THE RELATIVELY NARROW function of a bedroom or dining room largely dictates those spaces’ décor. A home’s communal chill chamber, however, has to be a lot of things to a lot of people: intimate and sophisticated enough for guests sipping aperitifs and cozy enough for family couch-potato Sundays. With so much asked of living rooms, the potential for decorating missteps can daunt even experts.

Nina Edwards Anker’s first principle: Start with ease—navigability, comfort, visual calm. “The worst error I see in living rooms is overcrowding,” said the founder of New York City’s Nea Studio. “Spaces, like paintings, need room to breathe.” Among her tips: Allow for ample storage to tuck away clutter. Meanwhile, Aileen Warren, of Jackson Warren Interiors in Houston, warns against filling the room with every stick of furniture on your wish list. “Be sure there’s enough space for traffic to move comfortably in and out of the seating groups,” she said.

Here, designers identify five other living-room gaffes they see far too often, and share their professional workarounds.

1. Conversation Pitfalls

“Don’t design a pretty space for a museum when living rooms are for socialising,” said Marissa Stokes, a Ramsey, N.J., designer. Novice decorators goof up here by leaving chasms between seats or, as Susan Jory points out, lining up all the furniture against the wall. “Seldom does one hold court,” said the London, Ontario, interior designer wryly.

Instead: Nurture intimacy with smart seating placement, says Kevie Murphy, of K.A. Murphy Interiors in Manhasset, N.Y. “Add [bonus-seating] ottomans under consoles, position chairs in the corners of the rooms.” A backless divan allows for “double-sided conversation,” she added. “And be sure each seated person has a table to place a drink or cocktail plate.”

2. Puny Rugs

Emily Del Bello, a New York City designer, looks askance at rooms where the carpet is too tiny to anchor more than a coffee table, while the rest of the room’s pieces float about visually untethered.

Instead: “The rug should go under all the furniture in that area, or at the very least, under the front legs of all sofas and chairs,” said Jen Samson, a Laguna Beach, Calif., designer. “This grounds the space and creates a frame [for] the area.”

For clients obsessed with vintage rugs too small for their living rooms, Katie Davis, an interior designer in Houston, layers the collectible pieces onto plenty-big neutral jutes. Some expansive advice: “Always go larger than you think,” directs Emily Williams of Z Properties, a design-build-interiors firm in Winter Park, Fla.

3. Monotony

Almost as unimaginative as a matching set of furniture is a scheme in which every piece conforms to one style, says Isabel Ladd. “When a living room is decorated completely traditional, or completely modern, the room feels stagnant,” said the Lexington, Ky., designer.

Instead: The décor should combine high and low aesthetics, says Paola Zamudio, founder and CEO at Npz Studio+ in New York City, who suggests, for example, “a designer statement piece combined with a vintage décor piece.” Disparate styles can blend within a single object as well. Linen upholstery and graphic embroidered trim can make a sofa with a traditional silhouette feel fresh, said Ms. Ladd.

4. Scale Fails

Dennese Guadeloupe Rojas, principal designer at Interiors by Design in Silver Spring, Md., warns that buying a one-and-done suite from a furniture showroom can saddle you with both a dull room and relentlessly overscale pieces. Benjamin Deaton has seen folks challenged by a small room err in the other direction, yielding to the false hope of “dollhouse furniture.” Said the Lexington, Ky., designer, “What you get is the opposite, a room that looks cluttered and still small.”

Instead: “Mixing the scale of furniture pieces can actually make the room feel larger and have more depth,” said Mr. Deaton. For those contemplating purchasing new furniture, Chicago design pro Bruce Fox recommends using blue tape to map out their footprint on your floor. To estimate their bulk in three dimensions, he suggests “piling other furniture or even empty boxes onto the footprint to mimic the height of the piece and get the full sense of scale.”

5. Dominating Overheads

One is less likely to curl up with a novel or chat for hours with friends under lights that are operating-theatre-bright.

Instead: “Lighting can change the entire landscape of the room,” said Mr. Deaton, who favours a combination of decorative lamps that double as sculpture, overhead lighting and shaded sconces that add texture and glow to a corner space.

Ms. Jory espouses dimmers: “Ambient lighting on tables and walls, paired with ceiling fixtures also on dimmers, provide a wash of warm, inviting light.”

Monster Chairs and More

Design pros recall egregious parlour schemes

“Recently, a client picked a single piece, an oversize armchair, then tried to design her entire living space around it. She realised, after hiring me, that she had to sell the armchair because it clashed with and crowded the sofa and storage cabinet we picked.” —Nina Edwards Anker, founder, Nea Studio, New York City

“The worst is when people purchase multiple pieces of furniture in the same, neutral upholstery fabric. Sure, the goal was to be cohesive, but the result is unfortunately a space that is sterile, slightly cold and without personality.” —Glenna Stone, interior designer, Philadelphia

“I usually see a massive sofa and a bunch of ditzy, underscale pieces because nothing else will fit properly.” —Liz Caan, interior designer, Boston

“I’ve seen large blowup pool toys laying about a primary living room and oversize shiny La-Z-Boys pushed into corners without anything else in the room.” —Melanie Hay, interior designer, Toronto

When Sterling McDavid’s parents bought a roughly 9,000-square-foot home on Aspen’s Red Mountain, the 33-year-old interior designer directed the architect and contractors to start digging.

