ADU Plumbing: What Every Homeowner Should Know Before Construction

Understanding ADU Plumbing from the Ground Up

Plumbing is one of the most critical and costly aspects of any ADU construction project. Whether you are building a detached ADU in your backyard, converting a garage, or adding a Junior ADU within your existing home, the plumbing system must be carefully planned and expertly installed to ensure reliable service, code compliance, and long-term durability.

Many homeowners underestimate the complexity and cost of ADU plumbing, only to face budget overruns and construction delays when hidden challenges emerge. In this guide, we will cover everything you need to know about ADU plumbing in California, including sewer connection options, water supply considerations, fixture selection, code requirements, cost breakdowns, and common pitfalls to avoid.

Sewer Connection Options for Your ADU

Connecting your ADU to the sewer system is typically the most expensive and complex part of the plumbing scope. Understanding your options will help you plan your project effectively and avoid costly surprises during construction.

Option 1: Connect to the Existing Main House Sewer Lateral

The most common approach is to run a new drain line from your ADU to the existing sewer lateral that serves your main house. This lateral is the pipe that connects your property to the city sewer main, typically located under the street.

Pros: Lower cost than a new connection; no separate sewer permit or connection fee; typically faster installation.

Cons: The existing lateral must be in good condition and have adequate capacity; long runs between the ADU and the lateral connection point can be expensive; may require cleanout access points.

In most California cities, connecting to the existing lateral is permitted as long as the lateral can handle the additional flow. A sewer camera inspection of the existing lateral ($150 to $350) is highly recommended before committing to this approach. If the lateral is old, cracked, or partially collapsed, you may need to replace it first, which can add $5,000 to $15,000 to your project.

Option 2: Install a New Sewer Lateral

If your existing lateral is in poor condition, is too far from the ADU location, or if the local utility requires a separate connection, you may need to install a new sewer lateral from the ADU directly to the city sewer main.

Pros: Independent connection; no impact on existing house plumbing; new lateral with long service life.

Cons: Significantly more expensive ($8,000 to $25,000); requires excavation in the public right-of-way; city permit and connection fees apply; longer installation timeline.

Option 3: Septic System (Rural Properties)

For properties not connected to a municipal sewer system, a septic system may be required. This is more common in rural areas of California. A new septic system for an ADU typically costs $10,000 to $30,000, depending on soil conditions and system size. Check with your local health department for septic system requirements and regulations.

Connection Type	Typical Cost	Best For
Existing lateral connection	$3,000 - $8,000	Most garage conversions and attached ADUs
New sewer lateral	$8,000 - $25,000	Detached ADUs far from existing lateral
Septic system	$10,000 - $30,000	Rural properties without sewer access

Water Supply Considerations

Your ADU needs a reliable water supply for all fixtures, appliances, and the HVAC system. Understanding the water supply options and requirements will help you plan your project efficiently.

Connecting to the Existing Water Service

In most cases, your ADU will connect to the existing water service that supplies your main house. California state law generally prohibits local agencies from requiring a separate water meter or connection for ADUs, which saves homeowners significant money.

The existing water main to your house is typically 3/4-inch or 1-inch copper or polyethylene pipe. For most ADU projects, a 3/4-inch branch line from the existing service is adequate, but your plumber will perform a flow calculation to confirm. Factors that affect this calculation include:

• Distance from the water main to the ADU
• Number of fixtures in the ADU
• Static water pressure at the main
• Elevation difference between the main and ADU
• Simultaneous demand from the main house

Water Pressure Considerations

California plumbing code requires a minimum of 15 PSI at each fixture. If your property has low water pressure or the ADU is located far from the main or at a higher elevation, you may need a booster pump. This adds $500 to $1,500 to the project but ensures reliable water pressure throughout the ADU.

Water Heater Options

Your ADU will need its own dedicated water heater. Here are the most common options for California ADU projects:

Water Heater Type	Cost (Installed)	Pros	Cons
Tankless electric	$1,200 - $2,500	Compact, endless hot water, efficient	High amperage requirement
Heat pump water heater	$2,000 - $4,000	Highly efficient, qualifies for rebates	Larger footprint, needs ventilation
Standard tank electric	$800 - $1,500	Low upfront cost, simple installation	Less efficient, takes up space
Tankless gas	$1,500 - $3,000	High flow rate, compact	Requires gas line, venting

For California ADUs built in 2026, heat pump water heaters are increasingly the go-to choice because they meet Title 24 energy requirements and qualify for substantial utility rebates through programs like the California Public Utilities Commission incentive programs.

