Question 1

What is the minimum plumbing required for a legal ADU in California?

Accepted Answer

To qualify as a legal Accessory Dwelling Unit, California Government Code requires that the unit provide complete independent living facilities — and for plumbing, that means permanent provisions for cooking, bathing, and waste disposal. Miss any one element and the unit is not an ADU under state law.The minimum plumbing fixture listFor a standard ADU (attached, detached, or a garage conversion), the plumbing scope must include:A full kitchen with a sink connected to both the potable water supply and the drain-waste-vent (DWV) system.At least one complete bathroom containing a toilet (water closet), a lavatory sink, and a bathtub or shower.A water heater sized to serve the unit — commonly a heat-pump, tankless, or standard tank unit (see mechanical plans for water-heating coordination).A potable water connection from the meter or an approved sub-meter.A connection to an approved waste disposal system — municipal sewer or an approved septic system.Every one of these fixtures must appear on the plumbing floor plan and fixture schedule, with flow rates and water heater specifications called out so the plan checker can verify code compliance.How a JADU differsA Junior ADU (JADU, up to 500 SF created within the walls of the existing house) has a lighter plumbing requirement. A JADU must have an efficiency kitchen with a sink connected to the DWV system, but a separate, private bathroom is not required if the JADU shares access to the primary dwelling's bathroom. That single distinction is one of the biggest reasons homeowners choose a JADU when they want to avoid cutting a slab or extending a sewer lateral.Why "minimum" is rarely the whole storyMeeting the bare fixture count is only the start. Each fixture triggers downstream design decisions — drain sizing, vent routing, the fixture-unit load on your existing sewer lateral, and CALGreen low-flow compliance. A bathroom group, for example, isn't just a toilet, sink, and shower; it's three traps that each need a vent, a properly sloped branch drain, and hot/cold supply lines sized for simultaneous demand.Requirements vary by jurisdiction and by your water/sewer district. Always confirm fixture and connection requirements with your local building department before finalizing design.Our permit-ready ADU plumbing plans document every required fixture, the water supply design, the full DWV layout, and water heater specs so your set passes plan check the first time. See California ADU requirements for the broader picture, or start an order when you are ready.

Question 2

What are California's water conservation requirements for ADU fixtures?

Accepted Answer

California enforces some of the strictest plumbing-fixture flow limits in the country through CALGreen (the California Green Building Standards Code, Title 24 Part 11). Every new residential fixture in an ADU must meet these maximum flow rates, and they are verified through plan check and inspection — not optional best practices.Maximum fixture flow ratesFixtureMaximum flow / flushTest pressureToilet (water closet)1.28 gallons per flush—Bathroom lavatory faucet1.2 GPMat 60 psiKitchen faucet1.8 GPMat 60 psiShowerhead1.8 GPM per head—Bathtub fill4.0 GPM—Note that older 1.6 gpf toilets are no longer compliant with current California code, even though they are still sold elsewhere. If you reuse a fixture salvaged from a remodel or buy out of state, check the engraved flush rating before installing it. The same caution applies to showerheads and faucets, which are stamped with their GPM rating either on the body or in the packaging — the figure printed there must match what your fixture schedule promises.Why California pushes the limits this hardThese rules exist because California is chronically water-stressed and adds tens of thousands of new dwelling units every year. An ADU is a brand-new household drawing on the same constrained supply, so the state treats every new fixture as an opportunity to lock in permanent savings. The cumulative effect is significant: a CALGreen-compliant ADU bathroom uses a fraction of the water a 1990s bathroom did, which is exactly why these limits are written into mandatory code rather than left to homeowner preference.How compliance is documented and verifiedOn the plans: the plumbing fixture schedule must list each fixture's make/model and rated flow so the plan checker can confirm compliance at a glance.At inspection: inspectors may verify installed fixtures match the approved schedule, and some flag non-compliant showerheads or faucets at final.For showers with multiple heads: CALGreen generally limits the combined flow, so a rain head plus a body-spray array can quietly violate the rule even if each individual head is rated 1.8 GPM.Why it matters for your budget and rebatesLow-flow fixtures aren't just a code hurdle. They reduce the demand load on your water service, which can be the difference between tying into your existing meter and paying for a costly service upgrade. Many California water districts also offer rebates for high-efficiency toilets and smart irrigation that you can stack onto an ADU project.Flow-rate enforcement and local amendments vary by jurisdiction. Some water-stressed districts adopt stricter limits — confirm with your local building department and water utility.Our plumbing plan sets specify CALGreen-compliant fixtures throughout and document the flow rates in the schedule. If you also need energy compliance, our Title 24 reports (a $240 add-on) coordinate water-heating efficiency with the rest of your MEP package.