Limited by zoning above ground, Ms. McDavid, who led the renovation, envisioned an expansive basement with a world-class gym, guest suites and hotel-caliber spa for her parents, former college track star Stacie McDavid, and David McDavid, a former owner of the Dallas Mavericks and car-dealership mogul.

“I love a basement,” says Ms. McDavid, who ultimately blew out her parents’ basement to more than double its size. “When you walk into a home, if all the magic is just within the first few steps, that’s no fun.”

Wealthy Londoners have long built basements reaching two, three, and even four stories below ground. Now, there is a cellars market in the U.S. among property owners facing restrictions on mansion sizes above ground. The McBasements of today have bars, bowling alleys, pools, climbing walls and whiskey-tasting rooms. To sidestep subterranean gloom, builders usher in natural light via grand staircases or skylights cut into the ground above.

“Down is the new up,” says Randy Correll, partner at Robert A.M. Stern Architects, who designs basements with luxe finishes. “Twenty years ago, basements were the ‘B word.’”

Ms. McDavid says the excavation for her parent’s 4,000-square-foot Aspen basement added about a year to the three-year renovation, since workers dug underneath the home and into a mountainside. The resulting basement has guest suites, a gym with white oak floors, a 12-person hot tub and an “absurdly large” steam room, she says. “You feel like you’re in a luxury cave.”

She says the prior owner, by contrast, had done little to maximise the basement, which was “absolutely heinous.” (“It had wine storage, not to be confused with a wine cellar,” she says.)

Still, some towns have cracked down on the chic crypts, worried about unsettling topography and having truckloads of excavated dirt roll through residential streets. In Aspen, a resident sued the city for permitting a neighbour’s two-story basement, alleging excessive noise and dust. Aspen now limits basements to one level. And the Beverly Hills Planning Commission is now mired in a down-and-dirty basement brouhaha.

The island of Nantucket, just 14 miles long and 3.5 miles wide, limits home sizes with more than 20 zoning districts. Some areas allow a footprint covering only 2 percent of the property.

One solution? Go low. “The possibilities are endless,” says Stephen Cheney, owner of Cheney Custom Homes, who is currently constructing a roughly 16,000-square-foot home and guest home with a 5,600-square-foot “bunker” below for a bowling alley, 3-D golf simulator, and spa.

Basements can dwarf homes above. Designer Andrew Kotchen, a principal at Workshop/APD, is working on a 5,000-square-foot Nantucket abode that will have a 10,000-square-foot basement with a basketball court, garage, bedrooms and a wellness space.

In beach towns, architects are deploying extreme waterproofing measures.

For the 10,000-square-foot grotto, workers are using the same waterproofing technique as Boston’s Big Dig highway project, Mr. Kotchen says. An emergency pump system and thick concrete slab underneath will prevent the foundation from floating up should water levels rise, he says.

In Aspen, a 5,000-square-foot basement at the confluence of two rivers required “dewatering,” says Ryan Walterscheid, a partner at architecture firm Forum Phi. Workers drilled wells around the site and pumped out almost a billion gallons of water before pouring the foundation. (The water was poured back into the river.)

Architect Charles Cunniffe, who designed the McDavids’ remodel, also did a basement with a tennis court. “You can’t hit big lob shots,” he says, “but you can play a decent game of tennis.”

Interior designer Bryan Graybill says a large basement at his home in East Hampton, N.Y., was the only way to fit all the amenities he and his husband desired in their 4,100-square-foot house. For frequent hosting, their 1,800-square-foot lower level has full guest quarters, a laundry room with three sets of washers and dryers, and a catering pantry with extra stemware and service for 48. “It’s like an instant party,” he says.

Beverly Hills residents are also keeping up, and down, with the neighbours.

Architect Paul McClean crafted a house there with 7,400 square feet above ground and another 12,000 below, including a 3,000-square-foot garage. The firm also designed developer Nile Niami’s roughly 105,000-square-foot Los Angeles megamansion “The One,” where about half the home sat below grade. First listed at $500 million, the property fetched $126 million at auction this year. (A relative bargain-basement deal.)

Beverly Hills leaders passed ordinances in recent years in response to what Craig Corman, former Planning Commission chairman, called a “pernicious” trend of mammoth dwellings with wedding cake-style retaining walls and massive basements. “They can be quite offensive,” he said during a recent commission meeting. Now, property owners in the Hillside area cannot remove more than 3,000 cubic yards of earth without special permits.

This has hampered real-estate investor David Taban, who is trying to build a 23,144-square-foot house, of which 9,829 would be a basement. (He wants to dig down to appease neighbours who worried his home would hurt their views.) But getting a basement of that size required removing 5,346 cubic yards of dirt, which one city planner said would amount to 594 truckloads.

“Violence to the land,” Planning Commission Chair Myra Demeter dubbed it at an August meeting.

Asked to revise, Mr. Taban’s team is set to return to the commission Thursday. As of late September, plans called for removing just 3,276 cubic yards of soil, according to attorney Ronald Richards, a representative for Mr. Taban, who also argued the project should be permitted since the proposal predates some restrictive rules.

“It’s not even that crazy of a project in our mind,” says Russell Linch, another Taban representative.

Spec developers know the more liveable space, the bigger the price tag, says Brett Loehmann, a project manager at McClean Design. “If you don’t have great amenities, you’re not going to be the coolest person on the block.”