Before you build an outhouse, you need to check with your local government. There are a lot of rules regarding any kind of waste system. This outhouse included a washing station.
Step 1: Digging the Hole
There are a lot of regulations about how close you can put an outhouse to water and to your lot line so check with authorities before choosing your site. Put a stake into the ground and then make a square with three other stakes so that they create a square that is three feet by three feet. Using the stakes as a guideline, dig a hole three feet deep. Keep the sides as straight down as you can. Try to make the hole as much of a cube as you can. If the outhouse is going to get constant use, dig the hole deeper.
Using 4 x 4 posts, make a rectangle that is eight feet long and four feet wide. Lay this so that the long sides are just beyond the sides of the hole and one short side is just beyond the side of the hole. Level this rectangle by shoveling away or adding dirt where needed. You need to keep all the sides of this rectangle above ground level or water run into your hole when it rains and fill it up.
Step 2: Floor
With this size of base, your outhouse will be four feet by four feet with a landing exactly the same size in front of it. Cut a piece of pressure treated 2 x 4 41 inches long and put a cross piece exactly two feet from the back of the rectangle. There should be a gap of exactly 21 1/2 inches from the inside of the back 4 x 4 base to the side of this cross piece. Then run 2 x 4 joists from this cross piece to the front of the rectangle. They should be 16 inch centers. Cut deck boards four feet long and screw them to these joists with deck screws to make the floor of the landing. The landing should end four feet from the front of the base. Remember to paint end cut on the ends of any board you cut.
Cut a piece of 3/4 inch pressure treated plywood 24 inches by 28 inches. Lay it so that it makes the floor of the outhouse itself, starting at the cross piece and going to the decking. Make sure it does not extend past the edge of the cross piece as this will create a ledge that could trap undesirable material. Cut strips of the 3/4 inch plywood and screw them to the top of 4 x 4 base anywhere that is not already covered by the plywood or the deck boards. This is so that when you build the walls, the base of the wall will be all the same height.
Back fill dirt along the 4 x 8 rectangle so that there are no gaps where critters could get underneath your floor when the outhouse is done.
Step 3: Walls and Seat
Build the walls in the usual fashion using top and bottom plates and making the total height eight feet. Don't put blocking between the two studs on the back wall. The vent pipe will run up between these two studs. Along the side walls, put a stud exactly 24 3/4 inches from the back. This is so you can nail the upright piece of the seat to it.
To make the seat, cut a piece of 3/4 inch plywood or other stock 18 x 48 inches. This will be the upright part of the seat. Nail it onto the stud. You will have to notch out the bottom plate. Make sure that the inside of this upright piece sits exactly even with the inside of the 2 x 4 underneath it. As I said before, you don't want to create a ledge to catch undesirable material. Cut a piece of sheet metal 22 x 41 inches and nail it to the inside of this upright plywood. This is so when guys are sitting and happen to pee also, the pee doesn't hit this upright piece of wood and soak in.
For the top of the seat, put a support that is one inch thick along the back wall at 18 inches high to support the top of the seat. It shouldn't be thicker than one inch because you will have to drill holes in the top board for the toilet seat and you need the space. Put supports on the side walls at 18 inches high also. Use plywood or other stock for the top of the seat.
Now you can start the sheeting or boards for the outside of the outhouse. You will need something that is resistant to the weather or you will have to paint it regularly or coat it with something. For the corners, run a piece of 1 x 3 board from the bottom to the top to cover up the unfinished edge.
Cut out a rectangle of about 14 inches by 10 inches on both side walls. Place a piece of nylon screen over the rectangle and then use molding to hold the screen in place. This is for ventilation.
Step 4: Roof
In this build, I made the overhang on one side of the outhouse longer. This is because I would be putting a wash station on that side of the outhouse and wanted protection from the rain while washing up.
Make the trusses in the usual fashion with the truss at the front and back of the outhouse lower so you can make a ladder for the front and back overhang. As this is an outhouse, I didn't bother with soffit and facia or eaves trough.
Block the spaces between the trusses at the top of the wall with pieces of wood and caulking so that it is relatively closed off to insects.
I had some leftover metal roofing so I used that but shingles work well for the roof. With metal roofing, you don't have to put sheeting on the roof and it is less prone to mildew than shingles.
Step 5: Door
Make a two section door. This allows you a beautiful view from the throne. (see picture) Make the bottom portion first. If you like, you can put door jams to cover up the rough 2 x 4 studs on the inside of the door but I didn't. Likewise, you could put door stop on the door jam to stop the door from swinging in but I just added an overlapping piece to the outside of the door to stop it.
Cut plywood or tongue and groove about three feet high by the width of the door opening (typically around 26 inches) minus about a half inch for clearance. Add bracing on the inside along the top and bottom and diagonally across the door. Attach the hinges.
Cut plywood or tongue and groove the rest of the height of the door for the top portion. Typically, doors are 80 inches high. Make the top portion the same way as the bottom portion but add a piece at the bottom that overlaps the bottom portion.
Step 6: Vent Pipe and Toilet Seat
The vent pipe should be three to four inches wide. Trace the outside of the vent pipe on the wood seat part right at the back. Cut the circular hole. Drill a hole all the way through the vent pipe about two inches from the bottom. Put a six inch spike through the hole so that it sticks out on either side of the pipe. Slide the vent pipe into the wooden hole. The spike will stop it from sliding all the way down into the pit.
If you were to run the pipe directly through the roof, you would have to cut away the top plate so cut the pipe below the top plate and then use two elbows to detour the pipe around the plate. The pipe should end up around two feet beyond the peak of the roof.
Toilet seats typically have two plastic bolts at the back. Line up the toilet seat so that it is in the middle of the outhouse and the two bolts will bypass the support you made for the back underneath the wooden seat. Mark where the bolts are going to go. Drill the holes and set the seat in place. With the lid up and the seat down, trace the inside of the seat. Remove the toilet seat and cut a hole 1/2 inch bigger than the traced oval. Put the toilet seat back and put the nuts onto the plastic bolts and tighten them snug.
Step 7: Wash Station (optional)
Make a platform about 33 inches high and attach it to the side of the outhouse. Find an old sink and trace the outline of the sink on the top of the platform. Cut a circle 1/2 inch inside the traced line and fit your sink it. Attach piping to the drain and run it underground to a drainage area or use a French trench. A French trench is a trench filled with rocks so that the water drains away and the ground is still even.
Make another platform right beside the first one but 10 inches higher. Buy a rectangular pail, a faucet with a threaded pipe, two thin nuts that fit on the pipe and some silicon. Cut a hole in the front wall of the pail near the bottom. You can't put it right at the bottom because you need some clearance for the inside nut. Put the first nut on the pipe thread as far as it will go. Put silicon on the inside and outside of the hole and push the pipe so that the nut is snug against the pail. Put the second nut on from the inside and tighten it until the faucet is firm.