Question 3

How do drain lines work in a concrete slab ADU or garage conversion?

Accepted Answer

In slab-on-grade construction — which covers virtually every California garage and most detached ADUs built on a concrete pad — the under-slab drain lines must be in place before the slab serves the fixtures above it. For new construction they are laid before the pour; for a garage conversion they require cutting and trenching the existing slab. This is one of the most underestimated parts of an ADU plumbing budget.The garage-conversion slab-cutting sequenceThe plumber transfers the exact fixture locations from the approved plumbing plan onto the slab surface.A concrete saw cuts trenches along the planned drain routes.PVC or ABS drain pipe is laid at a minimum slope of 1/4 inch per foot so waste flows by gravity.P-traps and vent stub-ups are set at each fixture location.A rough plumbing inspection is performed — and often a water or air test on the DWV — before any concrete goes back.The trench is backfilled, the slab is patched, and the concrete cures.Finish fixtures are connected above the slab once the floor is restored.For a typical two-bathroom-plus-kitchen ADU, expect roughly 40 to 80 linear feet of trenching. The drain depth at the far end is dictated by slope: every foot of horizontal run drops the pipe another quarter inch, so a long run can put the connection point surprisingly deep.The post-tension slab problemMany slabs poured since the 1980s are post-tension — they contain steel cables under thousands of pounds of tension. Cutting one blindly can sever a cable with violent, dangerous results and compromise the slab structurally. Before assuming a slab can be cut:Look for "PT" or "POST-TENSION" stamps at the slab edge or garage door.Look for circular grout plugs around the perimeter where cables were anchored.If in doubt, have the slab scanned (GPR) and get a structural engineer's sign-off before any saw touches concrete.Post-tension work requires specialized cutting, cable de-tensioning or careful avoidance, and engineered repair — adding cost and schedule that homeowners rarely anticipate.Crawlspace and stem-wall alternativesWhen an ADU sits over a raised foundation or crawlspace, drains hang below the floor framing instead of being buried — usually easier and cheaper to route and to modify later. If you have a choice during design, a crawlspace can dramatically reduce drainage risk.Slab-cutting permits, inspection sequencing, and post-tension review requirements vary by jurisdiction. Confirm with your local building department before scheduling concrete work.Our garage conversion MEP plans show drain routing, slopes, and vent stub locations precisely so your plumber cuts once and passes rough inspection.

Question 4

How does ADU sewer connection work in California?

Accepted Answer

Getting your ADU's waste to the public sewer comes down to one of three approaches — a tie-in to your existing lateral, a brand-new lateral, or a pumped (ejector) connection. Choosing the right one early, before architecture is locked, is one of the highest-leverage decisions in an ADU project.The three connection methods1. Tie into the existing sewer lateral (most common). The ADU drain connects to your home's existing building sewer through a wye or tee fitting. This works when the ADU is close enough to maintain the required 1/4 inch per foot gravity slope and the existing lateral has spare capacity. Always run a sewer camera before committing — old clay or cast-iron laterals with root intrusion, offset joints, or corrosion should be replaced before you add ADU load. Typical tie-in cost runs roughly $2,000–$8,000.2. Install a new sewer lateral. Required when the ADU is too far from the existing lateral, the lateral lacks capacity, or the city mandates separate sewer service for the new unit. This means excavating all the way to the public main, and cost climbs with distance, trench depth, and pavement or driveway restoration — commonly $8,000–$25,000.3. Sewage ejector / grinder pump. When the ADU's drains sit below the elevation of the gravity sewer (a basement bath, a downhill detached unit), a sealed ejector basin and pump lift the waste up to the gravity line. It works, but it adds a mechanical component, a dedicated electrical circuit, an alarm, and ongoing maintenance.Shared vs. separate serviceShared: the ADU ties into the existing lateral and is billed under the primary property. Simpler and cheaper, and adequate for most single-ADU projects.Separate: some sewer districts require an independent lateral and/or a separate connection fee for the new unit, especially where they bill per dwelling unit.Fixture-unit load and capacityEvery fixture you add contributes drainage fixture units (DFU) to the lateral. A standard ADU bathroom group plus a kitchen is a meaningful addition. If your existing lateral is a marginal 4-inch line already serving a large house, the district may require a capacity study before approving the tie-in.Connection methods, separate-service rules, capacity studies, and connection fees vary widely by city and sewer district — and can range from modest to tens of thousands of dollars. Confirm in writing with your local public works or sewer authority early.Our plumbing plans document the connection method, slope, and tie-in detail your plan checker needs. For a detached unit, pair them with the full MEP package.

