ADU Lot Size Requirements

Lot size is usually the first number a homeowner looks up, and it is the one most often misread. Two points before the table. Most minimums apply only to detached ADUs, because state law keeps cities from imposing much on an internal unit. And meeting the minimum only gets a lot through the first gate; setbacks, coverage and the size of the house decide whether a unit actually fits.

The state rules

Internal ADUs. Utah Code 10-21-303 bars cities from restricting total lot size for one internal ADU inside an owner-occupied home, with one exception: a city may prohibit an internal ADU if the lot is 6,000 square feet or less. That is the only lot size restriction the statute allows.

Detached ADUs. S.B. 284 (2026) creates Utah Code 10-21-304, which took effect October 1, 2026. It requires a specified municipality to permit a detached ADU on any lot of 11,000 square feet or larger that contains a single-family home, where the home is a permitted use. Cities may also allow detached units on smaller lots. "Specified municipality" is defined by city class and population, so check your own city's status rather than assuming it applies.

Minimum lot size by city

The county row covers unincorporated Salt Lake County only. Kearns, Magna, White City, Emigration Canyon, Copperton and Brighton have their own codes.

Why meeting the minimum is not the same as fitting

A lot can clear the minimum and still have no room for a unit. The usual reasons:

• Lot coverage.Millcreek's R-1 zones cap coverage between 25 and 50 percent depending on the zone. South Salt Lake limits an external ADU to 50 percent of the rear yard. Cottonwood Heights R-1 zones cap coverage at 50 percent including all structures.
• Rear yard depth.Where the unit must sit in the rear yard, a deep lot with a house set far back can have less usable space than a smaller lot with a shallow front setback.
• Setbacks.Side and rear setbacks come off every edge. On a narrow lot they can leave a strip too thin for a building. See ADU setback requirements.
• The house itself.Several cities cap a detached ADU by the house's size, so the lot is rarely the only limit. See ADU size.
• Easements.Millcreek and Sandy do not allow an ADU inside a recorded easement, which can take a slice out of the back of a lot.

Where lot size sets the size, not the eligibility

Holladay handles lot size differently. Since its November 2024 rewrite, the city says external ADUs can be added to any residential lot with a single-family detached home. Instead of a minimum, a chart sets the largest allowed footprint by lot size: 800 square feet on lots under 8,000 square feet, rising in steps to 1,400 square feet on lots over 80,600 square feet. The same chart sets a "no closer than" distance from property lines, from 3 feet on the smallest lots to 15 feet on the largest.

Cottonwood Heights sets no lot minimum specific to detached ADUs, but a detached unit is a conditional use that goes to a Planning Commission public hearing and must meet the zone's own lot size, setback and coverage rules.

Knowing your real lot size

Use the recorded lot area, not an estimate from an aerial photo. Fences often sit off the property line, and a few hundred square feet can decide a minimum. South Salt Lake's predevelopment checklist asks for a title report and ALTA survey issued within the last 30 days when a building permit is needed, which shows how seriously cities take the exact numbers.

What to do with the answer

If your lot clears the minimum, the next questions are placement, parking and utilities, in the order set out in Can I build an ADU? If it does not, an internal ADU or a basement ADU may still work. Our ADU feasibility review checks lot size against the current code for your address, together with everything else that decides fit. To start, See What Your Property Can Support.

Questions about ADU lot size