Unincorporated Boulder County is rewriting its accessory dwelling rules. This page is about the constraints that sit underneath that rewrite and will not move with it: water, wastewater, wildfire and access.
Those four are physical and legal facts about your parcel. A code change can widen where an accessory dwelling is allowed. It cannot create water rights, enlarge a septic field or move your property out of a wildfire zone.
Water#
Every dwelling needs a supply: Every dwelling needs water service or a permitted well, plus sewer service or an onsite wastewater system.
On a well, the permit is the thing to read, not the well. Colorado well permits are specific about use, and most rural residential permits are narrow: Most wells drilled since May 8, 1972 on parcels under 35 acres are permitted for household use only, inside the home.
Note the last three words of that rule. Whether a permit written for household use inside the home can also serve a second dwelling is precisely the question, and we will not guess at it: Being confirmed with State of Colorado.
Septic#
Adding a dwelling changes the load on an onsite wastewater system, and the county reviews that directly: 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.
That is a real engineering exercise with a real chance of failing. An older system sized for one house may simply not have the capacity, and the tank itself has to meet current standards: Tanks must meet current size and retention rules; a single-compartment tank needs a second tank in series.
Expect this to be the step that decides whether a rural ADU is feasible at all.
Wildfire and building standards#
Construction standards apply county-wide, and include ignition resistance: BuildSmart and wildfire mitigation requirements apply to tiny and modular dwellings, including sprinkler systems.
The code in force is Permit applications from July 1, 2026 use the 2021 I-Codes plus county amendments for energy, sustainability and ignition resistance.
Requirements vary by zone, and we are still recording them: Being confirmed with Boulder County.
What counts as a dwelling#
Two rules rule out a large share of what people propose. All structures must sit on permanent foundations. No wheels, no trailers. And RVs and travel trailers are not dwellings, and camping is limited to 14 days a year.
So a tiny house on wheels parked on the land is not a solution here, however long it stays.
Review paths today#
Adding a dwelling to an occupied parcel is not administrative: Adding any dwelling to a parcel that already has a single-family home requires Limited Impact Special Use Review.
Floor area triggers a second review: 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.
Where accessory dwellings are permitted today is one of the things the rewrite is examining, and the current categories are something we are confirming against the code text: Being confirmed with Boulder County.
Marshall Fire recovery units#
A separate allowance exists for disaster recovery: Up to 900 sq ft, expedited review, standalone or inside another structure, with no short-term or vacation rental. Its end date is something we are confirming: Being confirmed with Boulder County.
Access#
Emergency vehicles have to reach the building. Driveway width, grade, turning space and surface all come into it, and on mountain parcels this is frequently the constraint that decides where a unit can go. Requirements are set per parcel, so ask Boulder County Community Planning and Permitting about yours.
Where to go next#
The unincorporated Boulder County rules and rewrite tracker, or why the state ADU law does not reach this land.