There are a lot of simple 3D graphics programs that can help you to help people around you to understand where this is going. Put material list price tags on the back of the different designs and you might get the dreamers to come back to reality. You will probably want to avoid the 23 foot wide bottom step that my assistant designers forced me to build on my last deck.
If you are not good at drawing sketches but you want to know what that new or extended deck might look like, just take a photo of the existing deck space from as far and as high up as you can get -- a neighbours house, a tree, a shed roof or even just a ladder. Then make many photocopies of that photo blown up to a regular copy paper size. Now with a few straight lines you can draw your deck in complete perspective together with all the details of your house.
In the photo you can see that just a line parallel to the existing deck and back to the window makes a deck. Add some stairs and a couple of more rail lines and you are beginning to have a working drawing. Take a couple of dimensions, like the length, width and height and add them to the drawing. This is in fact enough for the guys as the renovation centre to make you full scale deck plans and materials lists. If it turns out to be too expensive, just sketch it a bit smaller and try again.
Don't know how to do computer graphics. Go back to the first technique, simply take two photos, blow them up, with an exacto knife cut out the bay window and cut around any objects that are in front of the window. Place the outdoor photo behind the indoor photo and voila. You can reduce the size of the outdoor photo at the photocopy store until it comes into proper perspective compared to looking through the window at the real thing.