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An accessory dwelling unit (ADU) can often share utility infrastructure with the primary house, but no connection should be assumed. Your building department, water and sewer authorities, electric utility, and gas utility determine whether each service may be shared, must be separately metered, or requires a new connection. Begin with a site survey, utility-capacity calculations, and an approved service plan before excavation or construction.
The same property may use a shared water line, sewer lateral, electrical service, or gas service only when the local code, utility tariff, capacity, access, and safety requirements are satisfied. A detached ADU usually adds trenching and route-length issues; an attached ADU can create access and separation issues inside the existing structure. The practical objective is a design that can be shut off, inspected, maintained, billed, and repaired without putting either dwelling at unnecessary risk.
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What to confirm before designing the ADU utilities
Utility rules are local. Confirm the governing jurisdiction, utility territories, sewer district, and whether the property is served by a public system or an approved private well, septic system, propane system, or other private infrastructure. A city building code may allow a shared arrangement while the utility’s service rules require a separate meter or a larger service.
Build a utility information file
- Locate the existing water, sewer, gas, and electric meters, main disconnects, panels, cleanouts, valves, and service lines.
- Obtain a boundary and topographic survey showing grades, structures, easements, driveways, trees, and likely trench routes.
- Request sewer invert elevations and connection requirements from the sewer authority. Ask where the public main or approved private system is located and what inspection points are required.
- Collect the existing electrical service rating, panel schedules, conductor sizes, gas meter capacity, water-line size, pressure information, and any previous permits or as-built drawings.
- Ask each authority whether a shared service, submeter, separate meter, new service, or utility easement is permitted for an ADU.
Do not rely on a contractor’s assumption that a nearby pipe or panel has spare capacity. A service that works for the main house may fail when the ADU adds simultaneous heating, water heating, cooking, laundry, refrigeration, and electric-vehicle charging.
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Choose the service strategy on the permit drawings
Show the proposed utility routes, fixture locations, panels, disconnects, shutoff valves, cleanouts, backflow or backwater devices, meters, equipment, and access clearances. Include calculations for water demand, sewer capacity and elevations, electrical load, and gas demand. The authorities review the complete arrangement, not just the point where a new pipe or cable enters the ADU.
Can an ADU share the house water line?
Often, yes, if the water authority and building department approve the design and the line has adequate capacity. Phoenix’s 2024 IRC-based Appendix BC states: “A primary dwelling and an ADU may share a common potable water system, provided that there are separate, accessible main shutoff valves allowing the water to be turned off on one side without affecting the other.” Portland’s guidance similarly calls for fixture shutoffs or a whole-dwelling shutoff in conversions and an accessible control valve for each unit in new construction.
Design the shared potable-water system
- Locate the public meter or private source and trace the private service line to the main dwelling and proposed ADU.
- List every fixture and appliance in both dwellings, including toilets, showers, tubs, lavatories, sinks, dishwashers, clothes washers, hose bibbs, water heaters, and any irrigation connection that shares the service.
- Have the plumbing designer calculate fixture demand, verify static and operating pressure, and determine whether the existing service must be upsized. Portland notes that the size of the line coming to the buildings depends on the fixtures.
- Place a clearly labeled main shutoff for each dwelling where occupants and emergency personnel can reach it without entering the other unit. Provide additional fixture or equipment shutoffs shown on the plumbing plan.
- Submit the sizing, routing, valves, backflow protection, and meter arrangement for plumbing and water-bureau approval.
- Install, pressure-test, flush, and inspect the system before walls, ceilings, or trenches are closed.
A shared meter measures combined consumption. That can be acceptable when the owner pays the bill, but it may complicate a tenant’s lease and future cost allocation. A separate water meter or approved submeter can simplify billing only where the water authority allows it; it is not a universal ADU requirement.
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Water shutoff and maintenance details
An ADU water shutoff valve must be listed for potable service, correctly sized for the approved pipe, installed in the location shown on the plan, protected from freezing or damage, and left accessible after finishes are installed. A hidden valve behind a fixed cabinet or under a permanently sealed floor does not provide the independent control that codes require. Label valves and photograph their locations for the owner and emergency responders.
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Can two buildings use one sewer lateral?
