How Boulder Handles Accessory Structures: A Practical Guide

Boulder maintains 143 local ordinances across all categories, and 5 of those deal specifically with accessory structures. Here is a breakdown of what the city actually requires, what is prohibited, and where Boulder falls on the strict-to-permissive spectrum compared to other cities.

ADU Rules

Boulder caps a detached accessory dwelling unit at 800 square feet, or 1,000 square feet if affordable, and an attached ADU at half the principal home's floor area or 1,000 square feet, whichever is less. Every rented ADU needs a city rental license, and short-term rental of an ADU is banned outright.

Key details: Detached ADU max: 800 sq. ft. (1,000 if affordable). Attached ADU max: Half of principal floor area or 1,000 sq. ft.. Small-home floor: 750 sq. ft. minimum cap for small principal homes. Rental license: Required under Chapter 10-3 to rent either unit. Short-term rental: Prohibited for ADUs except as authorized in § 10-3-19.

Renting an accessory dwelling unit or its paired principal dwelling without the Chapter 10-3 rental license, or offering either unit as a short-term rental in violation of Subsection 9-6-3(n)(1)(A)(iv), is enforced as a code violation subject to the Section 5-2-4 general penalty of up to $2,650 per violation, and independently conveying an ADU apart from the principal dwelling violates Subsection (n)(1)(A)(v).

ADU Impact Fees

Accessory dwelling units built to the standards of Boulder Revised Code section 9-6-3 are exempt from the city's affordable housing capital facility impact fee under section 8-9-3(c)(6). That is separate from the smaller exemption for additions of 500 square feet or less, and it applies specifically to the floor area that makes up the ADU itself.

Key details: Exempt fee: Affordable housing capital facility impact fee only. Basis for exemption: ADU built to section 9-6-3 standards. Separate small-addition exemption: 500 sq ft or less, once per dwelling unit. Small single-unit exemption: Whole dwelling 2,000 sq ft or less. Fee still due for: Parks, human services, municipal, police, fire, transportation.

Because paragraph (c)(6) is an exemption rather than a prohibition, there's no separate ADU-specific penalty; the general impact fee enforcement applies instead. An owner who fails to pay a capital facility impact fee that is actually due can face the fee being required before a certificate of occupancy or final building inspection is scheduled, and disputes over whether a project qualifies for the ADU exemption go through a Chapter 1-3 hearing rather than municipal court.

Boulder is more permissive than most cities when it comes to adu impact fees. That said, there are still limits.

Tiny Homes

Boulder allows site-built tiny houses as single dwelling units under Appendix BB of its Residential Building Code, BRC 10-5.5-2(jj). The catch: a tiny house must sit on a permanent foundation and connect permanently to utilities like any other dwelling, so a tiny house on wheels doesn't qualify as a code-compliant dwelling under this appendix. Off-site built tiny houses follow a separate state manufacturing statute instead.

Key details: Governing section: BRC § 10-5.5-2(jj), Appendix BB. Foundation rule: Permanent foundation required. Utility connection: Permanent utility connection required. Wheeled units: Not covered as a code dwelling. Off-site built units: Follow C.R.S. § 24-32-3311(6).

Constructing or occupying a tiny house that doesn't meet Appendix BB's permanent-foundation and utility-connection standard, or that skips the required building permit and inspections under Chapter 10-5.5, is a code violation prosecuted the same as any unpermitted residential construction: civil penalties starting at $150 for a first offense, rising to $1,000 for a third, plus possible criminal prosecution and a stop-work order until the structure is brought into compliance or removed.

Shed & Carport Rules

In Boulder's RR, RE, RL, and RMX-1 residential zones, sheds and other accessory buildings sitting in the rear yard setback area are capped at a combined 500 square feet of building coverage under Section 9-7-8(a), B.R.C. 1981, unless the property is an individual landmark or in a historic district.

Key details: Coverage cap: 500 sq ft combined, rear-yard accessory structures. Applies in: RR, RE, RL, RMX-1 zoning districts. Landmark/historic exception: One added structure, up to 500 sq ft more. Inside building envelope: No coverage limit applies there. Max fine: $2,650 per violation (Sec. 5-2-4).

A shed or accessory structure that exceeds the 500-square-foot coverage cap, or that is built without the required setback compliance, is a code violation subject to Boulder's general penalty under Section 5-2-4, B.R.C. 1981: a fine of up to $2,650 per violation, up to ninety days in jail, or both, with each day of noncompliance counted separately under Section 5-2-5(b).

ADU Rental Restrictions

Boulder requires a rental license before anyone occupies an accessory dwelling unit (ADU) or the paired principal dwelling as a paying tenant, bars short-term renting of either unit on an ADU lot unless separately licensed, and forbids selling the ADU apart from the main house. Each rule sits in Subparagraph 9-6-3(n)(1)(A) of the land use code.

Key details: Rental license required: Before renting ADU or principal dwelling to anyone. Rental license fee: $190 per accessory dwelling unit. Short-term rental: Prohibited unless licensed under Section 10-3-19. Independent sale: Never allowed to convey ADU separately from house. Max fine: $2,650 per violation under general penalty.

Renting an unlicensed ADU or principal dwelling, running an unauthorized short-term rental, or selling an ADU apart from the house is a code violation prosecuted in Boulder Municipal Court under the general penalty in Section 5-2-4: a fine of up to $2,650 per violation, up to ninety days in jail, or both, with every day of continued violation counted as a separate offense under Section 5-2-5(b).

This is not one of those rules that cities tend to ignore. Boulder actively enforces its adu rental restrictions requirements.

The Bottom Line

Boulder's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Boulder is broadly strict or permissive.

These rules come from Boulder's publicly available municipal code. For complete penalty schedules, exemption details, and answers to common questions, see the individual ordinance pages throughout this guide.