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

Some Restrictions

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

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

Key Facts

Number allowed: One ADU per property
Where: All ag & residential zones
Min lot (detached): 7,500 sq ft
Occupancy: Max 3 people; owner must live on site
Permit: Building permit 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 Jefferson County, CO rules below cover local size, height, and setback requirements.

Summary

Jeffco permits one accessory dwelling unit (ADU) on a single-family detached lot in every agricultural and residential zone. Owner must occupy one unit, max 3 occupants, and a building permit is required. Colorado's HB24-1152 now mandates ADU allowances in metro jurisdictions.

City-specific rules exist: Arvada, and Lakewood have their own adu rules rules that differ from Jefferson County's county-level regulations. If you live in one of those cities, check the city-specific page instead.

Planning & Zoning may permit an accessory dwelling unit (ADU) in all agricultural and residential zone districts, including comparable Planned Development zone districts. The accessory dwelling unit may only be allowed as an accessory use to a single family detached residence.

Source: Jefferson County - Accessory Dwelling UnitsView official code

Full Breakdown

Under Section 19.D of the Jefferson County Zoning Resolution, Planning & Zoning may permit one ADU (attached or detached) accessory to a single-family detached residence in all agricultural and residential zone districts. Detached ADUs need at least 7,500 sf of lot; on lots of 1 acre or more the ADU may be 1,200 sf or 40% of the primary unit, whichever is less (smaller caps on smaller lots). The owner must occupy the primary or accessory unit as their primary residence. Colorado HB24-1152 (effective June 30, 2025) separately requires many metro-area jurisdictions to allow one ADU wherever single-unit detached dwellings are permitted, removing owner-occupancy and extra-parking barriers.

Violations & Fines

Building an unpermitted second dwelling is a zoning violation enforced under C.R.S. 30-28-124; the County can require removal and pursue civil penalties. A recorded notice ties the ADU permit to the property.

Frequently Asked Questions

Can I rent out my ADU?

Yes, but the property owner must occupy either the primary home or the ADU as their primary residence, and the ADU cannot be sold separately from the main dwelling.

Does Colorado now force counties to allow ADUs?

HB24-1152 (effective June 30, 2025) requires many metro-area jurisdictions to allow one ADU where single-family homes are permitted; Jeffco already allows ADUs county-wide in unincorporated areas.

Sources & Official References

Other rules in Jefferson County

Noise Ordinances: Quiet HoursSome Restrictions Noise Ordinances: Construction HoursSome Restrictions Shed & Carport RulesSome Restrictions Garage ConversionsSome Restrictions Tiny HomesSome Restrictions Noise Ordinances: Leaf Blower RulesSome Restrictions Noise Ordinances: Barking DogsSome Restrictions Noise Ordinances: Vehicle NoiseSome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions

All Jefferson County rules

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

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ADU Rules in Cities Across Jefferson County

Lakewood, CO Arvada, CO