Property feasibility
ADU Sewer and Water
Water can go uphill under pressure. Sewage cannot. That one fact explains why the sewer line so often decides where a detached ADU sits and what it costs.
Of all the lines an ADU needs, sewer is usually the hardest. Most residential sewer runs by gravity, so the new drain has to fall steadily from the unit to the house's lateral, the private pipe that carries the house's wastewater to the public main. Water runs under pressure and is more forgiving, but it still has to be sized so two households get enough flow. Both are handled by providers that vary from street to street in this part of the valley.
The sewer line: fall, distance and condition
Three things decide whether a sewer connection is simple:
- Fall.How far below the unit's drains the lateral sits. A unit on low ground at the back of a lot may sit too low to drain by gravity, which means a pump or a different location.
- Distance.A longer run is a longer trench, more pipe and more cleanouts, the access points used to clear a line.
- Condition.An old lateral may need repair or replacement before it can carry a second home's flow. A camera inspection is the usual way to find out.
Salt Lake City's ADU handbook calls drain grade and slope a major up-front issue. Its guidance, echoed in South Salt Lake's handbook, is that the existing sewer and water service may be used for the ADU, but site conditions and the age and condition of the existing services may require additional work or separate connections.
Shared or separate laterals
Whether the ADU can share the house's lateral is the provider's call. Sandy Suburban Improvement District's design standards say that "each dwelling unit shall be served by an individual lateral," that stacked units may be served otherwise with district approval, and that shared laterals need a joint maintenance agreement recorded with the County Recorder. The same section requires a separate lateral where the ADU is on a separate legal parcel or has a separate address. Ask the district how that applies to an ADU on your lot before the design is fixed.
Who provides sewer
Sewer service in this part of the county is split among special districts and cities, and the city name does not always tell you which one serves a lot:
- Millcreek: the city's utility directory lists Mt. Olympus Improvement District.
- Holladay: the city lists Mt. Olympus Improvement District and Cottonwood Improvement District.
- South Salt Lake: the city runs its own wastewater system; residents south of 3300 South are served by Mt. Olympus Improvement District.
- Murray: Murray City's wastewater system serves over 9,500 connections and sends flow to Central Valley Water Reclamation Facility.
- Sandy: the city lists four sewer providers: Midvalley Improvement District, Sandy Suburban Improvement District, Cottonwood Sewer Improvement and South Valley Sewer District.
- Cottonwood Heights: Cottonwood Improvement District says its service area includes portions of the city.
- Salt Lake City: Salt Lake City Public Utilities provides sewer along with water and stormwater.
Mt. Olympus's own sewer availability letter form lists projects in Millcreek, South Salt Lake, Holladay and Murray. Confirm the provider for your exact address before assuming any fee or rule.
Mt. Olympus Improvement District's ADU charge
Mt. Olympus is one of the few providers that has published how it treats an ADU. Its Consolidated Rates and Fees Schedule for 2026, adopted December 16, 2025, has a line for "Property with additional Living Unit (ADU, Guest home, etc.)" with the entry "+1 RE & 1 Cap Fee." In plain terms, the property pays for one more residential equivalent each month and one more capacity fee:
| Charge | 2026 amount |
|---|---|
| Added residential equivalent (monthly sewer rate) | $30.00 per month |
| Added capacity fee (one time) | $758.00 |
The district sets fees each year, so confirm current amounts with Mt. Olympus before budgeting. Most other districts and cities have not published an ADU-specific rule, so ask the provider for your address how it counts an ADU.
Who provides water
Water is just as patchy. Salt Lake City Public Utilities serves Salt Lake City and customers in Cottonwood Heights, Holladay, Millcreek and parts of unincorporated county. Jordan Valley Water Conservancy District provides retail service in portions of Holladay, Midvale, Cottonwood Heights, Murray, Riverton, Millcreek, Sandy and South Salt Lake. Millcreek and Holladay also list Holliday Water Company. Murray runs its own municipal water utility. South Salt Lake's Water Division serves residents from 2100 South to 3300 South, with Jordan Valley Water serving south of 3300 South. Millcreek notes simply that it is served by several water companies and points residents to its map.
Letters, septic and the permit
In unincorporated Salt Lake County, an ADU application needs written verification from the water and sewer providers that service is available, and the building permit waits for the water supplier's written confirmation. Homes on septic need Health Department verification that the system meets regulations and is performing properly. State law lets cities prohibit an internal ADU where the house is served by a failing septic tank, and Sandy requires proof that the system works properly or a connection to sewer.
Before you design
Find where your lateral leaves the house and which way the lot falls. That, more than the floor plan, sets where a detached unit can go. The other utilities are covered in ADU utility connections, and the full order of questions is in Can I build an ADU? Our ADU feasibility review checks the sewer route and the providers for your address before design starts. To start, See What Your Property Can Support.
Questions about ADU sewer and water
Does an ADU need its own sewer lateral?
Not always. The Salt Lake City and South Salt Lake ADU handbooks say the existing sewer and water service may be used, although site conditions or the condition of the old lines may require separate connections. Sandy Suburban Improvement District's design standards say each dwelling unit shall be served by an individual lateral, with exceptions it approves. The district that serves your address decides.
What does Mt. Olympus Improvement District charge for an ADU?
Its 2026 fee schedule, adopted December 16, 2025, treats a property with an additional living unit such as an ADU as one more residential equivalent plus one capacity fee. For 2026 that is $30 a month and a $758 capacity fee. Fees are set yearly and can change, so confirm with the district.
How do I find out who provides my water?
Ask your city or check its utility map by address. In Millcreek, Holladay, Cottonwood Heights and South Salt Lake, more than one water provider serves different parts of the city, so the city name alone does not tell you.
Can I add an ADU to a home on a septic system?
Possibly, but expect extra steps. State law lets cities prohibit an internal ADU where the house is served by a failing septic tank. Sandy requires a professional's proof that the system is in proper working condition, or a connection to sewer. In unincorporated Salt Lake County, the Health Department must verify the system.