The booklet titled "Accessory Dwelling Unit Manual" may not hit The New York Times bestseller list any time soon. Published last month by The City of Santa Cruz, the booklet does not sound all that impressive: It is a how-to guide for the design, construction and operation of rental apartments, best known as "second units" or "granny flats," located in the back yards or atop the garages of conventional single-family houses.

Whether or not the ADU Manual makes the Oprah show matters little. This unprepossessing little booklet is dynamite because it promotes the least popular type of housing in California. Santa Cruz would be noteworthy simply for tolerating the units (see CP&DR, August 2003). Going further and actually advocating for granny flats is an enlightened gesture, and perhaps an act of political courage.

To understand why I am impressed, one need go to a neighborhood homeowner’s association meeting almost anywhere in California. Just as one does not shout fire in a crowded room, one does not dare suggest that granny flats are a good idea at a homeowner’s meeting, unless you want to wear a tiara made out of brickbats. Few issues are as irrational — or hypocritical — among certain homeowners as that of second units. Irrational, because people believe that their neighborhoods will be degraded by the presence of these extra units, as if a rental unit for a student or a single school-teacher in the backyard were equivalent to an apartment house. Hypocritical, because many California cities are awash in illegal second units. Los Angeles alone probably has tens of thousands, if not hundreds of thousands, of illegal second units.

Few homeowners will build legal second units, however, because most cities impose onerous conditions, such as two off-street parking spaces in addition to the two off-street spaces that the single-family residence is already required to have. So forget about converting the garage as a place for your divorced sister-in-law and her kid — unless you want the back yard to become a parking lot, which is probably against the zoning code anyway.

The bizarre emotions surrounding granny flats are all the more unfortunate because second units are arguably the most equitable way of increasing density across an entire neighborhood or city. The alternative is to create a two-tier society of people who live in very low density, single-family neighborhoods, and people who live in very high-density apartment complexes located on major thoroughfares. While there is nothing wrong with high-density apartments, renters should have a choice of housing.

So why are granny flats unpopular? As always, density is the bugbear. The conventional argument is that second units contribute to tight parking and traffic. Probably true. But the solutions to traffic problems lie in public transit, not in restrictions on housing. Second units provide an additional — and voluntary — housing resource in crowded California cities, and one that can help working families pay for ever-more-expensive homes with the help of a rent check each month.

The Santa Cruz booklet goes beyond the theoretical. The booklet itself is worthy of attention for both the clarity of its writing and the diagrams. (The booklet can be viewed online at Paid for, in part, by a $350,000 grant from the State Treasurer’s Sustainable Communities Grant and Loan Program, the manual is written in ordinary language. The topics cover everything from how to figure the size of the second unit based on the size of the house, to the range of different architectural styles that can be accommodated within a single envelope.

One particularly interesting diagram shows the ideal placement of granny flats along the three street types typical of Santa Cruz, including "traditional," "transitional" and "postwar." This diagram shows the depth of thinking that went into this planning exercise: This is a design for entire neighborhoods and districts, rather than for individual lots.

The intent is to protect the integrity and scale and streetscape of neighborhoods. In this way, the how-to book on fitting a second unit into the backyard becomes an exercise in urban design, or how to increase density in existing neighborhoods without destroying them. Accompanying the book is a second publication that features plans for seven different models of accessory apartments. The same city program offers technical assistance to homeowners, including low-interest loans to homeowners who agree to rent their granny flats at affordable rates.

Now, good design is not rare or hard to find. But it is rare to see intelligent design integrated into what is essentially a planning document.

This booklet comes into being at a time when a growing group of architects and city planners are talking about what is called "form-based" building codes, which are opposed to traditional "use-based" building codes. The basic idea is that planning should promote definite kinds of city form, rather than abstract zoning formulae that are concerned entirely with density ratios and uses. While the ADU Manual is not a planning document, the easy-to-understand approach to civic design contains a hint of what form-based planning might look like, wherein everybody, not just architects and developers, will be able to understand what is and is not permissible to build.

Even if the ADU Manual is not promoted on Oprah, it would be worthwhile for this booklet to circulate throughout the state as an example of critical thinking. People who are interested in preserving neighborhoods in the face of development pressures might do well to study the booklet and learn how cities can introduce new housing into built-out neighborhoods without destroying them.