Sometimes. Sewer approval depends on the number of fixtures and units, pipe size and condition, property slope, connection point, and the sewer authority’s rules. Portland’s code guidance allows certain shared sanitary-drainage arrangements, including a detached ADU on the same property when its stated toilet and line conditions are met. Raleigh likewise allows multiple buildings on one lot to connect building drains to a common building sewer while requiring separate building-drain treatment under its plumbing rules. These examples do not create a nationwide entitlement to share a lateral.
Plan the gravity connection
- Obtain public-sewer or approved-private-system elevations, including the upstream and downstream invert elevations and the next uphill manhole where applicable.
- Determine whether each building can drain by gravity while maintaining required slope, burial depth, cleanouts, and clearance from other utilities.
- Map each building drain separately, then show the point where the drains join the common building sewer or approved lateral.
- Check fixture counts, toilet limits, line-size requirements, and any restrictions on connecting a second dwelling to the existing lateral.
- Provide accessible cleanouts at building exits, changes in direction, and other locations required by the plumbing code.
- Where the ADU floor is below the next uphill manhole, add the required backwater protection. The backwater valve must remain accessible for inspection and maintenance.
Never connect a second building to an existing lateral merely because the pipe is physically nearby. The authority may require a capacity check, a new tap, a larger lateral, a separate building sewer, or an easement. Private septic systems require a separate evaluation of tank, drain-field, reserve-area, and health-department capacity.
When pumping is unavoidable
If elevations prevent gravity drainage, a listed sewage ejector or lift system may be necessary. The approved design must address pump capacity, alarm power, venting, check valves, cleanouts, emergency access, and protection against backflow. Pumped drainage adds equipment that must be maintained; it is not an automatic substitute for obtaining a workable gravity route.
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Not always. Local rules and the electric utility’s tariff determine whether the ADU can be supplied from the existing service, through a feeder to a separate panel, or through a completely new service and meter. Portland requires ADU circuits to be completely independent of the primary dwelling’s circuits and recommends a separate service or panel when feasible. If the existing panel is used, the service and load calculation must support the additional demand, and equipment must remain accessible.
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Calculate the electrical load before choosing a panel
- Inventory existing loads and the ADU’s proposed heating, cooling, water heating, cooking, laundry, ventilation, refrigeration, lighting, receptacles, pumps, workshop equipment, and EV charging.
- Apply the adopted electrical code’s demand calculation and account for continuous loads, motor loads, future equipment, and the service’s available capacity.
- Choose among an ADU feeder and dedicated panel, a larger service for both dwellings, or a new independent service. Coordinate conductor size, overcurrent protection, grounding, bonding, disconnects, and panel locations.
- Confirm meter-main, disconnect, working-clearance, and emergency-access requirements with the utility and building department.
- Have a licensed electrical contractor install the work. Portland’s Building Code Guide 00-10 states: “If a new electrical service is installed for the ADU, or for both the ADU and the primary dwelling, a licensed electrical contractor must perform such installation work.”
- Schedule rough and final inspections before concealing wiring and before energizing a new service.
Raleigh’s summary requires separate electrical equipment and says branch circuits in each dwelling serve only that dwelling’s loads. A shared utility meter can therefore coexist with electrically independent panels, but the exact arrangement must be approved. Separate meters may be required by local rules or the utility tariff, especially where the ADU will have a separate account.
Keep electrical equipment accessible
Do not place a disconnect or overcurrent device inside the other dwelling, behind a locked tenant area, or where stored belongings block the working space. Label the ADU panel and disconnect, provide a durable directory, and preserve access for utility workers and emergency responders. A subpanel does not create extra service capacity; it only distributes capacity that the service and feeder can safely provide.
Does an ADU need a separate gas meter?
A separate gas meter is a utility decision, not a universal ADU rule. A shared meter and branch piping may be allowed when the meter, regulator, service line, and downstream piping have adequate capacity and the utility approves the arrangement. A new meter or service may be required by the utility, by the location of the ADU, or by the need for separate customer accounts.
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- Choose fuel sources early. Compare gas equipment with an all-electric design before finalizing the electrical load and utility routes.
- List the ADU’s gas appliances and input ratings, including space heating, water heating, cooking, clothes drying, fireplaces, and any outdoor equipment.
- Have a licensed gas contractor and the gas utility verify meter capacity, regulator requirements, pipe sizing, route, burial depth, protection from damage, pressure testing, and appliance connectors.
