Congratulations on cleaning up the area. Few people would do such a thing. Where I work most people won't bother to clean up messes they leave in the shared microwave and refrigerator.
To start addressing your concerns: Are you certain the boss was not already planning to make a different use of the area? That he wasn't already in communication with these outside people who are going to help him achieve his vision for the area? It is possible there were plans already in place that you weren't aware of.
It seems there is a lack of communication here. While unfortunate, that seems common in IT. From what I see, any hope you have to resolve this issue in a way you find favorable will require that you improve your communications with your boss. That said, there's no guarantee that improved communications will get you what you want.
To start with, I suggest asking your boss if you can talk about the back yard. Say something like "Mary, Tom, and I worked a lot to clear that area up. We were hoping to use it as an area to get some sun while we take relaxation breaks. Now it seems you want to convert it to a glassed in office. Is there any way we can re-visit that plan?" Of course, you will change the details to fit the facts of your situation. The boss may say something like:
- "Oh, I didn't realize and hadn't thought of using it that way, I like that idea."
- Or maybe "Hmm, I really need that glassed in office, as that potential client loves things like that. However, we can compromise by splitting the space and making some of both."
- Of course, you might get something like "Sorry, it's my space and I've made my own plans for it."
The first reaction should be to your liking. The second will require some work on both your parts; maybe neither of you will be completely happy, but you'll each get something toward what you want. In the third case you're unlucky; regardless, you'll have tried and let the boss know your feelings about the matter. In the future, improved communications are likely to avoid situations like you are in now.
Another thought: Did you do this clean up on your own time or was it something you did during work? If it's the latter, the boss may be upset with you for doing something outside the scope of your job. (Now, I must get back to work myself!)