Boulder County’s B80 guide details the requirements for calculating residential floor area in unincorporated areas for site plan review. Based on the Boulder County Land Use Code, the 2021 International Residential Code, and the 2021 International Building Code, these guidelines specify which attached and detached spaces, such as garages and home offices, are included in the total floor area calculation, while outlining exemptions for structures like gazebos and agricultural barns
Boulder County Planning Publications
Determining Floor Area - B80
Determining Floor Area
Floor Area is measured in terms of square feet. The total square footage is everything within the exterior face of the exterior walls including garages and basements. See Figure 1: Floor Plan Examples below.
Figure 1: Floor Plan Examples
Residential vs. Non-Residential Floor Area
Residential Floor Area (defined in 18-189D of the Land Use Code) includes all attached and detached Floor Area (as defined in Article 18-162 of the Land Use Code) on a parcel, including principal and accessory structures used or customarily used for residential purposes, such as garages, studios, pool houses, home offices and workshops. Gazebos and carports up to a total combined size of 400 square feet are exempt. Barns used for agricultural purposes are not considered residential floor area. Covered porches that are attached to a residence and are not enclosed do not count as Residential Floor Area.
Note: If an existing wall(s) and/or roof(s) are removed and a new wall(s)/roof(s) are constructed, the associated floor area due to the new wall(s)/roof(s) are considered new construction and must be included in the calculation of floor area for Site Plan Review.
Definitions
Area, Carport or Roof Projection Cover
Carport area or a horizontal projection of roof or floor above, where a cover is provided with open sides below the cover, the area is measured to the drip edge of the roof projection on the open sides of the structure and to the outside wall sheathing of any shared exterior walls.
Covered area open on any side on a detached accessory structure.
Area, Floor
The area of the building, existing or new, under consideration including basements and attached garages calculated without deduction for corridors, stairways, closets, the thickness of interior walls, columns, or other features as measured from the exterior face of the exterior walls. This includes framed wall braced and unbraced wall sheathing.
Basement
That portion of a building that is partially or completely below grade (see story above grade plane). An under-floor space below the first story of the building that does not meet the definition of story above grade plane and has a ceiling height measured from the basement floor to the bottom of the floor joists above of 6 feet 8 inches or more.
Crawl Space
An under floor space below the first story floor of the building that does not meet the definition of story above grade plane, that has a ceiling height measured from the crawlspace grade or floor to the bottom of the floor joists above of less than 5-feet, and that does not contain interior stairs, fixed-in-place ladder systems, windows, wall and ceiling finish materials, trim or finished flooring.
Exception:
In newly constructed areas, an area of under floor space below the first story, not larger than 200 square feet, may have a ceiling height as measured from the crawlspace grade or floor to the bottom of the joists above of less than 6-feet 8-inches for the installation of mechanical equipment and automatic fire sprinkler installations. This area shall not contain interior stairs, fixed-in-place ladder systems, windows, wall and ceiling finish materials, trim or finished flooring.
Covered Porches
Covered porches that are attached to a residence and are not enclosed do not count as Residential Floor Area.
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