Boulder, CO Fence Regulations: Retaining Walls (2026)

2, 'Work exempt from permit,' and item 3 of that list specifically addresses retaining walls: they need no permit if they are not over three feet in height measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge or impounds Class I, II, or III-A flammable liquids. Anything over that three-foot threshold, or a shorter wall carrying a surcharge load or flammable liquid, needs a full building permit reviewed under the adopted 2024 International Building Code.

Boulder's land use code adds a separate height allowance where a fence sits on top of a retaining wall: Subparagraph 9-9-15(c)(2)(A) lets a fence or guard up to forty-two inches be placed on or within three feet of a retaining wall regardless of the combined fence and retaining wall height, as long as both are on the same property, an exception confirmed in Table 7-1 of Section 9-7-1, which lists the minimum height of a fence on top of a retaining wall as 42 inches citywide.

A retaining wall placed in the public right-of-way is treated differently still: under Section 8-6-8(e), 'Exempt Encroachments,' a right-of-way retaining wall needs no separate encroachment permit only if it is less than eighteen inches in height and does not require a subgrade foundation, a much stricter threshold than the three-foot building permit exemption that applies on private property.