- Provide an accessible dwelling gas shutoff and an accessible shutoff at each gas appliance where required. Phoenix requires ready access to shutoff valves serving the dwelling and to appliance shutoff valves.
- Address venting, combustion air, equipment clearances, carbon-monoxide alarms, and other fuel-gas and energy-code requirements in the mechanical and building plans.
- Complete the required pressure test and utility and building inspections before introducing gas.
New York City’s ADU FAQ also requires separate gas-shutoff access and notes additional code requirements for gas-served ADUs. These requirements illustrate why a generic parts list is unsafe: pipe, valve, regulator, connector, vent, and appliance selections must match the approved design and local code.
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Shared service versus separate meters
The choice affects construction cost, billing, maintenance, and future changes. Santa Monica’s handbook describes the same basic trade-off: separate electric, water, and gas meters can place service in a tenant’s name and simplify cost allocation, but they add permits, connection fees, trenching, and plumbing or electrical infrastructure.
| Approach | Advantages | Costs and complications | Questions to resolve |
|---|---|---|---|
| Shared meter or service | Usually less trenching and fewer utility connection points; may reuse existing infrastructure. | Combined billing; capacity upgrades may still be required; one failure can affect both dwellings; shutoffs and equipment access must be carefully separated. | Are independent shutoffs, panels, building drains, and lawful lease billing allowed? |
| Separate meter or service | Separate utility accounts, clearer tenant billing, and easier allocation of consumption. | New connection fees, meters, service equipment, trenching, easements, inspections, and longer schedules. | Will the utility install a second meter, and where must the meter and disconnect be located? |
| Shared utility with submeter | May provide usage information without a second utility connection. | Submetering rules, calibration, ownership, and lease disclosures vary; the utility may not recognize the submeter for billing. | Is submetering lawful and who maintains and reads it? |
Shared service is not the same as shared downstream equipment. Two dwellings can use one utility meter while retaining independent dwelling shutoffs, electrical panels, building drains, or gas controls. Put every separation point on the plans.
Attached versus detached ADU utility routes
| Issue | Attached ADU | Detached ADU |
|---|---|---|
| Water and gas route | Shorter runs may reduce trenching, but pipes can pass through occupied or fire-separated areas and need protected access. | Longer runs require trenching, burial, frost and damage protection, and coordination with landscaping, driveways, and easements. |
| Sewer | Existing building drains may be nearby, but elevation and cleanout access can be constrained by the foundation. | Grades and the distance to the common sewer control whether gravity drainage is possible; a pump or backwater protection may be needed. |
| Electrical | A feeder may cross the existing structure and must maintain required separation and access. | A buried feeder, new service, or utility easement may be needed, with trench inspections before backfill. |
| Equipment access | Panels, valves, and meters must not be trapped inside the other dwelling. | Outdoor equipment needs weather, impact, flood, and security protection while remaining accessible to utilities and occupants. |
Gas versus all-electric systems
An all-electric ADU can avoid a gas extension, gas meter, fuel piping, combustion-air openings, and appliance venting. It does not eliminate utility design: heat pumps, electric resistance heat, induction cooking, heat-pump water heaters, dryers, and EV charging can substantially increase the electrical load. The electrical service must be calculated for the actual equipment, and the design must meet the adopted energy code.
Gas can reduce some electrical demand, but it requires utility coordination, correctly sized piping, pressure testing, shutoffs, venting, combustion air, and carbon-monoxide protection where required. Compare the complete installed route and operating arrangement rather than the appliance purchase price alone.
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Gravity sewer versus pumped or backwater-protected drainage
| Option | Best fit | Benefits | Trade-offs |
|---|---|---|---|
| Gravity sewer | ADU floor and building drain can maintain approved slope to the connection. | No pump power or pump maintenance; simpler operation. | Requires suitable elevations, burial depth, cleanouts, and a permitted connection route. |
| Backwater-protected gravity | ADU is below the next uphill manhole or is exposed to surcharge risk. | Retains gravity flow while reducing reverse-flow risk when the authority requires a backwater valve. | Valve must remain accessible and maintained; incorrect placement can block drainage or defeat protection. |
| Pumped drainage | Gravity elevations cannot be achieved. | Permits construction where the site cannot provide a gravity slope. | Requires a pump, alarm, power, venting, check valve, maintenance, and a response plan for failures. |
Permit and construction sequence
Following the sequence below reduces redesign, utility rescheduling, and failed inspections.
