Better than codes are metrics: if you come up with a way to measure the sound conductance between units, it could be advertised (or maybe require labeling) with the unit, and I believe consumer behavior will strongly encourage the construction of quieter housing.

As for cramped/yard, the square footage and presence of a balcony are easily determined by the buyers. But building codes should accommodate balconies by exempting them from limits on floor-area ratio and building footprint.