Our Montclair addition was tiny, but it cured the house all of its layout problems in one shot. While only big enough to add a small mud room, laundry, and half bathroom, it let us take the back door out of the sitting room.
This made a huge impact on an unfurnishable room at the heart of the house. Because it had five door openings and no empty walls, there was nowhere to put a sofa. Now, we have a room made for lounging and TV watching.
The old kitchen had been an awkward size: too narrow for an island but too wide for easy use. Now, not only is it considerably more spacious, but everything is within reach. And we replaced its backsplash with a strip of windows to open up views to the backyard.