New houses are easy. Okay, so they are not exactly easy, but they constitute an entire ensemble from the same cloth (hopefully). All decisions to make it whole are up to the designer and the program. Additions and renovations, well, they are a little more difficult to find the right mix of 'keep' and 'add'. But that is what makes them fun.
In every addition, there is a decision to be made. 'We have this much house, we want to have an extra (fill in your favorite addition-bedroom, living room, garage, etc), how much will it cost?' A common question that you may have thought about, but it relates wholly to the real question, 'How much do we keep?'
In some projects, the delineation is straight forward. They are usually garage additions or some sort of new space that relates to the ground level (kind of tough to put a walk out patio on the second story). Others are not so straightforward. There are bearing lines, room adjacencies and massing and proportion effects that would add/e detrimental to the overall success of the project. It is my job as an architect to find out how to solve this. The three items above lead into the three main issues. Cost, function and form. Blowing any one of those may ruin the entire project.
Many times, the project budget helps make decisions, but there are occasions where removing something and rebuilding it new make the whole project fall into place. I recently had a project that was a garage addition to a traditional home in a Boston suburb. The original home had a two story previous addition that was used as a flex space that was extra space, but not really practical. In sketching the options, I moved the trace by accident and saw that the removal of this underutilized space could gain the project an easier framing detail, more buildable area that flowed with what the client wanted from the space, and allowed us to create a roof line that was in keeping with what was happening in the original house instead of looking like we had just tacked on a full addition with no respect to what was there.
Seeing the potential in adding by subtracting, we found a way to make the project better from all three of the design goals. Allowing our minds to open to the possibilities of what seems like more work, but in the end, is simpler, more easily planned and eliminates the massing nightmares is a fun result of looking at the project and its needs from all angles. This over built, underutilized area became the knuckle that the project needed.
So figure out what you have and what you want, then let your architect come up with the best way to create a whole project that brings the addition into the home, and not just tacks it on.