HACKENSACK, N.J. — For decades, American houses got bigger while family sizes got smaller. But lately, there’s been a shift. As the economy soured, more extended families began sharing space — whether it was unemployed young adults moving back with their parents, or financially strapped senior citizens moving in with their children.

Michael Litchfield thinks this trend will continue, as Americans increasingly question whether they’re making the best use of their space. In his new book, “In-laws, Outlaws and Granny Flats” (Taunton Press, $24.95), he shows how to create secondary living spaces — also called in-law units, granny flats, and accessory dwelling units — in ways that protect everyone’s comfort, budget and privacy.

Litchfield himself lives in a second unit on an old dairy farm in northern California, in a renovated building that used to contain tractors.

The Record spoke with him recently about turning a house into two homes. (Interview condensed and edited.)

Question: The projects you look at in your book are not just for the in-laws or Grandma. They’re also home offices, they’re places for adult children who are coming back.

Answer: In some cases, young people choose them so they can put an au pair in the unit, or maybe they need the rental income to qualify for a mortgage.

Q: Do you expect that as the baby boomers age, second units will become more popular?

A: Oh, absolutely. I think the AARP said it’s about a third of the cost to care for someone at home or in a community-based setting, as opposed to an institutionalized setting.

Q: A lot of the projects in your book seem to be about 400 square feet. What’s the minimum amount of space that works for one of these units?

A: Local zoning codes are going to determine minimum and maximum sizes, but my feeling is that if they’re carefully designed, 350 to 400 square feet can work.

Q: What are some of the tricks to making such a small space livable?

A: If you’re working with a small footprint, it’s really a good idea to have high ceilings. Somehow, that increase in volume just makes it feel bigger. Lots of natural light is also really important. It’s a more cheerful space, and if you’re incorporating the outside in your view or as a patio or deck, that gives you the sense of more space.

Also, furniture with multiple functions — such as a window seat or dining bench with storage underneath — is another good space saver.

Q: A lot of the units are handicapped-accessible. Why is this important?

A: Even if a homeowner is in relatively good shape now, it makes a lot of sense to think ahead, because you can count on getting older and infirm.

To make a house more accessible, you need doorways and hallways to be at least 36 inches wide. That allows someone in a walker or wheelchair — or for that matter, a mother with a baby on her hip — to get around much more easily. Also, eliminating or reducing the thresholds.

Sliding doors and pocket doors are generally much easier to open if you have limited mobility, and they take up less space than swinging doors.

Q: What are some of the biggest challenges in these projects?

A: It’s really important to check in with your neighbors and keep communications going. The less you can impose on your immediate neighbors and the neighborhood, the less likely that you’re going to encounter resistance.

Q: You have a chapter on getting approvals from the town, which can be difficult.

A: It depends on the town. Where there’s a compelling need for these things, people are going to build them anyhow — hence the “outlaws” in the title. So zoning is starting to change. Planning commissions don’t want to impose constraints on people that are really unpopular.

When cities allow private individuals to create second units, they’re actually creating affordable housing. And it’s not costing the taxpayer a dime. So everybody’s winning.

Q: Could you talk about the main kinds of projects you have in the book, and what are the challenges of each?

A: I’ll go from the simplest to the more complex.

The carve-out is basically when you appropriate a couple of rooms and a bathroom, and you pretty much just need to add a kitchenette to make it an autonomous living space. That’s really good for a caregiver or an elderly relative you want to have close. It’s less good for a rental unit because there are privacy issues.

A basement is another option. If you’ve got a damp basement, it’s probably a good idea to consider other alternatives. But if it’s a dry basement and you’ve got enough headroom — usually about 7 feet 6 inches is required by code — and especially if your house is built into a hill where you’ve got a large wall that could be used for windows to get light in, that’s a really good situation.

In basements or any in-law unit where you’re sharing a wall or a ceiling, you have to be concerned with soundproofing.

Then there are bump-outs —  that’s sort of like a carve-out, with another room attached to the house.

Q: Sort of a small addition?

A: Exactly. That can be a very cost-effective way to go because you’re taking advantage of existing framing and maybe sharing a wall or two. It eats up less of your back yard.

Garage conversions are very popular. They’re separate from the main house, so you’ve got good privacy. Also, garages tend to be minimally framed or unfinished, so you don’t have to tear out a lot of stuff.

Stand-alone cottages are probably the most flexible space in terms of who you can rent it to or put in there. But they can eat up a lot of your back yard. If you have a medium to large lot, they’re a great way to go.

An attic conversion is probably best for someone who’s a relative or someone who’s young, because stairs are an issue. Sound is also an issue. If the attic was originally framed to be a living space, then you’re probably OK, but if not, it can be a very expensive and disruptive way to go.

Q: What do you personally like about the lifestyle?

A: I decided to do this in my mid- to late-50s. Besides writing books, I’ve also worked as a creative director in advertising in San Francisco. I was just getting tired of corporate life. It was too much pressure, too much stress and I thought, ‘I’m healthy now — why do I have to wait till I drop in my tracks before I think about enjoying my life more?’

I’ve reduced the things that I have and the amount of money I need to live, and it’s just really agreeable. It also gives me flexibility to travel rather than having all my money tied up in a house.

Q: You have a quiz in the book with questions like, “How many siblings did you grow up with,” and “Do you share your meal at restaurants?” Why do people need to consider these kinds of social and psychological issues?

A: Even if a person has a compelling economic need to build a rental unit, if it’s not right for you, it’s going to make you unhappy and your tenant is not going to feel at home. This personality test is an unscientific way to get people to think about, will this really suit your temperament?

Q: The projects in the book, as well as the philosophy of not needing a huge amount of space, reminded me a little of the homes in Sarah Susanka’s “Not So Big House” books. Does your work have some of the same goals or principles?

A: I do think we’re going in same direction. She speaks about her “Not So Big House” phenomenon as being a reaction to the soullessness and literal emptiness of the McMansions. I think America has a huge amount of unused space, of houses that aren’t being used flexibly and cost-effectively. I’m thinking of a single person or couple rattling around a 3,000- or 4,000-square-foot house.

The book is asking people: You’ve got this enormous house; is it doing all you need it to do? Might the space be put to better use, either as a home office or to put up your boomerang kids or to provide a place for a caregiver? America probably has 30 to 40 million seriously underutilized homes. I don’t think most people can afford to be this wasteful.