it out!