Property feasibility
ADU Size: Limits and Layout
In several Salt Lake County cities, the house you already own sets the size of the ADU you can add. Here is each city's cap, what counts toward it, and how the number shapes the plan.
Size limits are where two homeowners with identical lots can get very different answers. One city caps a detached unit at a flat 1,000 square feet. The next caps it at half the house. A third caps the footprint by lot size. State law adds its own layer, and internal ADUs follow different rules again.
The state layer
For an internal ADU, Utah Code 10-21-303 bars cities from restricting its size in relation to the house. The state sets no number of its own. For detached ADUs, Utah Code 10-21-304 took effect October 1, 2026; it lets cities regulate size, height and lot coverage, and lets them prohibit a detached unit larger than the house.
Size and height caps by city
| City | Detached ADU size | Detached height | Internal or attached | Source |
|---|---|---|---|---|
| Millcreek | The lesser of the house's gross square footage or 1,000 sq ft | The lesser of 24 ft or the house's height | No size cap in the ADU chapter; zone coverage and height apply | Table 18.71-2; 18.71.080 |
| Salt Lake City | 1,000 sq ft gross floor area | 17 ft; up to 24 ft pitched or 20 ft flat with larger setbacks | No maximum floor area for internal ADUs | 21A.40.200.F, J |
| Murray | Less than 50 percent of the house and no more than 1,000 sq ft; the whole structure no more than 1,000 sq ft | One story, the lesser of 20 ft or the house's height | No maximum floor area for attached ADUs | 17.78.050; 17.78.040 |
| South Salt Lake | The lesser of 50 percent of the house (not counting the garage) or 1,000 sq ft; 2 bedrooms maximum | The lesser of the house's height or 20 ft; 2 stories maximum | No maximum size for internal ADUs | 17.07.070 |
| Holladay | Footprint of 200 sq ft minimum; maximum from 800 sq ft (lots under 8,000 sq ft) to 1,400 sq ft (lots over 80,600 sq ft) | 25 ft, plus a graduated height rule | Confirm with the city | 13.14.031; Chart 13.14.101 |
| Sandy | The smallest of: 2,000 sq ft gross; a footprint of 75 percent of the house's footprint; or the accessory structure limit (750 sq ft or 25 percent of the rear yard on lots up to 14,999 sq ft) | 15 ft, or 1.5 stories and 18 ft and no taller than the house | No cap stated in the ADU section; confirm | 21-11-1 |
| Cottonwood Heights | No ADU-specific cap; accessory buildings may cover no more than 25 percent of the rear yard and, outside rural residential zones, 70 percent of the house's floor area | 20 ft in R-1 zones | Must be within the existing footprint | 19.75.050; 19.76.030 |
| Unincorporated county | No maximum floor area set | 20 ft where a side or rear setback is under 10 ft; otherwise the house's height | No maximum floor area set | County Code 19.15 |
Sandy's figures come from Ord. 26-55, adopted September 15, 2026, which takes effect on publication of a summary. Holladay's chart is its accessory building chart, and it may count other accessory buildings on the lot; the same is true of Sandy's accessory structure limit. Ask the city how an existing garage or shed is counted.
What counts toward the number
Two caps with the same number can produce different buildings, depending on what is measured:
- Floor area or footprint. Salt Lake City, Murray and South Salt Lake cap floor area. Holladay caps footprint. Sandy caps both, and allows basements.
- Exclusions. Salt Lake City does not count stairs and landings to a second-level ADU, loft space with a ceiling under 7 feet, other accessory uses in the same building, or a storage-only basement with no internal connection to the ADU.
- Covered patios. Salt Lake City and South Salt Lake allow a covered patio roof of up to 120 square feet that does not count toward the ADU's size but does count toward lot coverage.
- The house's measurement. Millcreek measures the house's gross square footage; South Salt Lake excludes the garage; Sandy compares footprints.
How size drives the layout
A small unit still needs a full kitchen, a full bathroom, a water heater, a heating system and, in most cases, laundry. Those take roughly the same room in any unit, so the smaller the cap, the larger the share of the plan they consume. A few patterns follow:
- Caps tied to the house.On a modest house in Millcreek, Murray or South Salt Lake, the house's size may set the ADU below 1,000 square feet. The house's actual area is worth measuring before any plan is drawn.
- One-story limits.Murray's one-story rule and low height caps elsewhere push the unit to spread out, which uses more of the yard and lot coverage.
- Footprint caps.Where only the footprint is capped, a second level or basement can add space, within the height and total area limits.
- Stairs.A two-story plan gives up floor area on both levels to the stair, and Salt Lake City's stair exclusion is one reason a second level can make sense there.
Our page on ADU floor plans walks through the plan types that fit different sizes.
Choosing the right size
The largest allowed unit is not always the right one. A bigger unit costs more to build and heat, may cross a parking threshold (Sandy requires a second space for a detached unit at 650 square feet or more), and uses more of the yard. Start from who will live there, then check the cap. See in-law ADUs and multigenerational ADUs for household-specific needs.
Next steps
Measure the house, find your zone, and work through Can I build an ADU? Our ADU feasibility review calculates the cap that actually applies on your lot, including the house, coverage and height limits together. To start, See What Your Property Can Support.
Questions about ADU size
What is the maximum size for an ADU in Utah?
There is no single statewide number. State law stops cities from limiting an internal ADU's size in relation to the house. For detached ADUs, each city sets its own cap: 1,000 square feet is common among the cities we cover, but Millcreek, Murray and South Salt Lake can set a lower cap on a smaller house, and Sandy and Holladay use footprint-based limits.
Can an ADU be bigger than the main house?
Usually not for a detached unit. Millcreek, Murray and South Salt Lake cap a detached ADU below or at the house's size, and Sandy caps its footprint at 75 percent of the house's footprint. Since October 1, 2026, Utah Code 10-21-304 lets cities prohibit a detached ADU larger than the house.
Do a loft or basement count toward the size limit?
It depends on the city's wording. Salt Lake City does not count loft space with a ceiling under 7 feet, stairs to a second-level ADU, or a basement used only for storage with no internal connection to the ADU. Sandy caps footprint separately from total area and allows basements. Ask how your city measures before finalizing a design.
Is there a minimum ADU size?
Holladay sets a minimum external ADU footprint of 200 square feet. Elsewhere, building code requirements for rooms, ceilings, egress and bathrooms set the practical minimum, so ask your building department.