Over the last 39 years, Michael Litchfield has built one new house, acting as both designer and contractor, and extensively remodeled five others. He has written nine books on the design, construction and renovation of houses, including one on remodeling that runs to more than 600 pages, and he writes the Cozy Digz blog for Fine Homebuilding magazine, where he was a founding editor.

Clearly, Litchfield is an expert. But, as he demonstrates in his latest book, “In-laws, Outlaws, and Granny Flats: Your Guide to Turning One House into Two Homes” (Taunton, $25), he still remembers the befuddled perspective of a beginner. He has tailored his message accordingly, with lots of information, no jargon.

Knowing from his own experience that many readers may decide to do the entire project themselves, Litchfield begins “In-laws” with tasks that would ordinarily be undertaken by an architect or builder, including a primer on the planning review process and potential zoning issues that could derail your project before you start. He advises readers to attend one or two public hearings held by their local zoning commission to get a sense of what to expect. Success with getting your plans approved, he adds, can also depend on the support of your neighbors. Like most of us, they do not embrace change easily, so it’s important to bring them on board early, explaining what you want to do, how it might affect them and how you are trying to minimize this.

Should you decide to hire an architect and a builder, Litchfield advises that you have their contracts reviewed by a knowledgeable real estate attorney, advice I routinely offer but have never read elsewhere.

At the heart of the book are 39 examples of in-law units, technically known as accessory dwelling units or ADUs. Litchfield divides these into six approaches: going up (converting the attic); going down (converting or excavating to create a basement); carving up (reconfiguring the space within the existing building envelope); bumping out (adding an addition); converting the garage; and building a separate unit on your property.

From a planning and zoning perspective, what differentiates these projects from a typical renovation project is the addition of a kitchen. This enhancement creates the possibility that your unit may eventually become a separate rental, even if you intend it for your elderly parent who will hardly disrupt the neighborhood or add to the parking problems.

From a design perspective, Litch­field said in an interview, the major difference between this type of project and a typical renovation is the relationship of the owners to the person who will occupy the accessory unit.

If the occupant will be a renter, maximum privacy between the units and very separate entrances is paramount, subject to the constraints presented by your building lot, and local setback and height restrictions, Litchfield said. If the occupant will be an older parent, the owners will need a design that affords privacy while it facilitates communication and interaction. Although the granny flat may have its own kitchen, the parent may eat most meals with the family, so easy access to the main house will be important. The owners may eventually need to monitor the parent, so this will also have to be factored into the design.

On the other hand, given the changing demographics of American households, the family member who will occupy the “granny” flat may be an adult child who wants independence and privacy, especially for overnight guests. But the child will still welcome interaction with Mom and Dad, want to share the laundry and welcome old family rituals like having dinner together.

When the occupant is a family member, another issue is the degree of input and control that person can have in the design process, Litchfield said. It’s reasonable for the owners to take charge of the overall design, but leaving the final decisions, including such things as paint colors, to the occupant, can help that person to feel that it’s “his place.”

Of Litchfield’s 39 examples of granny flats, only one addressed the design issues involved when a parent has dementia, confusion and frailty, often cited as the reasons for moving a parent into a household. In this case, the 340-square-foot unit was physically separated from the main house because the mother, who has Alz­heimer’s, was given to singing and constant repetition. Tailoring the plan to the special needs of someone with this disease, the architect, Anne Phillips of Berkeley, Calif., simplified the space to minimize confusion. As she explains in the text: “People with Alzheimer’s can get lost in their own homes. So you have to reduce the number of choices they must make to get around.” All the interior doors were eliminated except for the bathroom, and the “hallway” is a tiny spot where it’s possible to see into the kitchen, bedroom and bathroom at the same time.

The units described in the book range in size from about 250 to 550 square feet — from tiny to merely small. Nonetheless, the designers have managed not only to include the necessities — kitchen, bathroom, and living and sleeping areas — but to do so with an inventiveness that can make the spaces look and feel twice as big.

The only serious omission is an example of how the enormous, 4,000-square-foot, five- or six-bedroom McMansions that dot the country could be creatively subdivided into separate living units. This strikes me as an obvious move because it would create affordable housing for renters while helping financially pressed owners to stay in their houses.

Litchfield concurred that such conversions seem obvious, but in most cases, he said, suburban residential zoning prohibits it.

Katherine Salant has a degree in architecture from Harvard. A native Washingtonian, she grew up in Fairfax County and now lives in Michigan.