There was some discussion of this. The issue is that the back left corner of the lot (where the parking lot is now) is very tight for a variety of reasons: the adjacent building has an egress onto the driveway that needs to be preserved, and the building on that side is built right to the lot line so if the library expanded on that side there would be a blank wall right in front of those abutters windows. Also, the existing building is not centered on the lot, so there is more space on the right side than the left.

On top of that, I think that the existing green space in the back and right side of the building (as opposed to in the front) was seen by the design team (and many at the meeting, including me, but not everyone) as pretty low value and low quality -- so there is not much detriment to replacing it with presumably higher quality green space where there is now asphalt.

Finally, there are apparently legal issues with the renovation of historic buildings that make expanding straight back problematic. At least according to what I was told, the standards for renovations of historic buildings generally speaking require that the new additions be architecturally distinct from the existing structure to preserve the historic integrity of the registered building as much as possible. This is much harder to achieve if the building were extended to the rear, which would require removing the rear facade. An addition to the side, therefore, is apparently much more likely to get approval from the various historic commissions that need to approve the plans (I am not an expert, this is just my take-away from what was said at the meeting). Relatedly, the goal of adding an accessible entrance is to make the accessible entrance on equal footing with the main entrance, which could be a hard standard to meet if the main accessible entrance were a back door.

So I think this was looked at in some depth and determined to be infeasible for various reasons.