Current designs for outhouses are exactly the same today as they were hundreds of years ago. Basically an outhouse is a simple wooden, plastic or cement shack built over a hole in the ground.
Plywood rafters are the most common building material used by builders of outhouses. Outhouse roofs are built typically between 6 feet and 7 feet high and sloped with the lowest portion situated over the sitting area leaving adequate headroom. A wooden door with a symbol, such as a half moon, is cut into the top portion to allow the light in and keep the odors out. Many outhouses have made use of Dutch doors (a door that splits horizontally in the middle) with screens, in order to keep the top portion open for light and ventilation.
The inside of an outhouse is also typically built of wood. It features a wooden bench with holes cut into it. Because people of various sizes and shapes will be using the privy, many outhouses offer one larger hole (for adults) and one smaller hole (for small women and children). In the past families were larger and it was perfectly natural to have up to 4 holes of various sizes in one outhouse.
The type of outhouse you build will largely depend on who's using it. If you're building an outhouse for a cottage or fishing shack you might choose to erect it on a grassy hill, overlooking a creek or valley for scenic reasons. It actually makes sense to build your outhouse at a high level to avoid being flooded during a storm.
Tales do exist of two story privies. They did exist throughout the rural mid-western United States, where winters were harsh and privy doors tended to get blocked by snow drifts. The solution was to build two story privies with the holes on the top level set back farther than the ones below. A false wall or channel would be built behind the bottom outhouse so that refuse from the top would fall down behind and not (God forbid) on top of someone.
Today, outhouses can also be ordered online for between $150 and $800. Outhouse kits that you can put together yourself are available over the internet in nylon/tent material with zip closures to more durable (and private) plastic, wood and even brick designs. Obviously, you will pay more for durability and privacy.
Another relatively new, environmentally friendly addition to the outhouse industry is the composting toilet - which requires no water and transforms the waste into hygienic fertilizer. Composting toilets range from $1,200 to $1,500 online.