Veronica Cervera-Goiseke's latest home-building project has been complicated. Just getting a permit took seven months. She had to specify where every bush was going, what colors she planned to paint the exterior and how the style would flow with the rest of the neighborhood. A new dream home? Actually, a new pool house.

With homeowners already transforming backyards across the country into "outside rooms," now many are focusing on that empty spot across the lawn, and adding pool houses. "It started with a grill and a barbecue," says New Canaan, Conn., architect Louise Brooks, who is currently working on three pool houses -- compared with three in the past eight years. "Now it's become a whole entertaining area."

Spiritelements.com, an online retailer that sells everything from bird baths to greenhouses, says its pool-house sales have increased about 50 percent since last year, while Summerwood.com, which sells customized pool-house "kits" over the Internet, has seen a 40 percent jump in business this year. Barbara Corcoran, chairman of real-estate brokerage firm Corcoran Group, compares it to the boom in wine cellars in the 1990s."If you drive up to a house with a four-car garage, you better see a pool house," she says.

It's part of the same home-improvement frenzy that has propelled remodeling into a $230 billion industry. Investment in exterior remodeling rose 70 percent from 1998 to 2002, according to Boulder, Colo., research company Primen Research. Pool and hot-tub supplies and services rose nearly 39 percent from 2002 to 2004. Meanwhile, and wood and plastic garden furniture increased 26 percent, according to online research company Bizminer.com. As pool houses have morphed from a place to shower and change to full-fledged entertainment centers with offices and bedrooms, the process of adding one has become akin to building a second home -- with many of the accompanying headaches.

Arnold Ditri, a Greenwich, Conn., financial executive, ended up paying 40 percent more for his than he had originally planned. "You think something is going to turn out one way, but it turns out another," says Mr. Ditri's son, Steve, who helped with the project. The elder Mr. Ditri initially wanted a place for his grandchildren to hang out that was less formal than his 1936 Belle Haven Tudor, and planned a space with an outdoor pizza oven, a big-screen television and stereo system, a dining area and a bar. It soon expanded to a 3,000-square-foot space adjoining the main house that also included an office on the second floor and a wine cellar that can hold 6,000 bottles in a loft space.

Zoning laws in some areas of the country that restrict the size of a main house are driving some homeowners to add extra rooms next to the pool. In Santa Barbara, Calif., architect Don Nulty sees his clients outfitting pool houses to be used as gyms or saunas, entertainment areas or retreats for private time outside the main house. In contrast, in much of the Hamptons in New York, accessory buildings (including pool houses) generally are limited in size to 250 square feet of enclosed space, meaning larger existing pool houses (which have been grandfathered) are truly coveted. "If they want to build a large pool house in Southampton or the town of East Hampton, N.Y., they have to buy an adjacent piece of property. It happens, but it adds millions to the cost of the pool house," says Jason Schommer, a real-estate broker with Allen Schneider Associates.

Architect Paul Marchese, who designed Mr. Ditri's building, puts the price of one of his pool houses at $350 to $1,000 per square foot or even higher, because a high proportion of these structures tend to be devoted to kitchen and bathroom space, the most expensive rooms in any house. For a bedroom or living room, prices are close to $200 a square foot, he says. With some of his pool houses measuring 1,000 square feet or more, prices can run $350,000 to $1 million each.

The cost is worth it to Marilyn Goldberg, who has been buying and selling homes in the Hamptons for 20 years. Ms. Goldberg has put in a pool house in each of the three houses she has bought in the past five years, as well as koi ponds and waterfalls. In the structures, which range in size from 250 square feet to 600 square feet, she uses ceramic-tile floors, marble countertops and handcrafted cherry-wood cabinetry, as well as Viking grills with rotisseries, and sometimes a small office with high-speed Internet access. Ms. Goldberg, who rents out her Hamptons properties, says putting in a pool house enables her to charge roughly 40 percent more for the Memorial Day to Labor Day season (one of her houses rents for $25,000 for the season, another for $70,000).

Pool houses had their heyday in the 1920s and were later romanticized by such movies as "Philadelphia Story." It wasn't until the real-estate boom of the 1990s and the extensive movement toward nesting that homeowners began seeing the backyard as an extension of the house, adding bigger decks and elaborate barbecue stations that amounted to outdoor kitchens, and loading up on expensive outdoor furniture. Even as the number of pools increased slightly, up 4.9 percent in 2003 from two years earlier, pool houses took off, seen as a way to extend a family's living space. "It's growing like all the other luxury items in the world," says Jim Notte, publisher of Coventry Pool & Garden, a catalog of pool-house designs, who says orders for his catalogs have quadrupled in the past five years.

When there's a boom in anything, it can mean a backup. When Julie Tiffee and her husband bought their new house in October, they decided the existing pool house -- what Ms. Tiffee calls a hideous shack -- had to go. Zoning issues slowed things down, however. Now, in the middle of July, instead of sitting by the pool watching the children splash, the Portland, Ore., stay-at-home mom stands in a backyard full of trenches and supervises while the children jump into the holes where the sewage lines will go. "I really thought we'd be swimming by May," she says.

Home Away From Home

Many homeowners are outfitting their pool houses with items normally found at the main house. Here's a sampling of items they're buying for spa, changing and entertainment areas:

ITEM: Frontgate Portable Massage Table Frontgate.com
PRICE: $395
COMMENTS: Padded with a memory foam layer that conforms to its occupant's body, this 73-inch-long table with maple legs comes with a nylon storage case that costs $59 extra.

ITEM: Margarita set Target.com
PRICE: $24.99
COMMENTS: Pool-house owner Charlene Wallace of Toms River, N.J., bought one of these six-piece acrylic sets (four glasses, one pitcher, one salter). "When we built our pool house, we wanted it to feel like you were walking up to a bar at the beach," she says.

ITEM: Marvel Ice Machine Ferguson Enterprises Inc.
PRICE: $1,790 to $2,150
COMMENTS: Manufacturer says it's seeing more ice-makers going to pool houses with full kitchens. Machine makes 3/4-inch cubes.

ITEM: Maytag Neptune stacking washer-dryer Aitoro Appliance Co.
PRICE: $1,800
COMMENTS: Front-loading machine with full-size capacity automatically controls the water levels coming in; comes in white or bisque.

ITEM: Pizza oven Earthstone Ovens Inc.
PRICE: $3,000 ($6,000 installed)
COMMENTS: Sales of these pizza ovens grew about 50 percent in 2004 from a year earlier and are expected to do the same this year, says a spokesman. Gas-fired ovens start at $9,000, not installed.