ADUs in unincorporated Boulder County
Restrictive today, and under review. The state ADU law does not reach unincorporated county land, and the county is rewriting its accessory dwelling rules.
Unincorporated Boulder County is the land outside any city or town. The county’s Land Use Code governs it, the state ADU law does not apply, and a rewrite of the accessory dwelling rules is underway.
Last reviewed September 18, 2026
The short version
What you need to know first.
Whether the state ADU law applies
No. HB24-1152 does not cover this land, so the county Land Use Code governs
Review for an additional dwelling
Adding any dwelling to a parcel that already has a single-family home requires Limited Impact Special Use Review
When Site Plan Review applies
On vacant parcels, for cumulative floor-area increases over 1,000 sq ft since September 8, 1998, and above 125% of the neighborhood median residential floor area
Water and wastewater
Every dwelling needs water service or a permitted well, plus sewer service or an onsite wastewater system
Foundations
All structures must sit on permanent foundations. No wheels, no trailers
Rules under review
Docket DC-26-0002 is reconsidering the accessory dwelling rules, including broader allowances
Every rule
The rules, grouped.
Each row links the official page it came from and the date we last read it. Where a rule is still being confirmed, we say so rather than guess.
Where and how
| Rule | What applies | Source |
|---|---|---|
| Whether the state ADU law applies | No. HB24-1152 does not cover this land, so the county Land Use Code governsCounty land is covered only inside a census-designated place of 40,000 or more people. Status: verified. | State of Colorado: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| What the county regulates | The county has no land use jurisdiction inside incorporated cities and towns Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| Common zoning districts | Most unincorporated mountain land is zoned Forestry (F); most plains land is Agricultural (A) Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| Where accessory dwellings are allowed today | Being confirmed with Boulder CountyBeing confirmed Status: being confirmed. | Boulder County: DC-22-0003: Marshall Fire accessory dwellings (opens in a new tab)Reviewed September 18, 2026 |
| Review for an additional dwelling | Adding any dwelling to a parcel that already has a single-family home requires Limited Impact Special Use Review Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| When Site Plan Review applies | On vacant parcels, for cumulative floor-area increases over 1,000 sq ft since September 8, 1998, and above 125% of the neighborhood median residential floor area Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
Size and shape
| Rule | What applies | Source |
|---|---|---|
| Foundations | All structures must sit on permanent foundations. No wheels, no trailers Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| RVs and travel trailers | RVs and travel trailers are not dwellings, and camping is limited to 14 days a year Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| Marshall Fire disaster recovery units | Up to 900 sq ft, expedited review, standalone or inside another structure, with no short-term or vacation rental Status: verified. | Boulder County: DC-22-0003: Marshall Fire accessory dwellings (opens in a new tab)Reviewed September 18, 2026 |
| When the recovery-unit allowance ends | Being confirmed with Boulder CountyBeing confirmed Status: being confirmed. | Boulder County: DC-22-0003: Marshall Fire accessory dwellings (opens in a new tab)Reviewed September 18, 2026 |
Utilities and safety
| Rule | What applies | Source |
|---|---|---|
| Water and wastewater | Every dwelling needs water service or a permitted well, plus sewer service or an onsite wastewater system Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| Septic change-in-use review | Adding an ADU triggers a change-in-use review: an engineer’s report with at least two test pits (which may be waived), flow calculations, a capacity check and a certified NAWT inspection Status: verified. | Boulder County: OWTS Change in Use Policy (opens in a new tab)Reviewed September 18, 2026 |
| Septic tank requirements | Tanks must meet current size and retention rules; a single-compartment tank needs a second tank in series Status: verified. | Boulder County: OWTS Change in Use Policy (opens in a new tab)Reviewed September 18, 2026 |
| What most household wells permit | Most wells drilled since May 8, 1972 on parcels under 35 acres are permitted for household use only, inside the home Status: verified. | Colorado Water Law and How to Garden Anyway: Colorado Water Law and How to Garden Anyway (opens in a new tab)Reviewed September 18, 2026 |
| Whether a household-use-only well can serve an ADU | Being confirmed with State of ColoradoBeing confirmed Status: being confirmed. | State of Colorado: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| BuildSmart and wildfire mitigation | BuildSmart and wildfire mitigation requirements apply to tiny and modular dwellings, including sprinkler systems Status: verified. | Boulder County: Planning Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| Building code in force | Permit applications from July 1, 2026 use the 2021 I-Codes plus county amendments for energy, sustainability and ignition resistance Status: verified. | Boulder County: Building Code Amendments (opens in a new tab)Reviewed September 18, 2026 |
| Wildfire zone construction requirements | Being confirmed with Boulder CountyBeing confirmed Status: being confirmed. | Boulder County: Building Code Amendments (opens in a new tab)Reviewed September 18, 2026 |
Other
| Rule | What applies | Source |
|---|---|---|
| HOA restrictions | Being confirmed with State of ColoradoBeing confirmed Status: being confirmed. | State of Colorado: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Rules under review | Docket DC-26-0002 is reconsidering the accessory dwelling rules, including broader allowances Status: verified. | Boulder County: DC-26-0002: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
Docket DC-26-0002
Accessory Dwelling Units Land Use Code update
Boulder County is reconsidering the accessory dwelling rules in its Land Use Code, including broader allowances. No draft code had been posted on the docket page when we last checked.
Commissioners authorize the rewrite
March 24, 2026
Completed.Commissioners’ work session on scope
May 5, 2026
Completed.Code development and community engagement
May–September 2026 (est.)
In progress now. This date is an estimate.Planning Commission public hearing
October 2026 (est.)
Upcoming. This date is an estimate.Commissioners’ public hearing
November 2026 (est.)
Upcoming. This date is an estimate.Adoption and effective date
Date not set
Upcoming.
Docket DC-26-0002. Last checked September 18, 2026.
Follow the rewrite
- Read the docket page for the current status, meeting notices and any posted documents.
- Public hearings before the Planning Commission and the Board of County Commissioners take public comment.
- Boulder County Community Planning and Permitting publishes how and when to submit written comment for each hearing.
We describe the process and link the documents. We take no position on the outcome.
Documents
- DC-26-0002: Accessory Dwelling UnitsBoulder County
It depends
What no page can answer for you.
These are the variables that decide what you can actually build. They differ lot by lot, and Boulder County decides them, not us.
Go deeper
Wells, septic, wildfire and access
The rural constraints that apply in unincorporated Boulder County whatever the rewrite says.
What changed recently
July 1, 2026
County building code moves to the 2021 I-Codes
Permit applications from this date use the 2021 I-Codes plus county amendments for energy, sustainability and ignition resistance.
March 24, 2026
Commissioners authorize the ADU rewrite, docket DC-26-0002
The Board of County Commissioners authorized a Land Use Code update reconsidering the accessory dwelling rules.
Not your jurisdiction? See the City of Boulder rules or work out which applies to you.
Sources
- Accessory Dwelling UnitsState of Colorado
- HB24-1152: Accessory Dwelling UnitsState of Colorado
- Planning Frequently Asked QuestionsBoulder County
- DC-22-0003: Marshall Fire accessory dwellingsBoulder County
- OWTS Change in Use PolicyBoulder County
- Colorado Water Law and How to Garden AnywayOther
- Building Code AmendmentsBoulder County
- DC-26-0002: Accessory Dwelling UnitsBoulder County
Last reviewed September 18, 2026.
This page summarizes public rules for general information. It isn’t legal advice or a permit determination. Confirm requirements for your property with the Boulder County before you design or build.
Get Unincorporated County updates
We email when a rule changes in unincorporated Boulder County, with the source. Nothing else.