Tiny-home plumbing

Short definition

Tiny-home plumbing is the compact plumbing system in an Accessory Dwelling Unit (ADU), Detached ADU (DADU), or stand-alone tiny home. On a permanent foundation, it’s regulated like any small residential structure under WA-amended UPC. On wheels, it’s classified as an RV and follows different standards. Off-grid versions often pair rainwater catchment, greywater, and a composting toilet.

What it is

The plumbing for a tiny home or ADU has the same components as a full-size home — water source, water heater, fixtures, drain, vent — but in a compact footprint that drives different equipment choices.

The components in a typical permitted ADU:

1. Water source:
   – Municipal hookup (most ADUs in cities).
   – Shared well with the main house.
   – Dedicated well.
   – Rainwater catchment (rural).
2. Water heater:
   – Electric tankless (small-footprint, no gas line, often the simplest install).
   – Heat-pump water heater (rebate-eligible through PSE, SCL, or Snohomish PUD).
   – Gas tankless if propane is available.
3. Toilet:
   – Standard 1.28-gpf high-efficiency toilet (Saving Water Partnership rebate-eligible).
   – Composting toilet for off-grid setups (verify county approval).
   – RV-style cassette or marine for tiny-home-on-wheels.
4. Drain / sewer:
   – Tied to the main house’s side sewer (most common urban ADU).
   – Separate connection (rare).
   – Septic if rural.
   – Greywater diversion to landscape (Tier 1, ≤60 gpd).
5. Kitchen and bath fixtures:
   – Compact mixer faucets.
   – Single sink.
   – Low-flow showerhead.
   – Standard P-traps for fixtures.

Pressure considerations: ensure adequate flow to a single shower at 40+ psi.

Why it matters to a homeowner

WA’s HALA (Housing Affordability and Livability Agenda) push has expanded ADU permitting in Seattle and other cities, making backyard cottages and AADUs/DADUs much more common. The plumbing for a permitted ADU has to meet full residential code — UPC, WA-amended residential code, and city-specific permit and inspection requirements.

A few practical patterns:

Seattle backyard cottage (DADU) — connects to the main house’s side sewer; HPWH for rebate; HET; low-flow shower. Typical permitted plumbing scope: $5,000–$15,000.

Vashon Island ADU — shared well plus Tier 1 greywater plus composting toilet for water savings. Verify composting-toilet approval with King County Public Health.

Methow Valley off-grid tiny — rainwater catchment plus greywater plus composting toilet. Each component requires separate verification with the local jurisdiction.

Multi-generational AADU in Bellevue — standard residential plumbing; the AADU shares the main house’s water service and side sewer.

When a designer or contractor proposes an ADU, three plumbing decisions drive most of the cost: (1) the water-heater choice (tankless vs. HPWH vs. gas), (2) the side-sewer connection (tie to main house vs. separate), and (3) the toilet choice (HET vs. composting). The Saving Water Partnership rebate stack and PSE/SCL HPWH rebates make some choices significantly cheaper than they look on the equipment-only sticker.

Common failure modes

• Undersized water heater for shower duration — tankless winter inlet at 45°F PNW reduces output.
• Frozen pipes in poorly insulated tiny-home wall cavities — common in tiny homes on wheels left in cold storage.
• Compact P-traps prone to evaporation in unoccupied tiny home — same issue as cabin trap evaporation.
• ADU drain undersized — slow drains, especially with longer fixture runs.
• Greywater system not WAC-compliant — Tier 1 ≤60 gpd applies the same as full-size.
• Composting toilet maintenance issues — odor or regulatory problems if not properly maintained.

Common variants

• ADU (Accessory Dwelling Unit, attached or detached, on foundation) vs. DADU (Detached ADU) vs. AADU (Attached ADU).
• THOW (Tiny Home On Wheels, RV-classified) vs. permanent-foundation tiny home (residential code).
• Off-grid tiny (rural, well or rainwater) vs. urban ADU (municipal hookup).
• Tiny vs. small home — definitions vary; for plumbing, ≤500 sq ft is typical “tiny.”

Washington note

WA regulatory layers depending on classification:

Permanent-foundation ADU or tiny home: Full UPC plus WA-amended residential code applies — fixtures, gas, water heaters, side-sewer connection. WA L&I plumber licensing required for permanent fixtures.

Tiny home on wheels (THOW): Often classified as an RV or park trailer, following ANSI A119.5 (Park Trailer) or NFPA 1192 (RV) rather than residential code. Permits depend on jurisdiction; some cities don’t permit THOW as long-term housing.

Composting toilets: Approval pathway through county health departments under WAC 246-274 plus jurisdiction-specific rules. Verify before designing.

Greywater systems: Tier 1 (≤60 gpd) rules apply to ADU and tiny-home greywater the same as for full-size homes. The square footage is smaller, but the flow rate is what matters for tier classification.

Energy code: WA Clean Buildings Act and WSEC affect water-heater choice in new permanent dwellings — heat-pump water heaters are increasingly favored over gas for new construction.

For Seattle ADUs specifically, the HALA-era plumbing-permit and inspection process has been streamlined relative to past years; verify current 2026 rules at the city’s permit center.