Base cabinets, while similar to wall cabinets, require a few special considerations. Chief among them is the decision how to build the back of the cabinet. Most economical minded cabinet shops opt for an open back instead of a solid one. This saves material. Let's take a look at how these cabinets are built.
Material saving construction
The back and the top of the cabinet are formed with 4" wide strips rather than solid pieces of wood. While this results in a plenty strong cabinet, the wall and the underside of the counter top will be visible. If that's not a problem for you, this method of construction is economical and fast.
Since the strip of wood are only 4 inches wide, the dadoes in the cabinet sides and the center piece must also stop short of running the entire length of the pieces. The table saw can't make this cut. The best tool for the job is the router table with a dado cutter. Remove the material just short of the final dimensions. Square the cut with a sharp chisel.
Keep in mind that the two side pieces are similar, but different. Don't create two left or two right cabinet sides. The cabinet needs one of each. Lay out your cuts carefully.
The vertical divider in the center of the cabinet does not receive dados. On that piece the cuts run all the way through the piece. This allows the horizontal strips of wood to run uninterrupted from one cabinet side to the other.
Simply remove the material from the center divider. The table saw and a sharp handsaw combine to make these cuts. If the pieces don't fit right, a chisel creates the perfect fit.
The pieces of the cabinet are held together with glue and screws. Drill pilot holes for all screws and countersink the head. The strips are held in place with two screws through the cabinet sides at each joint.
The dado joints also receive screws. This eliminates the need for clamping. Drill pilot holes from the inside in the center of the dado groove. This will locate the screws perfectly. Finish with a countersink bit from the outside.
The easiest way to install the center divider is to use a pre-cut measuring stick. Instead of measuring for the location of the divider, cut one or two relatively narrow pieces of cutoff stock to the exact interior width of the cabinet. Use the pieces to locate the divider in the carcass by feel. place the stick in the back of cabinet against one side of the cabinet. Push the divider against the other end of the stick until it can't move anymore. That corner of the divider will be in correct location. Secure that corner and move the stick to the next corner. This method is much more reliable than separate measurements at all corners.
The two shelves in this project are fixed to the sides of the cabinet by a runner in each corner. Every joint uses glue and screws. Each shelf is reinforced with a 1" wide polar strip across the front for stability and looks. This method of shelf installation was a design choice, not a necessity. The cabinet is strong enough to support floating shelves on pins, if that is the direction you want to go.
The face frame uses pocket hole joinery. It is matched to the cabinet dimensions overlapping on all sides. The frame is flush on the top to allow for a counter top to be installed. To account for the counter top overhang the top rail is wider than the bottom rail. If that was not the case, the doors of the cabinet would hit the overhang of the countertop. Make sure your design accounts for the swing of the doors.
The frame is attached to the cabinet carcass with glue and brads. Nail holes are not a concern on this project. The cabinet will eventually be painted. Biscuits are the preferred method for creating this joint on cabinets that will receive a higher quality finish.
If you want to add drawers to the front cabinet, start with choosing the size of opening in the face frame. The drawer box and runners will be built to match this opening. A height of 5"-6" is a good choice for drawers just below the counter top.
Building the drawer and installing them is whole separate topic. You can read about my approach in the thread How to build a drawer box.
The Cabinet Base
The base of the unit can be constructed separately or included in the main carcass. Both have advantages. A built-in base is part of the cabinet itself. It is guaranteed to fit the cabinet and it cannot be lost on the way to the job site. A separate base, on the other hand, can be installed before the entire unit is installed. The base is much lighter than the full unit, which makes leveling the base by itself much easier and faster than leveling the entire cabinet. Both solutions produce more or less identical results.
I chose to build a separate base, because I want the base to be slightly smaller than the cabinet itself. The base is made from several strips of plywood, which I seem to have an abundance of. I reinforced the corners with blocks of 2x4 dimensional lumber. Glue and nails hold it all together.
The front piece is 1/2" higher than the other sides of the base. This is done to allow the front to overlap a row of 3/4" pressure threaded sleepers under the base itself without touching the floor. The cabinet will rest on a concrete floor in an area prone to moisture. The pressure treated lumber is a safety precaution that will keep the bottom of the base from splitting due to water infiltration.
My design uses overlay doors. Overlay doors are fast to build and easy to fit. Slight mistakes in the drawer or the carcass won't affect the install like it would on an inlay door.
Base cabinets use many of the same techniques used to construct wall cabinets. Despite being bulkier, it is possible to save material by construction an open-back and open-top carcass. The result will be just as strong as a solid wall cabinet.
For more on the topic of cabinet building, check out the Build your own kitchen thread.