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Basement Measurement Formula

To determine how much of your basement will count toward your floor area, you must study pages 36 and 37 of the current proposed draft ordinance, particularly section 9-8-2 (e)(1)(D)(i) and figure 9-16 (see new ordinance language). This will instruct you as to how to measure.

Once you have your measurement, the formula is essentially

(PAG / P) * BSF

where P is measured perimeter of exterior walls, PAG is the measured perimeter of exterior walls exposed 3 ft above the adjacent finished grade, and BSF is your entire basement's square footage.

Definitions from City Code

"Building coverage" means the maximum horizontal area within the outer perimeter of the building walls, dividers, or columns at ground level or above, whichever is the greater area, including without limitation, courts and exterior stairways, but excluding:

1. Uncovered decks, stoops, patios, terraces, and stairways all less than thirty inches high;
2. The outer four feet of completely open, uncovered, cantilevered balconies that have a minimum of eight vertical feet clearance below;
3. Up to three hundred square feet of a single-family detached residence front porch that is adjacent to a street; and
4. Up to one hundred fifty square feet of additional covered porch area not located in the front yard for [a] single-family detached residence front porch; and
5. One accessory building, no larger than eight square feet size and no taller than ten feet in height, associated with a single-family detached residence

"Floor area" means the total square footage of all levels measured to the outside surface of the exterior framing, or to the outside surface of the exterior walls if there is no exterior framing, of a building or portion thereof, which includes stairways, elevators, the portions of all exterior elevated above grade corridors, balconies, and walkways that are required for primary or secondary egress by chapter 10-5, "Building Code," B.R.C. 1981, storage and mechanical rooms, whether internal or external to the structure, but excluding an atrium on the interior of a building where no floor exists, a courtyard, the stairway opening at the uppermost floor of a building, and floor area that meets the definition of uninhabitable space. FairFAR note: There are two definitions given for "floor area" in the city code. This appears to be the one that applies. There is also a newly added exemption for historic accessory buildings in the new ordinance.

"Floor area ratio (FAR)" means the ratio of the floor area of a building to the area of the lot on which the building is situated.

"Uninhabitable space" means a room or portion thereof that is six feet or less in floor to ceiling height, or a room solely used to house mechanical or electrical equipment that serves the building, including, without limitation, heating, cooling, electrical, ventilation and filtration systems, or any parking facility located completely below grade on all sides of the structure regardless of the topography of the site (see definition of "floor area"). FairFAR note: This definition would receive the minor change indicated in italics above.

Other than as indicated, these definitions come straight from the city code. There is also a revision to the definition of non-standard building in the currently proposed draft that brings its definition into sync with the new wall articulation and bulk plane requirements.

HELP! How can I determine what numbers go in each box?

Answer: Well, you can't really determine them without hiring expensive surveyors to do it for you as a part of your building plan. But we have some useful sources of this information. However, be aware they may be out of date or just plain wrong.

• The Boulder County Assessor's "Property Records Search" should give you the size of your home in sq ft, describe your basement size and any detached buildings. Unfortunately, it doesn't list your lot size.
• The Boulder County Assessor's "E-Mapping Tool" should give you the size of your home in sq ft, your basement size and describe any detached buildings AND give you your approximate lot size, but they are estimating your lot size from their GIS data and it is highly inaccurate, so the lot size estimates fluctuate (and don't necessarily agree with the City's estimates). Note you must click the report button to find your lot size.
• The City of Boulder's "Property Information Tool" should give you your zoning district, the size of your home in sq ft, your basement size and describe any detached buildings AND give you your approximate lot size, but they are estimating your lot size from their GIS data and it is highly inaccurate, so the lot size estimates fluctuate (and don't necessarily agree with the County's estimates).
• A better source is the appraisal and the land survey the bank insisted you have done as part of financing the purchase of your home. This is likely to be your most accurate source of information short of contacting a professional to do a new on-site survey.

Then there is the matter of how the City's new and existing regulations define what counts for terms like building coverage and floor area. You'll have to read both the existing regulations (particularly here and here) and the new ordinance language carefully to make sure you've found all the details that count--we were shocked, for example, to see that the concrete steps up to the front stoop of a split-level home on a flat lot appear to count toward building coverage, should those front stoops be more than 30 inches above the driveway. We've tried to cue you in to the commonest of the unexpected things that count in our instructions, but we didn't write these regulations.