Infill is one of those development terms that gets interpreted in a multitude of ways. I like to reference the definition supplied for the National Association of Realtors in the White Paper titled Best Practices to Encourage Infill Development written by Robinson & Cole in December 2002.  The definition supplied is as follows:
…not only the development of vacant parcels of land, but also may include the demolition, reconstruction, or substantial renovation of buildings or underutilized sites that may have been previously developed.”
This is a tremendous definition; however it still is quite vague in the first few words of the statement, “not only the development of vacant parcels of land”.  I believe that further clarification stating that existing infrastructure is in place should be added to the definition.  As it is stated, the term is often abused and includes all vacant parcels regardless of the presence of existing infrastructure.
Last weekend, the 2011 Colorado Springs Sustainability Bicycle Tour focused on a portion of the City where infill is the only manner of new development.  On this tour, we visited one extremely large-scale version of infill and other small modes of infill.  All scales of infill from the tour provide positive changes to the adjacent context.
Gold Hill Mesa is a neo-traditional neighborhood constructed in what was previously the tailing of a historic Gold Mine.  It is located between downtown Colorado Springs and Pikes Peak near Historic Old Colorado City.  Major environmental impacts were mitigated to create a neighborhood that will ultimately be a self-sufficient automobile-independent neighborhood.  Without the major commitment that Mr. Willard and his fellow partners have taken at Gold Hill Mesa, the hill would remain as a blighted eyesore near downtown Colorado Springs.
Smaller, incremental changes of recent infill along the tour included urban infill housing.  Two different areas were brought to the forefront for the participants of the Sustainability Bike Tour.  One of which was a site of two homes that I stumbled upon a few years back after visiting Old Colorado City.  The other was one that I recently stumbled upon in the preparation for the bike tour.  Both examples show the opportunity of modifying blighted, or vacated single-family lots, into multiple lots.  Due to the Euclidian zoning that has been applied to otherwise historic neighborhoods, these opportunities often seem unfeasible.  However, with a little research, it is often found that other homes and uses nearby also do not conform to the zoning code.  By incorporating residences that are now “harmonious” with adjacent uses, additional homes and greater profit margins can now be realized.
The following are the two infill sites included during the Sustainability Bike Tour.  The first set of three homes were purchased from a single lot for $82,000 in 2004 and subdivided into three lots which were constructed and sold for $199,000, $215,000 and $220,000 in 2006 and 2007.

Two new homes were recently constructed near Old Colorado City in Colorado Springs.

The second set of two homes include accessory dwelling units above the garage behind the homes.  They were constructed in 2005 and have a current market value of approximately $235,000 each.