I have been mulling this over for quite some time and have finally moved ahead with creating my first large carpentry project. I needed additional square footage and really only had the option of building up. As renters, I knew we could not make any modifications to the existing home but rather build a structure that could be taken apart without taking the place apart. It all started when I made a bed frame from recycled 2"X4" and one 2"X8"X12' (cut in half) and placed on casters. I knew that once I did build the deck I would want to be able to easily slide the bed underneath. To start the project, the first thing I did was measure and plan. My rough draft details dimensions of the room and the bed that slides underneath and possibly adding a twin bed on the deck, as well as, the measurements of the lumber that I will use. Things to take into consideration:

Measure just about everything. I needed to build at the correct height for my homemade bed and the size of the mattress I plan to buy. And also take into consideration any other obstacles, such as window trimming, cables, outlets, etc.
One thing I found out as I was building is that the floor is not level. I'm really surprised they were able to lay tile in the room. This "little" obstacle required I notch my posts a little differently and prolonged the project. Things can pop up which might require a slight change in plans.
Asking for help and having the right tools will unquestionably accelerate the process.

I hope this can give you an idea of how to add square footage to smaller spaces.

Step 1: Things You'll Need

These are just some of the tools you'll need to start building a deck. I had decided to build a 5'X10' area that would give me an additional 50 square feet of living space. With my plan in hand I went to the lumber yard and picked the pieces I needed:

FOUR 2"X6"X10' (all cut in half to measure 5' long)
ONE 4"X4"X12' (cut into 26" sections--the height of the deck)
TWO 4"X8" 15/32 PLYWOOD
THREE 2"X8"X10' (with one cut in half)
ONE 2"X4"X10' (Recycled piece and cut approximately into 18" pieces)
Fourteen 2"X6" joist hangers
Box of 3" gold screws
Box of 1-1/4" nails
Box of 2" nails

One of the reasons to measure exactly prior to going to the lumber store is so that if you are like me and don't have a well stocked tool shed you can have all of the lumber cut at the store.

The tools needed to build the deck are as follows:

A level
A square for measuring
Drill
Chisel
File
Screwdriver
Hammer
Clamp(s) (I was able to only use the one)
Hand saws
Circular Saw (Miter saw would work better)
Optional - If you have an oscillating multi-tool, this would work better in notching the posts and eliminate the need for a file and chisel, in addition to the large amount of time that could be saved!!

Step 2: Notching the Posts

The height to the top of the deck is 26 inches. The four posts at each corner must be notched to support the weight of the 2"X8"X10' beams. Important Note: Lumber dimensions actually measure differently to what they are called, i.e., a 2"X8" actually measures 1-1/2" x 7-1/4". I used the 2"X8" to mark the depth and width needed to cut on the 4"X4" post.You can see lines on the wood here where we used a circular saw to cut sections into the 4"X4" posts. After the cuts are made you can use a chisel with a hammer to cut the little sections out. There are a multitude of youtube videos on how to notch deck posts. It would be a good idea to watch as many as you can before starting. As you can see from the second picture here that one of the posts was accidentally cut further back from the markings. Two things happened, I tried to hurry the process by just knocking it with the hammer alone and there was a small knot in the wood here. This is where an oscillating multi-tool would be very handy. I used the chisel and file to even the cut surfaces of the wood. This was a long process. I also used the square to make sure I was making even cuts. As you are working this along you can keep placing it up against the 2"X8" to check that the 4"X4" post notch makes a good fit. I also used the hand saws sparingly for larger cuts that would otherwise take too long to achieve with a file and chisel.

Step 3: Putting the Frame Together

After all the notches on the 4"X4" posts are completed you can then start working to put the frame together. I also decided to add a 2"X4" to the side of the post to help support the weight of the 2"X8"X5' beam. I aligned the 2"X4" next to the 4"X4" post and pre-drilled three holes to screw in the 3" screws. I then aligned and leveled the 2"X8"X5' as it sat on the 2"X4" and drilled two holes for screws making sure it did not obstruct the notch where the other 10 foot beam will go. Make sure to use the level to make sure everything is aligning correctly. I then put the posts with the 2"X8"X5' up against the wall where it will go and used a C-clamp to hold the 10 foot beams in place. This helped me accurately drill the holes on the 10 foot beams to the posts. You must also take into consideration the placement of your screws. As you can see in the picture above the 5 foot beam had two screws placed far apart so that I could drill two screws closer together on the 10 foot beam.

Step 4: Adding the Joists

Once the frame is up you can start to add the joists. The important part here is that the joists be positioned in alignment to the plywood seams. So, since I purchased 4'X8' sheets of plywood and my deck is 5'X10', I knew that I would be cutting each sheet of plywood at 5 feet and 3 inches (the three inches is to adjust for the width of the 2" (actual is 1-1/2") of each 2"X8"X10' beam at each end of the 2"X8"X5'beams). You will want to make sure that each end of the plywood sits on a joist to add stability to the surface. The structure will be unstable and unsafe if the plywood seams are not on a joist. We used joist hangers to put up the 2"X6"X5' joists. We measured from the edge to the 4 foot mark and placed a joist there for the first seam. I then measured every 16" for the placement of the other joists. Here we used the smaller 1-1/4" nails to nail the hangers to the beam and the larger 2" nails for the angled insert point from the joist to the beam. You also must make sure that the joists are level at the top with the beams.

Step 5: Adding Plywood Surface and Finishing Touches

Upon completion of joist additions to the frame, you can now add the pre-cut plywood and nail into place. You can see here where we used smaller pieces of plywood to finish the surface of the deck. And once all the plywood is nailed in and slightly sanded we can add flooring. I found these glue-on tiles on clearance at a store and helped create a nice smooth finished floor. I first aligned the pieces along the surface before gluing them on to make sure they fit correctly and found they aligned really well along the 10 foot width of the deck which required no special cuts to the tiles. I did have to cut the the tile closer to the window to adjust for the five foot length. My dog likes the new easy access to the window. And that completes the deck. I might add some trim and paint to the edge but for now this is exactly what I was hoping for. If you all have any comments or advice let me know.