The first and most critical step is to create a detailed plan and obtain the necessary permits. Based on your finalized bathroom layout, draw a to-scale plumbing plan. This plan should show the exact location of the toilet, sink, and shower drains. Use a tape measure, framing square, and chalk line to transfer these locations onto the concrete floor. Mark the centerlines for each fixture's drain. For the toilet, the standard is 12 inches from the finished wall to the center of the drain. For the sink, it's the center of the vanity. For the shower, it's the center of the shower base. Research your local plumbing codes (e.g., IPC or UPC) to understand requirements for pipe sizing, drain slope (typically 1/4 inch per foot), and venting distances. Submit your plans to your local building department to secure a plumbing permit before any demolition begins.

This is the most physically demanding step. Wearing full personal protective equipment (safety glasses, hearing protection, dust mask, gloves, steel-toed boots) is non-negotiable. Use the chalk lines from the previous step as your guide to cut the perimeter of the trenches with a concrete saw for cleaner edges (optional but recommended). Then, use an electric demolition hammer or jackhammer to break up the concrete within the marked trench lines. A sledgehammer can be used to break up larger pieces. Use a pry bar and shovel to remove the broken concrete and a wheelbarrow to transport it out of the basement. Dig the trench deep enough to accommodate the diameter of your drain pipe (e.g., 3-inch pipe), plus a bed of gravel underneath, while maintaining a consistent slope of 1/4 inch per foot towards the main drain line.

This step involves laying the pipes that will carry waste away. Begin by locating and exposing a section of the main sewer line. Cut into the main line and install a wye fitting that will accept your new 3-inch drain line. Use a PVC pipe cutter for clean cuts. Before using any PVC primer or cement, lay out all your pipes and fittings in the trench—this is called a 'dry fit'. Assemble the 3-inch line for the toilet, connecting to a toilet flange set at the correct height. Run 2-inch pipe from the main line to the shower and sink locations, installing the appropriate shower drain assembly and a sanitary tee for the sink drain arm. Use a 4-foot level to constantly check your slope, ensuring it is at least 1/4 inch of fall for every foot of run. Once you are certain everything fits perfectly, disassemble and re-assemble the system permanently using PVC primer (usually purple) followed immediately by PVC cement. Add a layer of pea gravel under and around the pipes for support.

Every drain trap needs a vent to function. Extend the drain pipes vertically up into the wall cavities after the trap for each fixture. The sink drain arm (stub-out) will typically be 18-20 inches off the finished floor. Above the sanitary tee for the sink drain, a 1.5-inch or 2-inch pipe continues upward as the vent. The shower and toilet also need to be vented. Often, you can tie all the new bathroom vents together in the wall or ceiling into a single, larger vent pipe (e.g., 2-inch). This main vent line must then be connected to your home's existing main vent stack or, if that's not feasible, run independently up through the roof. Use a drill with hole saw or auger bits to run the pipes through wall studs and ceiling joists. Secure the pipes with pipe straps. Code dictates the maximum distance a fixture's trap can be from its vent, so consult your local code.

Before this step, you must have your under-slab plumbing inspected and approved. Once you have the green light from the inspector, carefully backfill the trenches with the excavated soil and gravel. Use a tamper or the end of a 2x4 to compact the fill in layers (every 6 inches) to prevent future settling. Lay down a plastic sheeting vapor barrier over the fill to match the rest of the basement floor. Mix ready-mix concrete in a wheelbarrow or mixer according to the manufacturer's directions. Pour the concrete into the trenches, ensuring it fills all voids. Use a trowel or screed board to level the new concrete with the existing slab, and a concrete float for a smooth finish. Allow the concrete to cure fully, which can take several days.

With the framing in place, you can run the hot and cold water supply lines. Turn off the main water supply to the house and drain the system. Tap into the main hot and cold water lines (usually 3/4-inch) with tee fittings. From these tees, run new 1/2-inch PEX tubing to each fixture location. Red PEX is for hot, blue is for cold. PEX is flexible and can be snaked through studs and joists easily. For the shower, secure a drop-ear elbow to a wood block between the studs at the correct height for the shower arm. Run separate hot and cold lines to the shower valve location. For the sink and toilet, run the tubing to the stub-out location and secure it to the framing. Use a PEX crimp tool to attach PEX fittings and shut-off valves (angle stops). Install stud guards (nail plates) over the framing wherever pipes pass through to protect them from future drywall screws.

The final step before you can insulate and hang drywall is the rough-in inspection. The plumbing inspector will return to check all of your work. They will verify that the DWV and supply lines are installed correctly, properly supported, and meet all code requirements. They will likely require a pressure test on both the DWV and the supply lines. For the DWV system, this involves sealing the system with a test ball or inflatable plug and filling it with water up to a certain level (or pressurizing it with air) to check for leaks. The supply lines will be tested under normal house pressure. Do not proceed with wall coverings until you have passed this final plumbing inspection.