An accessory dwelling unit is a second, smaller home on a lot that already has one. It has its own kitchen, its own bathroom and its own entrance, and it is legally tied to the main house rather than being a separate property.
That last part matters more than most people expect. An ADU is an accessory to the main home. In the City of Boulder, for instance, The ADU cannot be sold separately from the main home.
The four forms an ADU takes#
You will see these four words throughout both rulebooks, and the rules differ by form, so it is worth knowing which one you are describing.
Internal. A separate unit carved out inside the existing house, most often a basement with its own entrance. Nothing about the building's footprint changes.
Attached. An addition that shares at least one wall with the main house. The footprint grows, so setbacks, coverage and floor area all come into play.
Detached. A standalone building elsewhere on the lot: the backyard cottage people usually picture. This form carries the most requirements, because it is a new building.
Conversion. An existing structure that becomes a dwelling: a garage, a workshop, a barn. Whether this counts as attached or detached depends on whether the structure touches the house.
The City of Boulder sets different size limits for attached and detached units. An attached ADU is capped at The lesser of half the principal dwelling’s floor area or 1,000 sq ft; 750 sq ft if the principal dwelling is under 1,500 sq ft, while a detached one is capped at 800 sq ft.
Words you will meet in the code#
Setback. The required distance between a building and a property line. Every zone has its own.
Floor area ratio. The total floor area of everything on the lot, divided by the lot's area. Most zones cap it, which is why a large main house can leave no room for an ADU even on a big lot.
Building coverage. How much of the lot the buildings sit on, seen from above. Different from floor area ratio, and capped separately.
Bulk plane. An invisible sloping surface rising from the side property line. A building has to stay under it, which is what stops a tall wall right on the boundary.
By right. Allowed without discretionary approval, as long as you meet the written standards. The opposite is a review where someone can say no.
Owner occupancy. A requirement that the owner live in one of the two units. The City of Boulder dropped this: Not required since March 8, 2025.
Which rulebook applies to you#
Boulder County has two, and they differ sharply. Inside city limits, the City of Boulder's code governs and the state ADU law applies. Outside every city and town, the county's Land Use Code governs, and No. HB24-1152 does not cover this land, so the county Land Use Code governs.
What an ADU is not#
It is not a separate lot, and you cannot sell it on its own. It is not an RV or a tiny house on wheels: in unincorporated Boulder County, RVs and travel trailers are not dwellings, and camping is limited to 14 days a year, and All structures must sit on permanent foundations. No wheels, no trailers.
It is also not automatically a rental. Renting one brings its own rules, which differ by jurisdiction and by how long you rent it for.
Where to go next#
Read the rules for your jurisdiction: the City of Boulder or unincorporated Boulder County. If you are weighing how the unit gets built, factory-built vs site-built covers the certification differences. If you want the statewide picture, Colorado's ADU laws explains who HB24-1152 covers.