Urban design and historic preservation

Learning objectives

• Apply core urban design principles, including enclosure ratios and serial vision, and the lessons of observational research on public space.
• Explain placemaking and the principles of Crime Prevention Through Environmental Design (CPTED).
• Describe design review tools, including context-sensitive standards and the transect.
• Explain what National Register listing does and doesn't do, and walk through Section 106 review.
• Distinguish the four treatments in the Secretary of the Interior's Standards, and explain local historic districts and the rehabilitation tax credit.
• Use other preservation tools, such as demolition-by-neglect rules, preservation easements, and conservation districts.

Key concepts

Principles of urban design

Urban design shapes the physical setting for public life: the relationships among buildings, streets, and open spaces. Recurring principles:

• Human scale: buildings, signs, and spaces sized and detailed for people on foot.
• Enclosure: streets and plazas feel like outdoor rooms when building heights are in proportion to the space's width.
• Connectivity (permeability): short blocks and connected streets give people many route choices.
• Legibility: a clear structure of paths, edges, districts, nodes, and landmarks makes a place easy to understand (Kevin Lynch's elements; see Patterns of human settlement).
• Active edges: ground floors with doors, windows, and activity make streets feel safe and interesting. Blank walls and parking lots don't.
• Mix and diversity of uses, building types, and people.
• A quality public realm: sidewalks, trees, lighting, seating, and public spaces that invite people to stay.

Enclosure, sequence, and views

Enclosure can be described with a ratio: the height of the buildings lining a street to the street's width, wall to wall. A commonly cited rule of thumb is that ratios between about 1:1 and 1:3 feel comfortably enclosed, while a street much wider than about four times its building height starts to lose its sense of being a room. A very tall, narrow street can feel canyon-like, so the ratio works best with trees, setbacks at upper floors, and sunlight in mind.

The arithmetic is simple. Two-story buildings about 25 feet tall facing each other across 100 feet, wall to wall, give 25:100, or 1:4: loosely enclosed at best. Replace them with 50-foot buildings and the same street becomes 1:2, comfortably enclosed. Where the buildings can't grow, designers narrow the space instead: build closer to the sidewalk, or plant a row of street trees that acts as a second, nearer wall.

People also experience places in motion. British designer Gordon Cullen's Townscape (1961) described serial vision: a walk through a town as a sequence of views, with each bend, gateway, or opening revealing something new. Designers use it to plan arrival points and to protect view corridors to landmarks, water, or mountains, often with height limits along the line of sight.

9 more sections follow in the full lesson.