Question 5

What is a sewer lateral inspection and is it required?

Accepted Answer

A sewer lateral inspection sends a video camera down the sewer pipe to assess its condition from the inside — revealing root intrusion, cracks, offset joints, bellies, and blockages that are completely invisible from the surface. It is not always required by code, but on an ADU project it is one of the smartest few hundred dollars you can spend.What the camera revealsRoot intrusion at pipe joints — the leading cause of failure in older clay laterals.Cracks, fractures, and crushed sections from soil movement or tree pressure.Offset or separated joints where pipe segments have shifted.Bellies (low spots) that pool waste and cause chronic backups.Material and diameter — confirming whether you have clay, cast iron, Orangeburg, or modern PVC, and whether the line is large enough for the added ADU load.Why it matters before you build an ADUThe economics are the whole argument. If your lateral is deteriorated, replacing it during the ADU project — while trenches are open and landscaping is already disturbed — is dramatically cheaper than excavating again after the unit is finished, the patio is poured, and the yard is replanted. The inspection also confirms the lateral has the capacity to carry the new fixture-unit load, and it gives you documentation to budget repairs with confidence instead of discovering a failed line mid-construction.Is it required?It depends on where you are. A number of California cities — particularly in older neighborhoods with aging clay infrastructure, and many East Bay and coastal jurisdictions — require a point-of-sale or point-of-improvement lateral inspection (and sometimes a compliance certificate) as a condition of permitting. Even where it is not mandated, most experienced ADU builders treat it as standard due diligence.Lateral inspection and compliance-certificate requirements vary by city and sewer district. Confirm with your local building department or sewer authority before you finalize plumbing design.Cost and timingA camera inspection typically costs $150–$350 and takes about an hour. If the line is healthy, you get peace of mind and a clean tie-in. If it is failing, you have found the problem at the least expensive possible moment to fix it. Either way, schedule it during early planning — before architecture and site design are finalized — so the results can shape the plumbing design rather than blow up the budget later.

Question 6

What CALGreen requirements apply to ADU plumbing beyond fixture flow rates?

Accepted Answer

Low-flow fixtures get all the attention, but CALGreen reaches well beyond flush and faucet ratings. Several additional plumbing provisions are checked at plan review and at inspection, and overlooking them is a common source of corrections on ADU sets.Hot water pipe insulationCALGreen requires hot water supply piping to be insulated, particularly in unconditioned spaces such as attics, crawlspaces, and garages. Insulation reduces standby heat loss and shortens the wait for hot water at the fixture. The required insulation thickness scales with pipe diameter, and the requirement also typically covers the first run off the water heater and any piping buried in a slab.Hot water delivery volume limitsCALGreen Section 4.303.1 limits the volume of water allowed to sit in the hot-water supply pipe between the heater and the fixture. The intent is to stop you from running gallons of cold water down the drain while waiting for hot. In practice this constrains how far a single water heater can be from the farthest fixture. Long runs in a spread-out ADU may require:A recirculation system (often demand-controlled to save energy), orA point-of-use water heater near the remote fixture, orRelocating the water heater more centrally during design.This is why water heater placement is a design decision, not an afterthought — a heater tucked into a far corner can quietly force an expensive recirculation system that a more central location would have avoided entirely.Other CALGreen plumbing touchpointsGreywater-ready provisions — some jurisdictions require new residential construction to be stubbed or designed so a future greywater (laundry-to-landscape or branched-drain) system can be added.Pressure and pressure-reducing valves — fixture flow ratings are tested at 60 psi; high static pressure may require a PRV to keep fixtures compliant and protect the system.Construction waste and pollution-prevention measures that touch the plumbing trades during the build.How this ties into your plansThese requirements are easiest to satisfy when they are baked into the design rather than retrofitted. Water heater placement, pipe routing, and insulation notes all belong on the plumbing sheets from the start.CALGreen has mandatory and optional ("Tier") measures, and many cities add local amendments. Confirm which apply to your project with your local building department.Our plumbing plans call out insulation, hot-water delivery strategy, and CALGreen notes, and coordinate cleanly with our Title 24 energy reports so your water-heating and conservation requirements line up across the whole MEP set.

