Run Plumbing for Bathrooms
Adding a bathroom to your home addition is one of the most valuable upgrades, but it requires careful plumbing planning. Rough-in plumbing happens before drywall, so getting the supply lines, drains, and venting right now is critical.
Quick Summary
Rough-in cost
$2,000-$4,000
Time needed
1-3 days
Professional help
Licensed plumber
Why This Step Matters
Plumbing rough-in establishes the "skeleton" of your bathroom—the supply lines bringing water in, the drain lines taking waste out, and the venting that allows drains to flow properly. Getting this wrong means cutting into finished walls later, which is expensive and disruptive.
Critical Consideration
If you're on a septic system, adding a bathroom increases wastewater volume. Your septic tank may need to be pumped, inspected, or even replaced if it can't handle the additional load. Check this before starting plumbing work.
Bathroom Plumbing Components
Supply Lines (Water In)
Supply lines bring hot and cold water to each fixture. Modern plumbing uses PEX (cross-linked polyethylene) tubing, which is flexible, affordable, and freeze-resistant.
- Main supply: 3/4" line from existing plumbing
- Branch lines: 1/2" to each fixture
- Shut-off valves: Required at each fixture
Pro tip: Consider a "home run" PEX system where each fixture gets its own dedicated line from a central manifold. Costs slightly more but provides better pressure and easier repairs.
Drain Lines (Waste Out)
Drain lines carry wastewater to the sewer or septic. They rely on gravity, so proper slope is critical—typically 1/4" per foot minimum.
- Toilet drain: 3" or 4" pipe
- Shower/tub drain: 2" pipe
- Sink drain: 1-1/2" pipe
- Main drain: 3" or 4" to sewer/septic
Venting System
Venting allows air into the drain system so water flows smoothly. Without proper venting, drains gurgle, run slowly, and can siphon water from traps (allowing sewer gas into your home).
- • Each fixture needs a vent within a specific distance
- • Vents tie into the main vent stack that exits through the roof
- • "Wet venting" allows drain pipes to also serve as vents for nearby fixtures
- • Air admittance valves (AAVs) can replace vents in some situations
Warning: Improper venting is the #1 cause of plumbing inspection failures. This is where an experienced plumber earns their fee.
Septic System Considerations
If you're on a septic system rather than municipal sewer, adding a bathroom significantly increases wastewater volume. Here's what to check:
Before Adding Plumbing
- • Get septic tank inspected and pumped
- • Determine current tank size (gallons)
- • Calculate new daily flow (150 gal/bedroom)
- • Check drain field condition
- • Review local health dept. requirements
Potential Upgrades Needed
- • Larger septic tank: $3,000-$8,000
- • New/expanded drain field: $5,000-$15,000
- • Aerobic treatment system: $10,000-$20,000
- • Pump chamber (if needed): $1,500-$3,000
- • Health dept. permit: $200-$800
Good News
Many septic systems are oversized for their current use. If your tank was designed for a 4-bedroom home and you only have 3, you may be fine adding a bathroom. An inspection will confirm.
Plumbing Cost Breakdown
| Component | Half Bath | Full Bath |
|---|---|---|
| Rough-in labor | $800-$1,500 | $1,200-$2,500 |
| Supply lines & fittings | $200-$400 | $300-$600 |
| Drain/vent pipes | $300-$600 | $500-$1,000 |
| Permit & inspection | $150-$300 | $150-$300 |
| Total Rough-In | $1,450-$2,800 | $2,150-$4,400 |
Costs increase if you're far from existing plumbing or need to break through concrete slab.
Frequently Asked Questions
How close can a new bathroom be to existing plumbing?
The closer, the cheaper. A bathroom that shares a wall with an existing bathroom or kitchen can often tie into existing supply and drain lines with minimal new pipe. A bathroom on the opposite end of the house may need 50+ feet of new pipe runs, adding $500-$1,500 in materials and labor.
Can I run drains through the floor joists?
Yes, but with limitations. You can notch or drill joists within code limits (typically the middle third of the joist, no deeper than 1/3 the joist depth). Larger drain lines may need to run below joists, requiring a dropped ceiling or bulkhead in the room below.
What is a "rough-in dimension" for a toilet?
The "rough-in" is the distance from the wall to the center of the toilet drain. Standard is 12 inches, but 10" and 14" rough-ins exist. Your plumber needs to know which toilet you're installing before setting the drain location.
Do I need a water heater upgrade?
Possibly. Adding a bathroom increases hot water demand. If your water heater is over 10 years old or already struggles to keep up, consider upgrading. For distant additions, a tankless point-of-use heater ($150-$300) can supplement your main heater and provide instant hot water.
Ready for the Next Step?
With plumbing roughed in, the next step is extending your HVAC system to heat and cool the new space. This should happen before insulation so ductwork can be properly installed.