- Identify authorities. Confirm the building jurisdiction, water provider, sewer district, electric utility, gas utility, fire authority, health department for private wastewater, and right-of-way agency.
- Survey the site. Record property lines, grades, existing utility locations, meter and disconnect positions, sewer inverts, easements, and proposed ADU foundations.
- Select the service strategy. Decide which systems are shared, separately metered, or newly connected. Complete water-demand, sewer-elevation and capacity, electrical-load, and gas-demand calculations.
- Prepare coordinated drawings. Show routes, sizes, fixture and appliance locations, panels, disconnects, meters, shutoffs, cleanouts, backwater devices, equipment, burial depths, and access clearances.
- Obtain approvals. Apply for building, plumbing, electrical, mechanical or gas, right-of-way, utility connection, and private-sewer or septic permits as applicable. Obtain written utility approvals where required.
- Coordinate licensed trades. Use licensed plumbers, electricians, and gas contractors for work that the jurisdiction reserves to licensed professionals. Resolve any field condition through an approved revision rather than an informal change.
- Inspect open work. Schedule trench, underground, rough-in, pressure, electrical, sewer, and equipment inspections before covering pipes, conduits, or valves. Keep required separations and bedding visible.
- Complete final connections. Coordinate meter installation, service energization, gas turn-on, water activation, and sewer authorization with the relevant utilities.
- Retain records. Keep approved plans, test results, permits, inspection approvals, valve and panel directories, photographs, warranties, and as-built route information with the property records.
Utility fees, construction costs, and schedule
No universally applicable national connection-cost figure is established. Fees depend on the site, utility, jurisdiction, meter configuration, service size, trench length, road or sidewalk restoration, easements, impact or capacity charges, and whether existing infrastructure must be upgraded. Ask each authority for a written fee schedule and an estimate of deposits, inspection fees, tap fees, meter charges, and recurring account costs.
Separate services generally increase upfront work because they can require new meters, service equipment, trenching, utility design, easements, and additional inspections. Shared services can reduce connection work but may still require larger water lines, sewer upgrades, electrical-service upgrades, new panels, gas-capacity changes, or legal billing arrangements. Price the complete compliant installation, including engineering, permits, restoration, testing, and future maintenance access.
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- Insufficient water pressure or flow: Recheck fixture demand, pressure at peak use, service-line size, meter capacity, and pressure-regulating equipment. Submit an upsized design if the calculation requires it.
- One shutoff controls both dwellings: Add separate, accessible main valves and label them before finishes conceal the piping.
- Sewer backs up into the ADU: Verify elevations, downstream capacity, cleanout access, and required backwater protection. Keep the valve accessible for maintenance.
- Electrical panel fails the load calculation: Reduce coincident loads only when the approved design supports that approach, or upgrade the feeder, service, or equipment. A subpanel alone does not add capacity.
- Circuits serve the wrong dwelling: Trace and relabel wiring; provide dwelling-specific branch circuits and correct panel directories before final inspection.
- Gas pressure test fails: Stop work, isolate sections, inspect joints, regulators, connectors, and appliance valves, and have the licensed gas contractor repair and retest.
- Utility equipment is inaccessible: Relocate or redesign doors, cabinets, fences, landscaping, or locks so occupants and utility personnel can reach meters, disconnects, valves, cleanouts, and backwater devices.
- Work was covered before inspection: Do not energize or backfill concealed work until the authority gives direction. The inspector may require portions to be exposed or documented before approval.
- Field conditions differ from the plan: Pause the affected installation, document the condition, and obtain a revised drawing or written approval. Unrecorded changes can delay final permits and complicate future repairs or property transfer.
What a complete handoff should include
Give the owner a labeled site utility plan, as-built routes and depths, meter and shutoff locations, panel schedules, appliance and equipment data, pressure and electrical test results, sewer and backwater-valve maintenance instructions, permits, inspection approvals, and utility contact information. Explain which valves or disconnects isolate the ADU and which remain common to both dwellings. This record is essential when a leak, outage, sewer surcharge, renovation, or emergency occurs years after construction.
For a project-specific answer, have the design team submit the complete utility strategy to every authority before construction. The approved plan—not a generic assumption about shared lines or separate meters—controls what can legally and safely connect the ADU.
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