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Can you drain shower and sink water onto your lawn in Idaho?
What Idaho means by grey water — and what it does not
Laundry-to-the-lawn is the dream the phrase sells. The state’s septic technical manual defines something narrower and meaner, and it starts with the purpose:
“A grey water system is used to distribute grey water in the root zone of landscaping. Grey water is domestic wastewater that consists of used water from bathtubs, showers, and sinks used only for hand washing.”
— DEQ Technical Guidance Manual, December 2025, §4.13.1 (section revision May 6, 2020). Retrieved 5 October 2026
Three sources. That is the whole list — bathtubs, showers, and hand-washing sinks. Other sources can be argued case by case only where the water never touches blackwaste or food products (a drinking fountain or ice machine are the manual’s own examples). And the exclusions are printed by name:
“Grey water does not include wastewater from toilets, kitchen sinks, water softeners, dishwashers, or clothes washing machines, or nondomestic wastewater sources.”
— DEQ Technical Guidance Manual, December 2025, §4.13.1. Retrieved 5 October 2026
Read that list twice before planning anything: the washing machine — the fixture everyone pictures — is excluded in Idaho. So is the kitchen sink, so is the softener’s brine, so is anything from a business. What is left is shower and lavatory water, and only enough of it to matter in a big household.
Who may build one
The section’s own header block assigns the credentials in two lines: installer registration permit — property owner, or standard and basic (complex if pressurized), and licensed professional engineer required — no (yes if pressurized). Gravity keeps the job in the ordinary lanes; the moment the system pressurizes, it needs a PE design and a complex installer — the same complex-system credential split this site explains for drainfields. A homeowner who stays on the gravity side of that line is in the same position as any owner installing their own standard system.
The arithmetic: how much water that leaves
The manual sizes grey water by people, not plumbing. Occupants come from the bedroom count — “Residences will be based on the first bedroom with two occupants and each bedroom thereafter with one occupant unless higher usage is proposed by the applicant.” — and each occupant is budgeted by fixture, per day:
| Fixture connected | Gallons per person per day |
|---|---|
| Shower/bath | 18 |
| Hand sinks (faucets) | 12 |
| Other sources | Case-by-case determination |
The manual’s own worked example: a three-bedroom house planned for four people, with showers and hand sinks connected, assumes 30 gallons per occupant — 120 gallons per day of grey water. That is the realistic ceiling of the whole idea for a cabin: two weekend showers do not irrigate anything, and a gravity mini-leachfield is deliberately small and shallow to match.
Where it may go — and where it may never go
Three of the section’s approval conditions do most of the excluding. First, the garden:
“Grey water may not be used to irrigate vegetable gardens.”
— DEQ Technical Guidance Manual, December 2025, §4.13.2. Retrieved 5 October 2026
Second, the surface:
“Grey water must not be applied on the land surface or be allowed to reach the land surface.”
— DEQ Technical Guidance Manual, December 2025, §4.13.2. Retrieved 5 October 2026
Root zone means root zone — subsurface, in the landscaping, with no ponding, no spray, no hose. The system also has to meet the same effective soil depths and separation distances as a standard system — the separation tables apply. And third, the condition that ends most fantasies:
“Because the grey water system is intended for irrigation and not year-round disposal, the subsurface sewage disposal system must be designed for all wastewater flows including both grey water and black water wastes.”
— DEQ Technical Guidance Manual, December 2025, §4.13.2. Retrieved 5 October 2026
A grey water system buys irrigation, not capacity. The septic system must still be sized for every drop the house produces — grey and black — because in a frozen Idaho winter the irrigation side is dormant and everything flows to the tank. The septic side is still sized the way it always was.
What the system actually needs
The manual’s component list is longer than a barrel and a hose: a separate plumbing system keeping approved grey water sources apart from everything else — with the separate design approved by the Idaho Division of Building Safety, Plumbing Program; a dosing chamber or tank with surge capacity to absorb large drain flows; a filter against the particles that clog irrigation emitters; a pump where gravity will not do; and a drip irrigation field or a mini-leachfield to do the distributing. The overflow rule seals the design: the dosing chamber must overflow to the subsurface sewage disposal system, not to the yard.
The cabin version of the idea
- Inventory the fixtures honestly — if the plan assumed laundry water, the plan assumed water Idaho excludes.
- Run the occupant math first: bedrooms to occupants, occupants to gallons. If the number is small, the irrigation is decorative.
- Stay gravity-side unless a pressurized design is worth a PE and a complex installer.
- Size the septic system for all flows anyway — the overflow and the winter both depend on it.
- Set expectations against the calendar with (208) 656-1467 before touching plumbing.
Common questions
What counts as grey water in Idaho?
Domestic wastewater from bathtubs, showers, and sinks used only for hand washing. Other sources may be considered case by case where the water never contacts blackwaste or food products - a drinking fountain or ice machine are the manual's own examples.
Can a washing machine drain into a grey water system in Idaho?
No. The rule excludes wastewater from clothes washing machines by name, along with toilets, kitchen sinks, water softeners and dishwashers. The laundry-to-the-lawn version does not exist under this rule.
Who can install a grey water system?
A property owner, or a standard and basic installer - but a pressurized grey water system is complex, requiring a complex installer and a design by a professional engineer licensed in Idaho.
How much grey water does a house produce?
By the manual's table, 18 gallons per person per day from showers and baths and 12 from hand sinks. A three-bedroom house planned for four people with both connected works out to 30 gallons per occupant - 120 gallons per day.
Where may grey water be applied?
Only beneath the surface, in the root zone of landscaping - never on the land surface, never allowed to reach it, and never on vegetable gardens. The system must also meet the same soil depth and separation distances as a standard septic system.
Does a grey water system let you downsize the septic system?
No. Because the grey water system is intended for irrigation and not year-round disposal, the septic system must be designed for all wastewater flows, grey and black, and the grey water tank's overflow must route back to it.