Question 7

What is the DWV system and what happens if venting is improper?

Accepted Answer

The drain-waste-vent (DWV) system is the network of pipes that carries wastewater away from every fixture while keeping the drain system at atmospheric pressure. It has three integrated jobs: drains carry liquid waste from sinks, showers, and dishwashers; the waste portion carries solids from toilets; and vents run upward from the fixture connections and exit through the roof to admit air. Get the drains right but the vents wrong, and the system fails in ways that range from annoying to genuinely hazardous.Why venting is not optionalEvery fixture has a water-filled P-trap — the curved pipe under the sink — that holds a seal of water blocking sewer gas from the room. When water rushes down a drain, it pushes air ahead of it and pulls a vacuum behind it. Without a vent to relieve that pressure, the vacuum siphons the water right out of the P-trap. Once the seal is gone, there is nothing standing between your living space and the sewer.ComponentFunctionP-TrapHolds a water seal that blocks sewer gas from entering the living space.Vent PipeAdmits air and maintains atmospheric pressure so P-trap seals stay intact.Drain / Waste PipeCarries wastewater by gravity to the sewer lateral.What improper venting causesGurgling drains and slow flow as air fights its way through the water column.Sewer odors in the ADU once trap seals are siphoned dry.Exposure to sewer gas — hydrogen sulfide (the toxic rotten-egg smell) and methane (flammable).Repeated backups and fixtures that won't drain together.What proper vent design includesThe California Plumbing Code governs vent sizing, the maximum distance between a trap and its vent, and where vents can terminate. A complete plumbing plan shows:Vent stack locations and how each fixture is vented (individual, common, wet, or air-admittance valve where permitted).Vent pipe sizing based on the connected fixture-unit load.Roof penetration locations and required clearances from windows and intakes.Vent sizing, distances, and where air-admittance valves (AAVs) are allowed vary by jurisdiction. Confirm acceptable venting methods with your local building department.Missing or incomplete vent design is one of the most common plumbing plan-check corrections on ADU projects. Our plumbing plans include the full DWV layout — drains, waste, and a code-compliant vent system — so your set clears plan check and your fixtures drain quietly and safely.

Question 8

What is seismic strapping for water heaters and why is it required everywhere in California?

Accepted Answer

Seismic strapping is the set of metal restraints that anchor a water heater to the building so it cannot topple during an earthquake. Because every part of California sits in seismic territory, the California Plumbing Code requires water heaters to be strapped statewide — and inspectors check for it at final.Why it is required everywhereA full tank water heater can weigh several hundred pounds. In a quake, an unrestrained heater can walk, tip, and fall — and the consequences are exactly the ones you don't want during an emergency:Ruptured gas line at the heater, creating a fire or explosion hazard precisely when you can least respond to it.Broken water supply lines that flood the unit.Severed flue or venting on a combustion appliance, risking carbon monoxide.Injury to anyone nearby and a blocked egress path.What compliant strapping looks likeThe code requires the tank to be restrained against horizontal movement, typically with:Two metal strapping bands — one around the upper third of the tank and one around the lower third.Each band lag-bolted into wall studs or framing, not just screwed into drywall.Straps rated and listed for seismic restraint — not generic plumber's tape or pipe strap.The lower strap positioned to keep it clear of the controls but low enough to resist tipping.Best practice also includes a flexible (corrugated) connector on the water and gas lines so the connections can flex rather than snap if the tank shifts, and a proper drip pan with drainage where the heater sits over finished space.A note on heat-pump and tankless heatersTankless units are wall-mounted and bracketed to framing per the manufacturer, while heat-pump water heaters are tall and heavy and still need seismic anchoring. The two-strap rule is written for tank-type heaters, but every water heater needs appropriate, code-listed restraint.Where strapping fits in the bigger seismic pictureWater heater strapping is the most visible piece of a broader expectation that an ADU's mechanical equipment is restrained against earthquake movement. The same thinking drives requirements for anchoring tankless units to framing, restraining suspended ductwork, and securing equipment platforms. For a homeowner, the practical takeaway is simple: budget for proper seismic hardware up front, because adding it after the fact at final inspection means pulling the heater connections, drilling into finished walls, and re-testing the gas — far more disruptive than doing it once during rough-in.Strapping details and inspection practices can vary slightly by jurisdiction and with manufacturer instructions. Confirm with your local building department.Missing or inadequate strapping is one of the most common final-inspection corrections. Our plumbing plans note the water heater type, location, seismic strapping, and connection requirements so the installation passes the first time. Water-heating efficiency is coordinated with your mechanical plans and Title 24 report.

