This page names the things that make one Boulder ADU cost more than another. It carries no prices, no ranges and no per-square-foot figures, because any number we published would be wrong for most readers and stale within a year. Official published fees appear here when we have verified them.
Why we refuse to estimate#
Two ADUs of the same size on two Boulder lots can differ enormously in cost, and the difference is usually site work, not the building. A number that averages across those cases tells you nothing useful about yours. Get a builder to price your actual site.
What actually drives the number#
Attached, detached or conversion. A detached unit is a new building with its own foundation, its own utility runs and its own roof. An internal conversion reuses all three. This is generally the largest single factor.
Size, within the caps. The city caps you at 800 sq ft for a detached unit and 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 for an attached one.
Site conditions. Slope, access for equipment, how far the unit sits from existing services, what is under the ground, and what has to be removed or protected.
Utility connections. A detached unit needs its own: Detached ADUs need an In-Property Wet Utility Permit.
Fire sprinklers, for detached units. Detached ADUs need automatic sprinklers on a dedicated line through a bypass meter; equivalent mist systems can be approved. A dedicated line through a bypass meter is a real piece of work, not a fixture.
The energy code. New homes and additions must be all-electric, with no residential exceptions, under the code effective December 1, 2024. Compliance is simpler for a smaller unit: ADUs of 1,000 sq ft or less may use the prescriptive path instead of an energy model, which avoids the cost of an energy model.
Design and engineering. Stamped plans, a survey, and structural engineering. More complex ground or a tighter envelope means more of all three.
Finishes. The one component you control most directly after the shell is up, and the one most likely to drift.
Fees the city sets#
These are not builder costs. The city sets them, publishes them, and changes them on its own schedule.
Permit fees. Being confirmed with City of Boulder.
Plant investment fees. Being confirmed with City of Boulder.
Rental license, if you will rent it. Being confirmed with City of Boulder.
Where a figure above reads "being confirmed", it means we have not yet confirmed it with the city and will not print a number we have not checked. Ask Planning and Development Services for a current quote against your scope.
The affordability trade#
The city allows a larger unit for an affordability commitment: Attached up to the lesser of two-thirds of the principal dwelling or 1,200 sq ft (750 sq ft if the principal is under 1,125 sq ft); detached up to 1,000 sq ft, in exchange for Rent capped at 75% of area median income, with a Declaration of Use recorded against the property. More building, constrained rent, recorded against the property. Take legal advice before committing.
Where to go next#
The permit path, the full City of Boulder rules, or paying for an ADU.