Cache County · 15,018 residents
Smithfield ADU rules
Smithfield: internal ADUs allowed; detached ADUs allowed. Owner occupancy applies.
Internal ADU (inside the home)
Detached ADU (backyard building)
✓ Checked against city sources on September 24, 2026 See sources
Smithfield ADU rules at a glance
| Rule | Internal ADU (basement, attic, inside the home) | Detached ADU (backyard building) |
|---|---|---|
| Allowed? | Allowed | Allowed |
| Minimum lot | Not yet verified — confirm with the city | Smithfield's code does not set a separate minimum lot size for ADUs; a detached ADU is allowed in the A, RA and R-1 zones. |
| Owner occupancy | The owner must live in one of the units, except for documented temporary absences of up to three years (such as a job assignment, sabbatical, volunteer service, or a stay in a care facility). | The owner must live in one of the units, except for documented temporary absences of up to three years. |
| Parking | At least one dedicated, hard-surfaced off-street space per ADU. | At least one dedicated, hard-surfaced off-street space per ADU; a detached ADU over 650 sq ft needs two. |
| Size limits | No larger than 75% of the original home or 1,500 livable sq ft, whichever is less. A home may have up to one attached and one detached ADU. | No larger than 75% of the original home or 1,500 livable sq ft, whichever is less. A home may have up to one attached and one detached ADU. |
| Height | Not yet verified — confirm with the city | Must meet the height limits of the zone. |
| Setbacks | Any new construction or remodel for an ADU must meet the zone's setbacks. | Must meet the zone's setbacks, and every wall of a detached ADU must be at least 10 ft from any wall of the main house. |
| Zones | Allowed in the A, RA and R-1 zones, and in detached single-family homes in the MPC (Master Planned Community) zone. | Allowed in the A, RA and R-1 zones (not the MPC zone, where only internal ADUs are allowed). |
| Permit process | Apply to the zoning administrator (Accessory Apartment Application) with the fee from the city's fee schedule; the zoning administrator decides within 14 days. A certificate of occupancy is required before anyone moves in. | Apply to the zoning administrator with a new-construction zoning clearance and the fee from the city's fee schedule; the zoning administrator decides within 14 days. A detached ADU needs its own water and sewer connections, and a certificate of occupancy is required before occupancy. It cannot be sold separately from the main home. |
What the Smithfield council decided
Recorded votes and actions on ADU rules, linked to the public meeting record on OpenUtah.
· City Council
Approved Ord. 2025-11 amending 17.92.020 to allow ADUs up to 75% of the original home's size (5-0).
Read the meeting record
State law: detached ADUs from October 1, 2026
Smithfield is a “specified municipality” under Utah S.B. 284. Beginning October 1, 2026, it must permit one detached ADU as a permitted use on a lot of 11,000 sq ft or more that has a detached single-family home. The city can still apply setbacks, height, design, owner-occupancy and limited parking rules, and it may allow detached ADUs on smaller lots. How the state ADU laws work.
How our property map screens Smithfield lots
The map colors parcels using the rules on this page and the recorded lot size. A color is a planning screen, not a permit; Smithfield makes the decision.
- No numeric external minimum is recorded for Smithfield. The map keeps the statewide 11,000 sq ft external screen and shows the researched status (generally allowed) on the property report.
- "Likely" above 15,000 sq ft is this site's planning band for a large recorded lot. It is not a sentence in the Smithfield code. Owner occupancy, parking, setbacks, zones, and HOA rules below still have to be checked.
Planned communities, PRDs and PUDs
Reviewed on September 22, 2026 against this city's research file, its published ADU page where one was found, and Utah meeting packets. No adopted sentence was confirmed that allows or bans an ADU in every PUD, PRD, PRUD, or planned community. If the assessor class or subdivision name says PUD, PRD, PRUD, planned unit development, planned residential development, planned community, resort community, or master planned community, the map keeps that lot highly unlikely regardless of size. A plat, CC&Rs, or development agreement can still prohibit an ADU where the zone allows one.
Sources
We link only to government sources and public meeting records. Codes change; confirm the current version with Smithfield before you apply.
- U.S. Census Bureau — Vintage 2024 population estimates, Utah incorporated places
July 1, 2024 population estimates for the city and Cache County (145,487).
- Utah Code 17-60-104 — Classification of counties
A county with 40,000 to 259,999 people is a county of the third class; Cache County (145,487) is third class.
- Utah Code 10-2-301 — Classification of municipalities
City class population thresholds.
- Utah S.B. 284 (2026) enrolled — Utah Code 10-21-304 detached ADUs
Specified municipalities must permit a detached ADU on lots of 11,000 sq ft or more with a single-family home, effective Oct 1, 2026; caps detached-ADU parking at 1 space (<650 sq ft) or 2 spaces (650+ sq ft).
- Smithfield City — Forms & Permits
Links the Accessory Apartment Application and zoning clearance forms.
- Smithfield City — Accessory Apartment Application (updated 06/01/2026)
Owner-signed application; inspections by Cache County Building Department and Smithfield Fire. Its checklist still lists 3 parking spaces, which is stricter than the current code (1 per ADU).
- Smithfield City Council minutes — May 14, 2025
Ordinance 2025-11 (ADU size up to 75% of the home) approved 5-0.
- Official Smithfield website
Common Smithfield ADU questions
Can you build an ADU in Smithfield?
Smithfield: internal ADUs allowed; detached ADUs allowed. Owner occupancy applies. This is planning guidance, not a permit. Confirm the zone on your parcel with Smithfield.
What lot size does Smithfield require for an ADU?
Internal ADU: Not yet verified — confirm with the city. Detached ADU: Smithfield's code does not set a separate minimum lot size for ADUs; a detached ADU is allowed in the A, RA and R-1 zones.
Does Smithfield require the owner to live on the property?
Internal ADU: The owner must live in one of the units, except for documented temporary absences of up to three years (such as a job assignment, sabbatical, volunteer service, or a stay in a care facility). Detached ADU: The owner must live in one of the units, except for documented temporary absences of up to three years.
How much parking does a Smithfield ADU need?
Internal ADU: At least one dedicated, hard-surfaced off-street space per ADU. Detached ADU: At least one dedicated, hard-surfaced off-street space per ADU; a detached ADU over 650 sq ft needs two.
What does it cost to build an ADU in Smithfield?
Wasatch Front planning bands are often about $55,000–$145,000 for an internal suite, $70,000–$175,000 for a garage conversion, and $95,000–$250,000 or more for a new detached unit, before unusual site work. These are planning ranges, not bids.
Can an HOA block an ADU in Smithfield?
City approval does not override CC&Rs or plat notes. Read the HOA documents and your subdivision plat before you design. A private restriction can stop a unit the city would otherwise allow.
Before you design: general checks for any Utah ADU
General planning advice, not Smithfield code.
- Confirm the zoning district on the city zoning map. The county assessor’s “residential” class is not zoning.
- Read your HOA CC&Rs and subdivision plat notes; private restrictions can block a unit the city allows.
- Sketch existing and proposed parking. Driveway geometry and access often decide what fits.
- Ask early about sewer, water and electrical capacity; utilities can force a redesign.
- Expanding a building’s footprint usually triggers full setback and height rules.
Nearby city ADU rules
Want to know what fits on your Smithfield lot?
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Utah ADU Builder provides educational planning guidance. This page is not legal advice and not a permit. Population: U.S. Census Bureau population estimate (2024). Editorial standards.