Plumbing Rough-In: What Happens Behind the Walls

The rough-in phase is when all of the supply and drain pipes are installed before walls, floors, and ceilings are closed up. This is one of the most critical phases of your ADU project because plumbing mistakes made during rough-in are expensive and disruptive to fix later.

Drain, Waste, and Vent (DWV) System

The DWV system carries wastewater from your fixtures to the sewer connection and provides venting to maintain proper drainage. Key components include:

• Drain lines: Sloped pipes (minimum 1/4 inch per foot for most fixtures) that carry wastewater by gravity to the sewer connection. These are typically 1.5-inch to 4-inch ABS or PVC pipe.
• Vent pipes: Vertical pipes that extend through the roof to allow air into the drainage system. Without proper venting, drains can siphon trap water and allow sewer gases to enter the living space.
• Cleanouts: Access points for clearing blockages. California code requires cleanouts at specific intervals and locations, including one near the ADU's sewer connection point.

Supply Lines

Hot and cold water supply lines run from the water service connection to each fixture. Modern ADU construction typically uses PEX (cross-linked polyethylene) pipe, which is flexible, freeze-resistant, and faster to install than traditional copper. PEX manifold systems, where each fixture has its own dedicated supply line from a central distribution point, are increasingly popular because they allow independent shutoff of each fixture and reduce the risk of leaks at fittings.

Slab Work

For garage conversions and slab-on-grade ADU construction, running drain lines often requires cutting through the concrete slab. This is one of the most expensive aspects of ADU plumbing because it involves saw-cutting the concrete, excavating the trench, installing pipe with proper slope, backfilling, and patching the slab. Typical slab plumbing costs range from $3,000 to $10,000, depending on the number and length of cuts required.

To minimize slab work, position the kitchen and bathroom as close together as possible and align them with the nearest sewer connection point. This reduces the amount of trenching required and can save thousands of dollars.

Fixture Selection for Your ADU

Choosing the right plumbing fixtures for your ADU involves balancing quality, aesthetics, water efficiency, and budget. California's water efficiency requirements mean that all fixtures must meet specific flow rate standards.

California Water Efficiency Requirements (CALGreen)

Fixture	Maximum Flow Rate	Notes
Toilets	1.28 GPF	WaterSense labeled required
Lavatory faucets	1.2 GPM	At 60 PSI
Kitchen faucets	1.8 GPM	At 60 PSI
Showerheads	1.8 GPM	At 80 PSI

Best Fixture Choices for ADU Bathrooms

For space-efficient ADU bathrooms, consider these popular fixture choices:

• Wall-hung toilets: These save 6 to 10 inches of floor space compared to standard floor-mounted toilets. They also make cleaning easier. Cost: $400 to $1,200 for the bowl and carrier system.
• Compact vanities: A 24-inch vanity provides a sink and storage in a minimal footprint. Cost: $200 to $800.
• Walk-in showers: A 36x36-inch or 32x48-inch walk-in shower with a glass panel is the most space-efficient shower option. Cost: $1,500 to $4,000 installed.
• Shower-tub combos: If you want to include a bathtub for tenant appeal, a standard 60-inch tub with shower surround provides both options. Cost: $800 to $2,500 installed.

Best Fixture Choices for ADU Kitchens

ADU kitchens are typically compact, so choose fixtures that maximize functionality in minimal space:

• Single-bowl undermount sink: A 25-inch single-bowl sink provides more usable washing space than a double-bowl sink in a smaller footprint. Cost: $150 to $500.
• Pull-down faucet: A pull-down sprayer faucet adds versatility to a small sink. Look for models with magnetic docking for durability. Cost: $100 to $400.
• Compact dishwasher: An 18-inch dishwasher fits in tight kitchen layouts while still handling a full day's dishes for one or two people. Cost: $400 to $800.

California Plumbing Code Requirements for ADUs

All ADU plumbing work in California must comply with the California Plumbing Code (CPC), which is based on the Uniform Plumbing Code with California-specific amendments. Here are the key requirements that affect ADU projects.

Minimum Fixture Requirements

A legally habitable ADU must include, at minimum: one toilet, one lavatory (sink), one bathtub or shower, one kitchen sink, and a water heating system. Additional fixtures like a dishwasher and laundry connections are optional but recommended for tenant appeal.

Backflow Prevention

California requires backflow prevention on all potable water connections to protect the municipal water supply from contamination. Your plumber will install appropriate backflow prevention devices at the water service connection to the ADU.

Earthquake Strapping

Water heaters in California must be strapped to the wall to prevent toppling during an earthquake. This is a simple requirement but an important one that inspectors always check. Strapping kits cost $15 to $30 and take about 20 minutes to install.

