I don't know if you are all aware, and forgive me if you are, but there is a very powerful Google application which you can download for free and which allows you to perform your own spacial planning, in 3D, before you get an architect involved. The tool is called Sketchup and it also comes with on line tutorials to get you started.
It's very handy for newbuilds and extensions alike. I have used it simply for space planning my house, but I find it very useful in my professional work too.
Have a play and enjoy.