My wife has similar desires for fresh air, which bodes ill for us reducing our carbon footprint in future years
Your problems aren't ours only because our bedroom is to the front of the house, not the rear and you might consider relocating your Master Bedroom.
You're still not seeing the logical fallacy which I seem to I see.
If you sleep with your windows open
now you are taking a risk.
As it stands anyone could come up to your house and get in with a ladder.
Once you extend
anything to the rear you will have a single storey building outside your rear elevation regardless of what it looks like.
Short of making the roof out of steel spikes or sloping glass covered with oil [or conversely, Superglue], thieves can use it to gain access.
Perhaps you could consider a security grille inside the 1st floor windows, but lets face it, they could simply come in the front door or window.
ONQ.