Summit County, UT Accessory Structures: ADU Rules (2026)

Summit County is unusual because it is governed by THREE separate ADU regulatory frameworks depending on which side of the county the property sits in. (1) The Snyderville Basin Planning District covers the urbanized unincorporated west side of the county - the area around Park City including Kimball Junction, Pinebrook, Silver Springs, Jeremy Ranch, Silver Creek, Summit Park, and Promontory - and is administered under Title 10 of the Summit County Code (the Snyderville Basin Development Code). Section 10-8-5 ('Accessory Dwelling Units, Internal Accessory Dwelling Units and Live/Work Dwelling Units') sets the core rules. ADUs must be incidental to and on the same lot as a Single-Family Detached Dwelling Unit, or associated with an allowed, low-impact, or conditional Commercial Use in the Service Commercial Zone. The maximum floor area of an ADU is one thousand (1,000) square feet, measured from the inside face of the perimeter walls and including all wall partitions, mechanical rooms, hallways and stairwells. Only one ADU OR one Internal ADU may be located on a lot - a lot cannot contain both. The ADU may not be sold separately from the principal dwelling. ADUs may be created by conversion of an existing residence (in conjunction with a new primary single-family dwelling), within or attached to the principal dwelling, attached to or within a larger accessory structure (such as a barn or garage), or as a standalone detached structure on the same lot. Applicants must submit a floor plan and site plan showing location, size, layout and any proposed changes. If the lot is on a septic system, the applicant must demonstrate that the system can accommodate both the primary dwelling and the ADU without failure - a critical issue in the unsewered portions of the Basin. Section 10-2-10 lists ADUs as a Conditional Use within the Service Commercial Zone, and the County Planning Department recently approved a Conditional Use Permit for an ~880-square-foot ADU on the third floor of a commercial unit at Altitude Loft Condominiums under this provision. (2) The Eastern Summit County Planning District covers the rural unincorporated east side - around Coalville, Henefer, Echo, Oakley, Kamas, Francis, Peoa and Hoytsville - and is governed by the Eastern Summit County Development Code (a separate document published at summitcountyutah.gov). The Eastern code likewise allows ADUs incidental to a One-Family Dwelling Unit on the same lot, including conversions within the footprint, attachments to the primary or to an accessory structure (such as a barn), or stand-alone structures, and applies the same 1,000 square foot maximum floor area cap. The Rural Residential (RR) zone allows existing residential uses to continue and new single-family dwellings on legally platted lots, with ADUs subject to the same Eastern code criteria. (3) Park City Municipal Corporation (the incorporated city) is excluded from both county codes and operates under the Park City Land Management Code (LMC), Title 15 of the Park City Municipal Code. LMC Section 15-4-7 (Accessory Apartments) requires the apartment to be no less than 280 square feet and no more than 1,000 square feet, with no more than two bedrooms, with one parking space per bedroom (tandem allowed under specific conditions for driveways 25+ feet long, vegetation preservation, or with administrative permit in historic districts). Property owners must live on site and record a Notice to Purchaser (deed notice) with Summit County prohibiting rental of the accessory apartment for periods less than ninety (90) days - effectively banning short-term/nightly rentals in the accessory unit. Accessory Apartments are permitted in most Park City zones via administrative permit, with HOA notification required. LMC 15-4-7.1 (Internal Accessory Dwelling Units) allows one IADU on lots of 6,000 square feet or greater, requires the single-family dwelling to be occupied by the owner, prohibits separate sale of the IADU, prohibits rentals of either unit for less than thirty (30) days, and requires a recorded notice. Park City also regulates Guest Houses (LMC 15-4-8), which are only permitted on lots of one (1) acre or greater and prohibited entirely in the HRL, HR-2, HCB, ROS, POS, RCO, GC and LI Zoning Districts. (4) Statewide preemption: Utah Code Sections 10-21-303 (municipalities) and 17-80-303 (counties), enacted as H.B. 82 in 2021 and amended by subsequent bills, make IADUs a permitted use in any area zoned primarily for residential use - a single-family home with an internal accessory unit cannot be banned outright by either Summit County or Park City. The state law allows local restrictions including a 6,000 sf minimum lot size, design standards preserving the appearance of the primary dwelling, additional parking, permits/licenses, and prohibition of short-term (under 30 day) rental of the IADU. Local governments may exempt up to 25% of their primary-residential zoned area from the IADU requirement (or up to 67% for jurisdictions hosting a university with 10,000+ students - which does not apply to Summit County). The state law also exempts IADUs from impact fees and prohibits HOAs from banning compliant IADUs. Both the Snyderville Basin and Eastern Summit codes have been amended to incorporate these state IADU mandates, while the County retains discretion to require Conditional Use Permits and impose the 1,000 sf cap on detached/attached external ADUs that are not IADUs. Property tax 'primary residence' exemption rules and STR licensing are governed separately from the ADU permits.