Quick answer
An ADU water connection is the pipe that brings drinking water to an accessory dwelling unit, usually as a branch off the main house's water line, or as a new service with its own meter where the water provider requires or allows one.
The choice shapes the trench, the water pressure at the new taps, the fee the water provider can charge and who gets the bill.
Quick facts
- Two routes
- A branch off the house line, or a new service and meter from the street
- Who decides on a meter
- The water provider, within state law
- Model code pressure cap
- Static pressure not over 80 psi; a pressure-reducing valve above that (2024 IRC P2903.3.2) 3
- California: separate connection for a conversion ADU
- Cannot be required for units under Gov. Code 66323(a)(1), with two exceptions 1
- California: other ADUs
- A fee must be proportionate to the burden of the unit 1
- Who does the work
- A licensed plumbing contractor (CSLB C-36 in California) or a licensed general contractor 5
Key takeaways
- Most ADUs get water from a new branch tied into the main house line.
- Some water providers want a separate meter, mostly for new detached units.
- California bars a separate connection for ADUs built inside an existing home or accessory structure, with two exceptions.
- Where a fee is allowed, California says it must match the burden the ADU adds.
- Pipe size depends on how many fixtures the two homes run at once, not on floor area.
- Licensed plumbers do the tie-in. Digging needs a utility locate first.
What it means, in plain English
Your ADU needs clean water for its sink, shower, toilet and water heater. The water connection is the pipe that gets it there.
There are only two places that water can come from. It can split off the pipe that already feeds your house. Or it can come straight from the public main under the street, through a new meter.
A few words come up a lot. The water service is the pipe from the street meter to the house. A branch line is a new pipe that splits off it. A meter measures how much water passes so the provider can bill for it.
Why it matters to you
Water is often the cheapest utility to run, but the meter decision can swing the budget. A new meter brings a provider fee and a longer trench. A branch line keeps the work on your lot.
It also decides comfort. If the house line is small, two homes running showers at once can drop the pressure in both.
Last, it decides billing. With one meter, one bill covers both homes. If you rent the ADU, the lease has to say how water is shared. The separate utility meter page covers that rental choice.
How the water gets from the street to the ADU
- The public main runs under the street. The provider owns it.
- A service pipe runs from the main to a meter, usually near the front property line.
- From the meter, your private water service runs to the house. You own that pipe.
- For a branch connection, a plumber taps the private service or the house supply and runs a new line to the ADU, often with its own shutoff valve.
- For a separate connection, the provider sets a new meter and a new private line runs from it to the ADU.
- Inside the ADU, the line feeds the water heater and every fixture.
- The building department inspects the pipe and pressure test before the trench is closed.
Three ways to supply water to an ADU
| Route | What it looks like | Best fit | Watch for |
|---|---|---|---|
| Branch inside the house | Pipes extend from the house plumbing into an attached or interior unit | Junior ADUs, basement and attached units | Pressure at the far fixtures |
| Branch off the yard line | A tee on the private service in the yard, with a trench to the ADU | Most detached and garage conversion units | Trench route, tree roots, the sewer trench |
| New meter and service | The provider sets a second meter; a new line runs to the ADU | Units the provider says must have their own service, or owners who want split billing | Provider fee, longer trench, street work |
Branch line is not a “separate connection”
A branch line is private plumbing past your existing meter, so the provider sees one customer. A separate connection is a new service and meter from the provider’s main. California’s fee rule in Gov. Code 66311.5 is about that second kind: when a provider may require it, and what it may charge 1.
Example: will one water line serve two homes?
A hypothetical lot, used only to show how a plumber thinks about it. Real sizing follows the plumbing code your city adopted and the provider's rules.
| Input | Main house | Detached ADU |
|---|---|---|
| Bathrooms | 2 | 1 |
| Kitchens | 1 | 1 |
| Laundry | 1 | None |
| Water use counted by | Fixture count (water supply fixture units) | Fixture count (water supply fixture units) |
| Result | The combined fixture count sets the size the shared line must be | If the existing line is too small, it is upsized or a new service is used |
The point: the plumber adds up fixtures in both homes, then checks the existing pipe size, its length and the street pressure. Floor area alone does not answer the question. The 2024 IRC sizes water service mains in Section P2903.8 3.
