This photo provided by Rill Architects shows the inside of a garage in Emmitsburg, Md. (Photo: AP)

What if your home had a spare room you’d never noticed before? Your garage, even if it has one or more cars in it, can pull double-duty as a gym, a crafting room and even a place to socialize.

Designers and architects tell us that gaining more living space without putting an addition on your house can make the effort worthwhile, even if it means investing in things like upgraded lighting, flooring and heating.

We shouldn’t be “treating the garage as a big box,” says Bethesda, Maryland-based architect Jim Rill. “Make it another room. It’s a lost opportunity if you don’t.”

Marina Case, founder of the Warwick, New York-based design firm The Red Shutters, agrees: “A garage,” she says, “can be anything you need it to be.”

We’ve asked Rill, Case and interior designer Anna Maria Mannarino of New Jersey-based Mannarino Designs for advice on creating a well-organized garage that can also function as a flexible spare room.

FLOORS AND WALLS

Upgrading the look and feel of your garage can start at the bottom: Paint the floor, says Case.

Painting a cement floor a dark taupe or gray can have a big impact, she says, or choose an even bolder color. “You’ll feel like you’re in this fresh, fun space,” she says.

But do test the color by painting a piece of foam core that’s at least a few square feet, she says, and leaving it on the garage floor for a few days to make sure you like it.

Another option: Showroom flooring is available for as little as $5 per foot, says Rill. And if you won’t be parking cars in the garage and are instead using it as a “man cave or a she-shed,” Mannarino says, consider upgrading the flooring with something you’d normally use inside the house.

Walls come next: “Why is the garage always just a drywall box?” Rill asks.

If your garage walls aren’t sheet-rocked, Mannarino says you can add that and give it a coat of paint. Or put up paneling, Rill says, making it easier to hang items like rakes or hoses. You can add a flat hanging system that includes space for hanging baskets and brackets for shelves. Many closet-design brands offer flat systems that will hold heavy outdoor items.

If you prefer freestanding storage along the walls, add several tall, sturdy shelving units. You can line them with large, clear bins neatly labeled, or fancier storage bins, Case says.

Or go an extra step and have built-in cabinetry installed.

And if your garage ceiling is high and has ample space away from where the garage door opens, consider adding storage on the ceiling, Mannarino says.

“It gives you that much more real estate,” she says. But don’t cut corners: Have ceiling shelving or storage racks mounted properly by a professional.

HEATING AND LIGHTING

If your garage gets cold in the winter, you can add a separate heating system that’s inexpensive to run. These “mini-split” heating systems can be turned on only when you’re spending time in the garage. Adding insulation also helps control the climate, making the garage feel more like an indoor room.

And don’t settle for a bare bulb in the ceiling. Replacing it with a larger, more attractive fixture can dramatically change the way a garage feels.

ENTERTAINING OPTIONS

Although it’s common to have a workshop in a garage, and many people use the space for messy crafting projects or as a home gym, a garage can also become an entertainment space.

If you’re a car enthusiast who works on a vintage car or hotrod, Rill says, why not use part of your garage as a place to hang out with friends talking about cars?

Case suggests adding a bar area with comfortable seating, even if it’s small, to make the garage an inviting place to hang out with guests. You can also hang up a flat-screen TV and add a refrigerator.

Rill has a vintage cooler, reclaimed from a supermarket, in his garage for soft drinks, water and beer. It’s used all summer when the family is outdoors.

Along with year-round entertaining inside a garage, these designers point out that an open garage can be a great place to set up a buffet table during an outdoor summer party.

Case suggests adding ceiling-mounted tracks for curtains in an indoor/outdoor material like Sunbrella, so you can draw them behind a serving table in your open garage.

Barn doors or other types of upgraded garage door can make the space more attractive and accessible during parties.

And upgrading your garage door does more than just add beauty to the exterior of your home, Mannarino says. It also gives you the option of adding more windows, bringing natural light into your garage.

IS THERE AN ATTIC?

Many detached garages have a tiny second-floor attic or loft space, Rill says. Even if its ceiling is low, that space can become a furnished clubhouse for younger kids, a place to practice musical instruments or even a cozy guest suite.

On one garage project, Rill replaced the solid wooden ceiling in a large detached garage with a perforated metal floor. That gave added natural light to the attic space above, which was then transformed into a kids’ clubhouse.