Often builders give little thought to the location of the crawl space access. Over the years, as plantings grow and modifications are made, the location of the original access could become extremely inconvenient. The only solution is to create a new access door. Doing so, however, requires cutting through cement block and building a new door. Although at first glance this seems intimidating, it isn't really as hard as it sounds.
Cutting the Hole
Choose a spot where you will only have to cut one block in half on each side. Assuming that your crawlspace is three courses high, you will be removing a total of six blocks. Since the courses are staggered, you will remove two full blocks from the top row directly under the sill plate, two full blocks from the bottom row and two half blocks and one full block from the middle row.
Snap a chalk line vertically where you want the hole to begin and end. You want a straight line flush with the edge of the blocks that will be staying. In the top and bottom courses your line should be directly on the joint between the existing mortar and the block that will stay.
Cut along the two vertical chalk lines using your circular saw with the masonry blade. You don't need to cut all the way to the top or bottom because that will just be mortar. Just cut through the two blocks that need to be halved. Of course, you won't be able to cut all the way through the block, only through the outside as deep as the blade will go.
Smash out the block that is exactly in the center with a large sledge hammer. Blocks are usually hollow so they actually break fairly easily with a big sledge hammer. Try not to get too close to the edges of the hole, you don't want to crack any blocks that will remain as part of the wall.
Chisel out the mortar between the other blocks that you want to remove with the ball-peen hammer and chisel. The top two should be the easiest to loosen. You may need to reach inside (or go inside) to remove the mortar on the inside of the blocks. Be careful not to break the inside of the two blocks you cut from the outside. Now you will know exactly where to cut the inside.
Snap another chalk line on the two center inside blocks and cut them just as you did on the outside. Clean off any remaining mortar around the edge of the hole with your hammer and chisel.
Building the Door and Frame
Measure the hole. Standard cement blocks are nominally 8" by 8" by 16" but are really 7 5/8ths by 7 5/8ths by 15 5/8th to allow space for mortar. If you removed two blocks (plus mortar), your hole should be about 32 inches wide and three rows high or 24 inches.
Measure and cut the 2 x 8 x 8 into four pieces. Two will be the right length to fit across the top and bottom. The other two will be the length of the sides minus roughly 3 inches i.e. the thickness of the top and bottom boards (measure to be exact). You want them to fit very tightly, even if you have to hammer them in a bit.
Reinforce the top sill by framing all around. Remember the two sides will go inside the other two boards. In other words, they will rest on the bottom board and hold up the top board. They will be reinforcing the sill plate. Once you have all four boards in place, toe-nail the corners.
Build a lip all the way around the inside of the frame with the 1-by-6 boards. Measure the thickness of your plywood and leave that much space from the outside edge so when you place the plywood door in, it will be flush with the outside.
Insert the plywood into the hole. You will need to have something to hold the door closed. It can be as simple as two blocks of wood with a hole drilled in the center that you can loosely nail in place and swivel to keep the door closed or use sliding barrel bolts.
Seal everything by painting the door and caulking between the 2 x 8 and the cement block. You want to keep moisture and critters out. Be sure to caulk the top, bottom and both sides and provide a good coating of paint to prevent water from warping the door.