If you own the home that you're living in, any changes you make to its design will be largely aesthetic unless you are doing large-scale renovations. If you are looking to buy a home, or if you're just interested, thinking about a home's design in relation to energy efficiency can be very important. Energy efficient design isn't just a matter of good insulation and an efficient HVAC system; the actual form of a building has a lot to do with how much energy it uses, especially when it comes to heating and cooling.
The most relevant design factor to a home's heating and cooling efficiency is the area of its exterior walls. When your home is a different temperature than the outside air, warmth (whether it's outside or inside) will flow toward a colder area. In the summer, this probably means that warmth from outside will flow into your home, which is why you can't just turn on the AC for an hour and have a cool home for the whole day. In the winter, this probably means that all that heating you paid for is flowing outside. So how does the heat flow? Well, primarily through the walls and in the form of air escaping through the drafty areas of your home (even if you think your home isn't drafty, every home allows for some airflow because it's important for indoor air quality, among other things). If you consider that air must escape through the envelope of your home, it makes sense that the lower the area of the outside of your home, the easier it is to control the air escaping.
Given this fact, we can now think about what shapes are most efficient for a home or a building in general. An efficient home will maximize the volume of usable space in comparison to the area of the exterior walls. The best way to do this would be to build a spherical building, but this is not practical and probably wouldn't sell very well. Excluding spheres, a perfectly square house would maximize usable interior space compared to exterior surface area. An L-shaped home, or some other less-compact design would not lend itself to efficiency because it would have extra exterior wall space that a compact design wouldn't have. Regardless of its particular shape, a row home would likely be more efficient than any detached building because it shares two of its walls with the home next to it, so the external surface area is very small. Obviously, these aren't the only possible designs, but these three give you an idea of efficiency in different designs.
The shape of a home's roof can also have an effect on the efficiency of it's design, but it's harder to make a general rule about roofs because there is so much variation. A lot depends on whether or not the home has a flat roof or pitched, an attic or no attic, what type of climate you live in, etc. The only definite thing that can be said is that the roof, like the rest of the house, should help to maximize usable space in comparison to the area of the exterior walls (including the roof). Some roof shapes like flat roofs are also talked about as being more efficient than other shapes like traditional pitched roofs for heating and cooling, but little data is readily available to support this.
Grisso, Robert. "Energy Series: What does the Shape of the House have to do with Energy Efficiency?." Virginia Polytechnic Institute, last modified May 1, 2009, accessed 5/30/2013, 2013,http://pubs.ext.vt.edu/2901/2901-9013/2901-9013.html.
Mitchell, Mukti K. "Heat Loss Calculator." The Resurgence Trust, last modified February 2009, accessed 5/30/2013, 2013, http://www.resurgence.org/education/heac.html.
Straube, John. "Air Flow Control in Buildings." Building Science Corporation, last modified 5/9/2008, accessed 5/30/2013, 2013, http://www.buildingscience.com/documents/digests/bsd-014-air-flow-c....
Straube, John. "The Function of Form—Building Shape and Energy." Building Science Corporation, last modified 6/20/2012, accessed 5/30/2013, 2013, http://www.buildingscience.com/documents/insights/bsi-061-function-...