In http://thegamesjournal.com/rules/BigCityExpansion.shtml Greg Aleknevicus says:
The Hospital receives an increasing bonus based on the number of occupied
adjacent properties (including parks and factories):
(It's the number of occupied properties that count not the actual buildings so a double residence still counts as 2 properties in this regard.)
How could a building of 1 space have more than 4 adjacent properties?
In other words, if the adjacent building has 2 or 3 properties (for instance, a residence with a value of 10), it counts not only the adjacent property?