Total ADU Plumbing Cost Summary

Plumbing Component	Cost Range
Sewer connection	$3,000 - $25,000
Water supply line	$1,500 - $5,000
Rough-in (DWV + supply)	$4,000 - $12,000
Slab work (if applicable)	$3,000 - $10,000
Water heater	$800 - $4,000
Fixture installation	$2,000 - $6,000
Gas line (if applicable)	$500 - $2,000
Total plumbing budget	$10,000 - $35,000

For most California ADU projects, plumbing accounts for 10% to 20% of the total construction budget. Detached ADUs at the far end of the property with new sewer connections fall on the higher end, while garage conversions close to existing plumbing infrastructure fall on the lower end.

Frequently Asked Questions About ADU Plumbing

Can my ADU share plumbing with my main house?

Yes, in most cases your ADU can connect to the existing water service and sewer lateral that serve your main house. California law generally prevents local agencies from requiring separate utility connections for ADUs. However, the existing systems must have adequate capacity to serve both the main house and the ADU. Your plumber will evaluate this during the design phase.

Do I need a separate water meter for my ADU?

No. California state law prohibits local agencies from requiring a separate water meter or connection for ADUs in most cases. Your ADU can share the existing water meter with your main house. However, some homeowners voluntarily install a separate sub-meter to track ADU water usage for tenant billing purposes. A sub-meter costs $200 to $500 installed.

How much does it cost to run a sewer line from a detached ADU to the main sewer?

Running a new sewer line from a detached ADU to the existing sewer lateral typically costs $3,000 to $8,000, depending on the distance and the need for excavation. If you need a new connection to the city sewer main, the cost can range from $8,000 to $25,000, including the city connection fee and excavation in the public right-of-way.

What size water heater does an ADU need?

For a one-bedroom ADU, a 30 to 40-gallon tank water heater or a tankless unit rated at 3 to 5 GPM is typically adequate. For a studio or JADU, a smaller point-of-use water heater may be sufficient. Your plumber will size the water heater based on the number and type of fixtures in your ADU and the expected occupancy.

Can I do any of the ADU plumbing work myself?

In California, a homeowner can perform plumbing work on their own property with a homeowner permit, but this work must still meet code and pass inspection. However, for ADU projects, we strongly recommend hiring a licensed plumber (C-36 license). ADU plumbing involves complex code requirements, precise grading for drainage, and coordination with other trades. Mistakes can lead to failed inspections, water damage, and costly repairs.

Frequently Asked Questions About ADU Plumbing

Can my existing sewer line handle the additional load from an ADU?

In most cases, yes. A standard 4-inch residential sewer lateral can typically handle the wastewater from both a primary residence and an ADU without requiring an upgrade. However, older homes with clay or cast iron sewer lines may have issues with root intrusion, corrosion, or reduced capacity that should be addressed before connecting an ADU. Your plumber should perform a sewer camera inspection to assess the condition and capacity of your existing lateral before construction begins. If your sewer line needs replacement, expect to pay $3,000 to $10,000 depending on the length and depth of the line. Some California cities also require a sewer capacity study for new ADU connections, so check with your local public works department early in the planning process. For properties not connected to municipal sewer, review our guide on ADU septic system requirements.

Should I install a tankless water heater or a traditional tank water heater in my ADU?

Both options work well for ADUs, but each has advantages depending on your situation. Tankless (on-demand) water heaters are compact, energy efficient, and provide unlimited hot water, making them ideal for small ADUs where space is limited. They cost $1,500 to $3,500 installed but can save 20% to 30% on water heating costs over time. Traditional tank water heaters cost less upfront ($800 to $2,000 installed) and are simpler to maintain, but they take up more floor space and can run out of hot water during heavy use. For rental ADUs, tankless heaters are often the better investment because tenants appreciate endless hot water and lower utility bills. For ADUs used as guest suites or home offices with minimal hot water needs, a small 20 to 30 gallon tank heater may be the most cost-effective choice.

How far can my ADU be from the main house and still connect to existing plumbing?

There is no strict maximum distance, but longer plumbing runs increase costs and can create performance issues. For water supply lines, runs up to 100 feet are common and manageable with proper pipe sizing. For drain and sewer lines, the distance affects the required slope (typically 1/4 inch per foot for 3 and 4 inch pipes), which means longer runs may require deeper trenching. Most ADU plumbing connections run 20 to 60 feet from the main house, costing $2,000 to $6,000 for the underground piping. Beyond 80 feet, you may need larger diameter supply pipes to maintain adequate water pressure, and sewer runs may require cleanouts every 50 to 75 feet. Your licensed plumber will evaluate your specific layout and recommend the most efficient routing for supply and drain lines.