Splish, splash. Water spills down a curving fiberglass slide into the swimming pool built by Tracy and Clint Heiden behind their McLean home. The pool forms the centerpiece of a $1 million backyard transformation, changing an unusable hillside into an outdoor living area offering indoor-style conveniences.

At one end of the yard, a metal-roofed pavilion shelters a comfortable seating area centered on a limestone fireplace and 60-inch flat-panel TV. A barbecue grill, sink and refrigerator drawers set below granite countertops provide an outdoor kitchen.

Next to the pool are more features for the Heidens, their four children and friends to enjoy.

Built into the flagstone patio are a glass mosaic-tiled spa and a gas-fueled fire pit. Umbrella-shaded tables, chairs and chaises provide places to dine and relax.

“Before we had the pool and landscaping, we spent all our time in our kitchen and family room,” says Tracy Heiden, 45, an advertising executive. “Now we spend about six to nine months out here relaxing and entertaining. This outdoor space doubled the size of our house.”

As evident from this back yard and others in the Washington area, pools have become more than utilitarian places for swimming and cooling off. “People spend more time around the pool than in the water, so the design of the surroundings is important,” says Don Gwiz, principal of Lewis Aquatech, a pool builder in Chantilly.

A pool often acts as a springboard to a backyard landscape filled with complementary amenities.

“We have more requests for structures around the pool — small pool houses, pergolas, outdoor kitchens and bathrooms, fire bowls, sculpture,” says Walt Williams, co-owner of Alpine Pool in Annandale, who designed and built the Heidens’ pool and pavilion. “The area around the pool is getting more detailed and elaborate.”

At the same time, the in-ground basin for the water is getting simpler. “What’s becoming popular is a contemporary design, mostly geometric with very crisp lines,” says Williams.

Rectangular pools are now more common than free-form or kidney-shape designs, experts say, because their simple geometry can more easily accommodate an automatic pool cover to keep the water clean and provide a safety barrier. This heavy-duty vinyl sheet, sturdy enough to walk on, extends to the perimeter of the pool and rolls up into a box at the deep end.

“When you have kids, you want to have the cover so you can close and lock it in between the times the pool is in use so there are no accidents,” says Harry Mulford, 51, a lawyer-turned-horseback riding instructor.

Mulford and wife Mindy hired Town & Country Pools of Springfield to build a 22-by-46-foot rectangular pool behind their Centreville home for their eight children to enjoy. Mindful of the kids’ safety, they clearly marked the depths of the water on the sides of the pool and installed a “No diving” sign at the shallow end.

Along with pools in simpler shapes has come easier pool maintenance. “It’s drastically different than it was 10 years ago,” says Gwiz. “You can push a button from a remote control or use a cellphone app to adjust the temperature or turn on the cleaner.”

Reducing the day-to-day upkeep are saltwater chlorination systems. They are not actually chlorine-free: A generator produces the sanitizing agent instead of requiring chlorine to be directly added into the water. The systems create softer water and eliminate conventionally chlorinated water’s risk of skin and eye irritation.

“With the saline systems, the days of checking chlorine levels are practically over,” says Williams, although he adds that “you still have to watch the pH balance of the water.”

Although easier maintenance may make the idea of a backyard water feature more attractive, building even a simply shaped swimming pool can be a lengthy, costly process. “Permitting time is getting longer and longer as local governments get more involved in backyard construction,” says Duncan MacKeever of Crystal Pools in Rockville.

Securing a building permit for a pool in Fairfax County, for example, requires filing a grading plan — a drawing showing topography and planned construction — when an area of 2,500 square feet or greater is to be disturbed.

The Mulfords spent about $15,000 for the civil engineering services and county fees required of the grading plan in addition to $120,000 for the pool construction.

In terms of overall costs, homeowners should be prepared to spend about $125,000 to $150,000 for a 20-by-40-foot pool with a flagstone deck, saltwater chlorination system, LED lighting, automatic cover and heater, says Williams.

Costs can quickly escalate, pool builders say, when a spa, shade structures, fencing, landscaping, outdoor furnishings and other elements are added.

Sitework and infrastructure for the pool — gas and electrical lines, drainage, fencing — can also substantially add to the cost, especially in urban neighborhoods.

Buyers beware: The investment might not be recouped when it comes time to sell the house. “Swimming pools are not a selling point for about 90 percent of home buyers in the D.C. region,” says agent Morgan Knull of Re/Max Gateway. “This isn’t Florida or California, where a pool is usable all year. Unless a seller finds a buyer who views the swimming pool as a benefit, it actually represents an overimprovement and a depreciating asset that most buyers don’t appreciate.”

Georgetown homeowners Julie and Marty Doerschlag spent about $750,000 to overhaul their back yard into a garden oasis centered on a pool and fountain. The project required cutting down trees, underpinning an adjacent house and carrying dirt through the house, since there was no street access to the back yard.

“We incorporated the pool into the yard as a garden element, like the terrace and plantings,” said landscape architect Sheila Brady of D.C.-based Oehme van Sweden and Associates, who designed the two-level outdoor room and water features in consultation with Alpine Pool.

Tall magnolia trees and hydrangea-covered fencing screen the neighboring houses, and a copper wall at the end of the yard hides the pool equipment. Seating areas on the limestone-paved terraces provide places for the homeowners to dine and relax with a steady stream of friends.

“The pool is the centerpiece of the outdoor living space where we entertain, decompress and escape,” says Julie Doerschlag, 47, an illustrator. “You have to be ready with drinks and food, because the people we invite always show up.”

Deborah K. Dietsch is a freelance writer.