Question 9

What are the gas line requirements when an ADU uses gas appliances?

Accepted Answer

Where natural gas is permitted — meaning your city has not adopted an all-electric ordinance for new construction — gas piping in an ADU must be properly sized, valved, trapped, tested, and inspected before any wall is closed. Gas work is unforgiving, so the code requirements are specific and strictly enforced.Core gas piping requirementsProper sizing for aggregate demand. The piping must deliver the combined BTU/hr load of every gas appliance running simultaneously while maintaining adequate pressure (commonly about 7 inches water column for standard residential appliances). Sizing depends on the total connected load, the developed length of pipe, and the pipe material.A manual shutoff valve at each appliance, accessible and within sight of the unit.A sediment trap (drip leg) ahead of each appliance gas valve to catch moisture and debris before it reaches the burner controls.A pressure (leak) test before concealment — typically holding air or inert gas at around 30 psi for a minimum period (often 30 minutes or more) with no pressure drop.Inspection of all joints by the building department before the piping is covered.Common ADU gas appliancesThe usual gas loads in an ADU are a tankless or tank water heater, a furnace or wall heater, a range/cooktop, and occasionally a gas dryer. Each adds to the BTU total the line must carry. Note that as California pushes toward electrification, many homeowners now choose a heat-pump water heater and heat-pump HVAC instead, which can eliminate the gas line entirely — often the cheaper and faster path.The utility's roleThe gas meter and the service connection belong to your gas utility (SoCalGas, PG&E, Southwest Gas, etc.), not your plumber. Adding ADU gas load may require a meter upgrade or relocation, which means a separate utility application, its own timeline, and its own fees. Start that coordination early — utility lead times routinely outlast construction.A California Public Utilities Commission (CPUC) ruling eliminated certain gas line-extension subsidies for some mixed-fuel new construction. All-electric ordinances and gas-infrastructure cost rules vary by city and utility and continue to change — confirm current requirements with your building department and gas provider.Our plumbing plans show gas piping, sizing, shutoffs, and sediment traps where gas is used, and our mechanical and electrical sheets coordinate the all-electric alternative when that is the better route for your project.

Question 10

What are the most expensive plumbing surprises on ADU projects and how do you avoid them?

Accepted Answer

The plumbing line items that blow up ADU budgets are almost always the ones hidden underground or buried in a utility fee schedule. The good news: nearly every one is discoverable during early planning, before architecture is finalized and money is committed. Here are the big ones and how to get ahead of them.The costly surprisesSurpriseTypical costHow to get ahead of itSewer lateral replacement$5,000–$15,000Run a sewer camera inspection ($150–$350) before design. Replace a failing lateral while trenches are open.Sewer capacity study$1,000–$3,000Ask public works during early planning whether a capacity study is required for ADU connections in your area.Water service / meter upgrade$3,000–$8,000 + utility feesConfirm with your water district that the existing meter and service can carry the added ADU demand.Post-tension slab cutting$2,000–$5,000 extraIdentify slab type before assuming it can be cut. Look for "PT" markings and end plugs; get an engineer's sign-off.Utility connection feesHighly variableGet the water and sewer fee schedules in writing early. Some districts charge modest fees, others tens of thousands.Long hot-water runs / recirculation$800–$2,000Place the water heater centrally; a recirculation pump may be needed to meet CALGreen delivery limits.A pre-design investigation checklistCamera the sewer lateral and document its condition, material, and slope.Call public works and your sewer district about capacity studies, separate-service requirements, and connection fees — in writing.Verify water meter and service capacity with your water district for the added fixture load.Identify the garage/ADU slab type (standard vs. post-tension) before planning any slab cutting.Locate the water heater centrally so hot-water runs stay short and CALGreen-compliant.Check for an all-electric ordinance that could make gas-line work moot.Connection fees, capacity-study rules, and lateral requirements vary dramatically between cities and districts and can swing a budget by five figures. Confirm everything with your local building department, water district, and sewer authority during early planning.The pattern is simple: investigate underground and with the utilities before architecture is locked. Designing around what you find is cheap; discovering it mid-construction is not. Our plumbing plans document the connection method, slope, and water heater placement to keep your build predictable. See the cost of ADU MEP plans and our complete ADU MEP guide for the full picture, or see how it works to get started.

ADU Plumbing Guide

Related pages

Ready to get permit-ready MEP plans?