How urban is urban?

Self-driving cars have 'levels'. How about a set for urban detached housing?

• L3 Urban: when Monopoly-style aggregation of parcels makes sense, because it's cheaper to buy your neighbors house and expand than to tear down your own house and build a bigger one.
• L4 being when you buy an existing house, tear it down, and build a new bigger house in its place.
• L2 being when you still have enough property to add another extension onto your house.
• L1 being...the first house built on the lot.
• L5 is London-style 'iceberg' houses when 80% of the house is underground, because you can't get planning permission to build anything bigger than the house you just tore down
• L6 is when you start building tunnels between Iceberg houses. 

This assumes that the only thing you can build is a single family house, for 50%+ of urbanized land, that's probably true.