Upcycle Countertop Into Workbench

I had an old counter top that had previously been used as a desktop. But I have been wanting a workbench for a long time. So, I built this workbench and incorporated the counter top to use as my work surface. All of the dimensions were dictated by the size of the counter top. You may have to modify your workbench to fit whatever you use. Total cost was around $75, about 1/3 of which was the wheels.
I consider myself an advanced-beginner. Anyone who is reasonably comfortable with a circular saw and miter saw can probably complete this in an afternoon. My son and I spent about 6 hours on the entire project. 

Using 2x4 lumber, I first built a frame the same dimensions as my counter top. At each connection, I pre-drilled pilot holes, and then used two 3.5" screws to secure the lumber.

Since my counter top is 24", I made my lower shelf to be half that depth, so 12". Notice that the side rails extend the full 24". Again, I pre-drilled the pilot holes, and used 3.5" screws at each connection.

Next I added legs. My workbench needs to be on wheels as a result of space restrictions in my garage. Since I wanted my work surface to be 36", I subtracted the 3.5" height of the wheels, as well as the 1.5" thickness of a 2x4, for total leg length of 31". Then I joined the work surface and the lower shelf to the legs. You may want to adjust the dimensions for your particular project.
Having two cordless drills helped. One was used to pre-drill the pilot holes, and the other was used to drive the crews. Having four batteries also helped, allowing us to constantly have fresh batteries in our tools.

I wanted to ensure that the wheels would handle the weight of any future project I might undertake. The wheels were rated for a little less weight than I had hoped, so I use a total of 6 wheels. Therefore, I also used 6 legs, so that each wheel would be directly under a leg. Here is the workbench with the 6 legs attached.

Notice that my corner legs are inside the frame of the workbench. When adding the plywood on the lower shelf, I notched out the two rear corners to accommodate the rear pair of legs. I used a jig saw for this part, but a circular saw would also have worked. The shelf was attached with 1.5" screws.

I know it is a workbench, but I wanted the surface to be as clean as possible. So, I attached it from below using these 6 pieces of 2x4. When I attached the counter top, I screwed in from below using 2.25" screws.

I next joined the three legs on each side with another 2x4. Then I could attach the wheels directly under each leg, and the entire structure is rock solid. The legs don't wiggle, and the cross-braced structure at the bottom also gives me more "meat" to attach the wheels.

Hopefully this allows you to see the way I attached the wheels to the legs using the 2x4 cross-brace. The wheels are attached with lag screws.

Here is the completed project, and you can see the space restrictions. The most expensive part was the wheels, which can be eliminated if your workbench does not need to be mobile. A handful of 2x4s, a sheet of plywood, 3.5" screws and 2.25" screws, 1.5" screws and lag screws were all that I used. It was a great father-son project that stretched out over a few days. We could have completed the entire thing in one day, but we had other things going on as well.