Trust me, if she's anything like my wife, she'll go ballistic
Under no account should you tell her, as this will give her too much time to think about what you have done. She'll come back on Tuesday, blood boiling 'cause she doesn't know what to expect, along with a very good viable and justified reason for it to be gone. Simple as that.
When you do let the cat out of the bag, tell her while she's watching all of her soaps on TV
you can still be with her in the same room, instead of being separated by the ceiling
The only problem that I can see with a setup like this, is who's going to make the cups of tea and coffee all night and who's going to do the dishes (Perish the thought)
Apart from all of that, it's perfect where it is and your wife should be thankful that you've utilised the space in a very thoughtful manner