What home services could be centralised? I think in a block of units (or a group of townhouses) it would be easy enough to fit one large air conditioning system for the whole building, and probably more efficient too. Same goes for hot water, in theory, though you might need internal hot water meters to divide that part of the electric bill fairly. I know some places have communal laundry rooms and when you get right down to cheap student housing you start getting shared bathrooms and lounges.
My point is that if we start overcrowding and also try to conserve energy and other resources, our current ideals of free-standing houses with private gardens and separate amenities is going to start looking pretty wasteful of both space and energy.
Mokalus of Borg
PS - Most eco-house concepts still seem to be free-standing.
PPS - Green living in a society really wouldn't look like that.