Where you will see it
- The site plan. It shows the meter, the path of the new line and any new valves.
- The plumbing sheet. It lists pipe sizes and the water heater.
- A contractor bid. Trench, pipe, tie-in and pressure valve show up as line items.
- A provider letter or fee notice. It says whether a new meter is needed and what it costs.
- An inspection card. The inspector signs off the underground pipe before backfill.
California's rules for ADU water connections
| Situation | What the law says |
|---|---|
| ADU or JADU built inside an existing or proposed home or accessory structure (66323(a)(1)) | A local agency, special district or water corporation cannot require a new or separate connection, or charge a connection fee or capacity charge, unless the unit is built with a new home or sold separately |
| Other ADUs, such as a new detached unit | A separate connection may be required. The fee must be proportionate to the burden, based on square feet or drainage fixture units, and not exceed the reasonable cost of service |
| Any ADU | Not a "new residential use" for connection fees, unless built with a new single-family home |
Sources: [1]
What the plumbing code checks
The 2024 International Residential Code (IRC) is the national model code. In Chapter 29 it caps static water pressure at 80 psi. Above that, a pressure-reducing valve is needed (P2903.3.2). It also has a rule on keeping the water service apart from the building sewer in a shared trench (P2906.4.1).
California does not use the IRC plumbing chapters. It adopts its own California Plumbing Code as part of Title 24. The 2025 edition took effect on 1 January 2026. Section numbers differ, so ask your building department which rule applies.
Outside California, states adopt their own plumbing code. See ADU laws by state for the ADU rules that sit on top.
What drives the cost
No single price fits every lot, so this page gives drivers, not dollars. The length of the trench is the first one. A unit at the back of a deep lot needs more pipe and more digging.
The second is what is in the way. Driveways, patios and large trees add cutting, patching or hand digging.
The third is the provider. A new meter brings its own fee schedule. In California the fee must follow the proportionate rule above. The fee estimator helps you budget fees, and the ADU cost hub places water among the other line items.
Branch line versus new meter
The cards weigh the two routes on the points owners ask about most: billing, shutoffs, fees and who you deal with. The lists below them cover the branch line in more detail.
Branch line: benefits
- Shorter trench in most yards
- No new meter fee from the provider
- Fewer agencies involved
- Often faster to finish
Branch line: limits
- One bill for both homes
- Shared pressure when both homes use water at once
- A leak on the shared line affects both homes
- May need a bigger pipe from the meter
Branch off your existing service
- One meter and one water bill for both homes
- Work is private plumbing, checked by the building department
- Both homes share pressure and the main shutoff
- Pipe from the meter may need upsizing for two homes
New meter from the provider
- A second meter, so each home gets its own bill
- The provider sets the meter and its own fee, within any state limit
- Each home can be shut off on its own
- Often a longer trench and some street-side work
Which route your ADU can use depends on the provider and, in California, on Gov. Code 66311.5 1. Source: [1]
Water connection compared with related terms
| Term | What it covers | Where to read more |
|---|---|---|
| Water connection | Drinking water supply to the ADU | This page |
| Sewer lateral | Wastewater leaving the lot | ADU sewer lateral |
| Utility connection | All services together, plus fees | ADU utility connection |
| Separate meter | Billing and tenant use | Separate utility meter |
Common misreadings
- Myth "Every ADU needs its own water meter."
- Reality Not in California for a unit built inside an existing home or accessory structure. For other units the provider decides 1.
- Myth "A bigger water heater fixes low pressure."
- Reality No. Pressure depends on the supply pipe size, its length and the street pressure, not the heater.
