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Adams County, CO Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified July 2026

ADU rules in Adams County, CO, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Units allowed: One ADU per single-family home
Minimum size: 500 square feet
Max detached size: 1,500 square feet
Max height: 25 feet to roof mid-pitch
Owner-occupancy: Not required

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Colorado under recent ADU streamlining laws. The Adams County, CO rules below cover local size, height, and setback requirements.

Summary

Unincorporated Adams County permits one ADU per single-family home in any zone district. Minimum 500 sq ft; detached units cap at 1,500 sq ft, up to 25 ft tall. A building permit is required and the ADU cannot be sold separately.

City-specific rules exist: Westminster has its own adu rules rules that differ from Adams County's county-level regulations. If you live in Westminster, check the city-specific page instead.

The minimum size for any ADU is 500 square feet. Detached ADUs must be set back 10 feet behind the front structure line. ADUs can be up to 25 feet in height, measured to the roof mid-pitch.

Source: Adams County - Accessory Dwelling UnitsView official code

Full Breakdown

Adams County's Development Standards and Regulations (Sec. 4-03-03-02-01) allow one accessory dwelling unit accessory to a single-family home on any legal lot, in any zone district. The minimum ADU size is 500 square feet; an ADU may not exceed 40% of the home's floor area or 750 sq ft (whichever is greater), and a detached ADU tops out at 1,500 sq ft. Detached ADUs can rise to 25 feet measured to roof mid-pitch and must sit at least 10 feet behind the front structure line. There is no owner-occupancy requirement, but a building permit is mandatory (including for modular units) and the ADU stays legally accessory to the main home. Incorporated cities set their own ADU rules.

Violations & Fines

Building without the required permit triggers stop-work orders and zoning enforcement by Community & Economic Development; unpermitted units must be legalized or removed.

Frequently Asked Questions

Can I sell my ADU separately from the house?

No. An ADU stays legally accessory to the primary single-family home and cannot be sold or subdivided as its own property in unincorporated Adams County.

Do I need a building permit for an ADU?

Yes. All ADUs, including modular or prefabricated units, require a building permit from Adams County before construction or placement.

Sources & Official References

Other rules in Adams County

Parking Rules: Street Parking LimitsSome Restrictions Animal Ordinances: Dog Leash LawsSome Restrictions Shed & Carport RulesSome Restrictions Tiny HomesSome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSignificant Restrictions Fence Regulations: Fence Permit RequirementsSome Restrictions Animal Ordinances: Wildlife FeedingHeavy Restrictions Fire Regulations: FireworksHeavy Restrictions

All Adams County rules

How Adams County compares: Easiest Cities to Build an ADU·Compare Adams County to another location·View the Colorado accessory structures overview

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ADU Rules in Cities Across Adams County

Thornton, CO Westminster, CO