Does the lengthening summer day -- or even the memory of last winter -- have you shopping for an energy-efficient home?

If you're looking in the District, something you might want to look for -- even before checking the density of the attic insulation or the ratings on the furnace and water heater -- is an address on a "letter-named" or alphabetically arranged street rather than on a "number-named" street or avenue honoring a state.

That's because surveys conducted in various parts of the country show that houses built on streets running east and west use less energy -- sometimes substantially less -- than identical ones facing on north-south streets.

This is because detached houses on east-west streets generally have their long exterior front and back walls and largest window areas facing north and south, while those on north-south roads, such as the numbered streets in the District, have more wall and window area facing east and west.

South-facing windows allow low-angle winter solar gain, even in houses that are not particularly designed for solar energy, which can balance out the heat loss through north-facing windows. The soon-to-be-cursed summer sun in Washington, meanwhile, is to the east in the morning, overhead during the hottest part of the day (not coming in through south windows), and to the west in the afternoon.

Houses on east-west streets generally have nearly every built-in advantage here; those built on the perpendicular cross streets face their long walls and large windows to the east and west, picking up unwanted summer sun and missing the helpful winter sun's heat and light.

And the savings can be considerable, according to studies conducted in cities from Champaign-Urbana, Ill., to Davis, Calif., Toronto, and Toledo, Ohio. The latest of this research, conducted by Parkland College in Champaign, found that homes on east-west streets use 15 percent less natural gas for heating during winter and 10 percent less electric air conditioning during summer.

For Parkland's study, researchers identified 300 single-family detached homes less than five years old and located on both north-south and east-west streets. The investigators obtained permission from 150 of the homeowners to see copies of their bills from the local utility, Illinois Power Co. The homes were similar in age, size and style, and were evenly split between the two compass orientations. Homes intentionally designed to make use of solar energy were not considered.

While there may be nearly no open land in the District left for innovative zoning, there are at least some possibilities in the suburbs. However, many streets run neither east-west nor north-south. Still, solutions can be found. A street cutting diagonally across the map usually ends up with houses that face somewhere between the four main points of the compass and become shaded by their neighbors or get no particular benefit from their orientation.

But if side lot lines -- the dividing lines for lots that run from the street to the back of the property -- can run north and south regardless of street direction, houses can still be made to face in an advantageous way. That, of course, means that the side lot lines are not perpendicular to the street and the boundaries become less obvious without a fence. Lots also can be sited behind one another in a flag shape -- square for the house with a long, thin strip for the driveway -- or arranged to cluster the homes in a north/south-facing group.

In Montgomery County, says Robert Beall, a planner with the Silver Spring office of the Maryland National Capital Park and Planning Commission, developers planning sidelines of interior lots (lots not on the end of a block) that are not perpendicular to a straight street need to prove to the county that a variation from the norm "will result in a better layout."

However, once a lot is established, placement of the house on it is governed largely by the setback limits that keep any part of a structure from coming too close to a neighbor or to the street. Still, not all lots are big enough to allow a house to be built facing any particular desired direction.

In Prince George's County, says principal development coordinator Dean Armstrong of MNCPPC's Upper Marlboro office, subdivision regulations do allow for side lot lines that are not perpendicular, although the conventional method is preferred.

When a subdivision proposal goes before the planning board, he said, "energy conservation is one thing to be considered." If there is a good reason for doing something out of the ordinary, then optional road frontage, flag lots and cluster ideas can be helped along once necessary information is provided, he said.

Alexandria, on the other hand, probably would not consider energy conservation a good enough reason for a subdivision developer to deviate from perpendicular side lot lines, says Charles Moore, zoning administrator. City regulations require perpendicular side lot lines whenever possible, allowing exceptions generally only for reasons of topography and grade.

"I don't know that trying to save energy would be a reason to deviate from the code," he said, adding that if it could be shown that the energy savings were significant enough he might ask the city council to change the ordinance. Still, houses can be rotated to any angle on the lot, provided setbacks are observed.

Meanwhile, Arlington County has nothing in its codes to prevent angled side lot lines, but lots would still be required to meet the zoning widths of the area, and that measurement is made from side lot line to side lot line, not along the frontage, a zoning official there said. While this might not be a problem for designers of unusual lots, he noted that the resulting longer street frontage could produce higher tax assessments.

Some jurisdictions around the country go even further in recognizing this aspect of home energy use. Municipalities in California, Colorado, Nebraska, New Mexico, Oregon, Texas and Vermont have enacted ordinances requiring energy-efficient street orientation in new developments. Many of these laws specifically mandate that lots and new streets be laid out so that at least 80 percent of the buildings on them can be solar-oriented.

And University of Florida researchers hope to take advantage of a new state growth-management law directed at encouraging better planning by cities and counties to revamp the concept of zoning to save energy, money and the environment.

As part of a $150,000 project under contract to the state Department of Community Affairs, the university is working on a two-year project to craft a land development code to serve as a model for local governments, with an emphasis on different side lot lines and clustered houses.

Until these innovations are made easier, complains David Coffey, an attorney and researcher at the Center for Governmental Responsibility at UF's college of law, builders have no choice, "so each house is situated in the middle of a square lot, and you end up with . . . monotonous tract housing" that wastes energy.