- Myth "Higher pressure is always better."
- Reality The model code caps static pressure at 80 psi and calls for a reducing valve above it 3.
- Myth "Water and sewer can go in any trench together."
- Reality The code has a separation rule for water service and building sewer pipes 3. Your plumber and inspector apply it.
What to ask your plumber and water provider
- Is the existing service big enough for both homes, by fixture count?
- What is the street pressure at my meter?
- Will the provider require a separate meter for this ADU type? Under which rule?
- If a fee applies, how is it worked out: square feet or fixture units?
- Where will the shutoff valves for the ADU be?
- Can the water and sewer lines share a trench on this lot?
- Is the plumber licensed for this work? (In California, check the CSLB license.)
Checklist for your project
- Find your meter and note the size of the line leaving it.
- List every fixture planned in the ADU.
- Ask the water provider in writing if a separate meter is needed.
- Have a licensed plumber size the line and check pressure.
- Before any digging, the excavator calls 811 so buried utilities are marked 2.
- Show the route, valves and any reducing valve on the plans.
- Book the underground inspection before backfill.
- Write water billing into any lease.
Outside California
Many water providers in other states treat an ADU as a new account and charge a full tap fee. Others allow a shared meter. No state limit like California's applies unless your state passed one.
Questions about adu water connection
Can an ADU share water with the main house?
Yes, in most cases. A plumber runs a branch off the house service to the ADU. The shared line must be big enough for the fixtures in both homes, and the water provider's rules still apply.
Does an ADU need a separate water meter?
Not always. In California a provider cannot require a separate connection for an ADU built inside an existing home or accessory structure, with two exceptions. For a new detached unit, the provider may require one.
Who decides if my ADU needs its own meter?
The water provider, which may be a city, a special district or a private water company. State law, where it exists, limits what the provider can demand. Ask for the answer and the rule in writing.
Will adding an ADU lower my water pressure?
It can, if the shared line is too small or very long. A plumber sizes the line by the fixtures in both homes. If the line is too small, it can be upsized or a new service used.
What size water line does an ADU need?
It depends on the fixtures in both homes, the pipe length and street pressure. The plumbing code your city adopted sets the method. A licensed plumber works it out for your lot.
Can a water provider charge a fee for an ADU in California?
Sometimes. It cannot charge a connection fee or capacity charge for a 66323(a)(1) unit unless it is built with a new home or sold separately. For other ADUs, the fee must be proportionate to the burden.
Can the water line and sewer line go in the same trench?
The code has a separation rule for these two pipes. In some cases they can share a trench with set conditions. Your plumber and inspector decide how it applies on your lot.
Do I need to call 811 before trenching for an ADU water line?
Yes. In California, Gov. Code 4216.2 requires an excavator to notify the regional notification center before digging, except in an emergency. Buried lines are then marked.
Can I run the ADU water line myself?
This page does not give DIY steps. Water lines need a permit and inspection, and a mistake can contaminate drinking water. Hire a licensed plumber or general contractor.
Does a garage conversion ADU need a new water connection?
Usually not. A garage is an existing accessory structure, so in California a conversion under 66323(a)(1) cannot be made to take a separate connection, unless built with a new home or sold separately.
Sources
- California Legislative Information, Gov. Code § 66311.5, retrieved .
- California Legislative Information, Gov. Code § 4216.2, retrieved .
- International Code Council, 2024 International Residential Code, Chapter 29 Water Supply and Distribution, retrieved .
- California Building Standards Commission, California Building Standards Code (Title 24), retrieved .
- Contractors State License Board, License Classifications, retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Gov. Code §§ 66311.5 and 4216.2 were opened on leginfo, and 2024 IRC Chapter 29 (P2903.3.2, P2903.8, P2906.4.1) on codes.iccsafe.org, on 8 October 2026. The sizing example uses a hypothetical lot and no prices.
Suggest a correction. We fix errors and say what changed.