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

Some Restrictions

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

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

Key Facts

ADUs per lot (A-1): One, plus principal dwelling
Detached ADU, central water: One-acre minimum lot
Detached ADU, on a well: Two-acre minimum lot
Second ADU: Only on 35+ acres
Governing code: DCZR Sections 303, 306, 325

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 Douglas County, CO rules below cover local size, height, and setback requirements.

Summary

Unincorporated Douglas County allows one accessory dwelling unit (ADU) per lot in the A-1 Agricultural district. A detached ADU needs at least one acre on central water, or two acres on a well. A second ADU is allowed only on parcels of 35 acres or more.

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

Accessory Dwelling Unit (ADU) – one per lot, except as restricted by a Rural Site Plan, subdivision plat, or other similar land use approval. A second ADU may be allowed subject to Section 325, Second Accessory Dwelling Unit Standards.

View official code

Full Breakdown

The Douglas County Zoning Resolution (Section 303.01) lists an ADU as a permitted accessory use in the A-1 district, one per lot, once a principal dwelling exists. Section 306.01 requires a one-acre minimum lot for a detached ADU on central water; 306.02 requires two acres if served by an individual well. A second ADU is possible only on 35+ acres, subject to Section 325 standards and Board approval. Incorporated towns (Castle Rock, Parker, Lone Tree) set their own ADU rules. Colorado's 2024 ADU law (HB24-1152) also requires many metro-area jurisdictions to permit one ADU where single-family homes are allowed.

Violations & Fines

Building and using an ADU without required zoning approval and a building permit is a zoning violation subject to county enforcement, stop-work orders, and abatement.

Frequently Asked Questions

Can I put an ADU on my Highlands Ranch lot?

Highlands Ranch is unincorporated but zoned Planned Development; ADU allowance depends on your PD guide and HOA. Check with Douglas County Planning and your metro district before building.

Does a detached ADU have a minimum lot size?

Yes. In A-1, a detached ADU requires at least one acre on central water or two acres if served by an individual groundwater well.

Sources & Official References

Other rules in Douglas County

Noise Ordinances: Construction HoursSome Restrictions Shed & Carport RulesSome Restrictions Garage ConversionsSome Restrictions Tiny HomesSome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Noise Ordinances: Aircraft NoiseSignificant Restrictions Noise Ordinances: Decibel LimitsSome Restrictions Parking Rules: Loading ZonesSome Restrictions Fence Regulations: Material RestrictionsSignificant Restrictions

All Douglas County rules

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

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ADU Rules in Cities Across Douglas County

Castle Rock, CO