Moreover, she says, in many of these areas, the small, narrow lots (think 30 by 100 feet) and homes without driveways or other modern amenities are not as appealing to future residents. "Having a driveway and a garage is a big improvement in one's property," Dewar says, and a feat that can be accomplished in most cases only by adding the lot next door. "It makes (these blocks) a better 2012 area, as opposed to 1912 (when many were developed)."

How blotting works
The process of acquiring vacant lots around an owner's property is different in every city and can take anywhere from 90 days to nine months, depending on the process and approvals necessary.

Owners, in most cases, must demonstrate ownership of their own property and prove that it is up to code and that they have the means to maintain it. They also must inform the city of their plans for the lot they wish to acquire. Many cities require these lots to be fenced in, and some will provide fencing material.

Under the New Orleans Redevelopment Authority's Lot Next Door program, purchasers may also qualify for a grant that provides up to $10,000 in landscaping materials and plants to improve the look of these spaces.  

The cost of gaining title to these extra lots is only a couple of hundred bucks in some areas of Detroit and Cleveland. In New Orleans it can cost $4,000 or more, after a city rebate. If both neighbors want the lot, cities simply divide it and split the cost. Because most of these blots do not hold a separate structure and property values are low, the tax impact is minimal.    

"There's not a lot of value in these properties," says Tobias Armborst, a partner in the Brooklyn design firm Interboro Partners, which coined the term "blot" almost a decade ago in a research paper. "Their greatest value is in their use."

While this type of land grab has been going on for decades, the past few years have seen many owners making big investments in the lots next door, as Harris did. That's because it has gotten a lot easier for people to gain clear title to these abandoned properties through city land banks.

For decades, cities did not pursue tax foreclosures on vacant properties and could only give neighboring owners quitclaim deeds, which did not convey clear title but merely released the city's interest in the property. Others were squatted on and fenced in with a neighbor's yard — a kind of urban homesteading. The land banks have made it easier for residents to claim title quickly and to gain financing from banks for meaningful improvements, rather than simply fencing the lots in to keep vagrants out.