Adding on basement space is a valuable asset compared to a crawlspace. You gain more usable square footage and maintenance is much easier and tends to be less expensive. One aspect of adding onto an existing basement is that somehow, a hole has to be cut through the foundation in order to connect the spaces. This is a messy job, but you can save yourself over $1,000 by doing this project yourself.
Preparing For The Opening
Figure out where you want the center of the hole to be and make a mark on the bottom of the wall on the new addition side. Use the rough opening of the door as your main reference. This is the opening needed to install a prehung door (door with the jamb already attached). Add 6 inches to the rough opening size and this is the size of the hole that needs to be cut. Divide the distance you figured by 2 and mark this measurement from the middle door mark on each side. Use a level to transfer these marks vertically up the wall, all the way to the bottom plate that the floor joist sit on. This is the amount of foundation you will be cutting.
Cutting The Hole
From the new addition side, score the pencil lines with the saw so you have clear marks on where you are cutting. Be careful not to hit the bottom plate. Put on a respirator and eye protection. Have a friend hold a garden hose when you are cutting into the foundation with the large concrete rental saw. This will keep the dust down. Bury the blade all the way into the foundation. Take frequent breaks to avoid asphyxiation from the dust and to recuperate from the weight of the saw.
When you have cut both lines, go in the old portion of the basement, and hit the top of the portion where you cut with a sledge hammer to break the top of the cut portion off. After you have beaten the top a bit, hit along the cut lines to crack the foundation all the way through. Give the whole chunk a push and the piece should come crashing down in the new addition basement. Clean up the concrete by breaking it into chunks. Hit the edges with the sledge hammer, letting the weight of the hammer do the work.
Framing For The Door
Measure up from the floor the height of the new door. Make a mark and measure up 3 inches. This will be the bottom of the header and or header framing if the gap between the bottom plate of the house framing and the bottom of the header is more than 13 inches. First, measure and cut two 2×4 or 2×6 studs that go from the floor to the bottom plate of the house. only attach them to the plate with screws. Screw a 2×6 or 2×4 (whatever the width of the bottom plate is) to the the bottom plate between the studs the entire width of the opening. make your header out of double 2x8s, 2×10,s, or 2x12s depending on your height requirements. Screw the header to the top plate you installed on the bottom plate of the house. Add two 2x4s or 2x6s to the bottom of the header with screws. Cut two 2x4s or 2x6s to support the header and screw them to the initial studs that were installed. Use your level to make sure that the door framing is plumb and opening is consistent . Use a hammer drill to drill into the foundation through the sides of the studs so concrete nail ins can attach the door frame to the concrete foundation.
You now have an opening that is framed out for a door, or can be left open, depending on your preferences.