Parking Lot Layout Design
In Chicago Suburbs, IL

How many accessible spaces your lot needs

This is the first question worth answering on any layout, because it is the one with a published number attached and the one most likely to be wrong on an older lot.

The federal count comes from section 208.2 of the 2010 ADA Standards for Accessible Design, and it scales with the total number of spaces in the lot:

Total spaces in lot	Minimum accessible spaces
1 to 25	1
26 to 50	2
51 to 75	3
76 to 100	4
101 to 150	5
151 to 200	6
201 to 300	7
301 to 400	8
401 to 500	9
501 to 1,000	2 percent of the total
1,001 and over	20, plus 1 for each 100 or fraction over 1,000

Two rules ride along with that table. Under section 208.2.4, at least one of every six accessible spaces — or fraction of six — must be van accessible, which means a lot with a single accessible space needs that space to be the van one. And under section 208.3.1, accessible spaces must sit on the shortest accessible route from parking to an accessible entrance. A lot can hold the right number of accessible spaces and still fail, if they are grouped at the far end of the building because that is where there was room to paint them.

When the table binds. The count above is not a standing charge against every existing lot. The 2010 Standards attach to new construction and to alterations — 28 CFR 36.402 defines an alteration as a change that “affects or could affect the usability” of the facility, which is what a new layout does to a parking lot — and the section requires the altered portion to be made accessible “to the maximum extent feasible.” Running alongside that, 28 CFR 36.304(a) obliges a public accommodation to remove barriers in an existing facility where doing so is “readily achievable,” meaning without much difficulty or expense. Elements untouched on or after March 15, 2012 that met the 1991 Standards sit under a safe harbor. So the practical position for a layout project is that the table above is the standard the new layout is measured against.

In Illinois there is a second layer. The Illinois Accessibility Code, adopted by the Capital Development Board under the Environmental Barriers Act, applies on top of the federal standard and sets the state minimum. It is a floor, not a ceiling — a governmental unit may enact stricter requirements. So the practical answer for a lot in DuPage, Cook, Kane, Will, or Kendall County is that the layout has to satisfy the federal count, the state code, and whatever the village has adopted, and the binding number is whichever of those is highest.

Village zoning is where the rest of the geometry usually comes from. Stall dimensions, minimum aisle widths, required parking ratios for a given use, landscape islands, and setbacks are set by each municipality’s own zoning ordinance across the twenty-one towns we serve. There is no single suburban standard, and a number that is correct in Naperville is not automatically correct in Schaumburg. We check the ordinance for the town the property sits in rather than assuming.

Stall geometry and drive-aisle circulation

Once the accessible count is settled, layout design becomes a question of fitting the most usable parking into the space you have without making the lot unpleasant to drive.

Stall angle. Ninety-degree stalls hold the most vehicles per square foot and allow two-way aisles, which is why they dominate retail and office lots. Angled stalls at sixty or forty-five degrees are easier to enter and exit, need a narrower aisle, and force one-way circulation — a good trade on a long narrow site or where turnover is high and drivers are unfamiliar. The choice drives everything downstream, so it is made first.

Drive aisles. The aisle has to serve the stall angle it faces and stay wide enough for the vehicles that actually use the lot, including delivery trucks and, on many suburban properties, a fire apparatus access route that has its own minimum width from the municipality’s adopted fire code. Aisles that dead-end need somewhere to turn around; aisles that carry the main entry flow need enough length between the street and the first conflict point that arriving drivers are not making decisions in the right-of-way.

Circulation and conflict points. A good layout minimizes the number of places where a driver looking for a space meets a driver leaving, and where either meets someone on foot. That means putting the pedestrian route somewhere other than the main drive aisle where the site allows it, marking crosswalks at the points people already walk rather than where it is convenient to paint, and keeping the accessible route clear of the path vehicles reverse into.

Loading, waste, and fire access. Dumpster enclosures, loading zones, and the fire apparatus access route all consume lot area and all have to be reachable while the lot is full. They are easier to place at layout time than to retrofit after the stalls are painted.

Capacity versus usability. It is nearly always possible to add a few more stalls by tightening dimensions. Whether you should is a different question — a lot squeezed to its theoretical maximum is a lot where doors get dinged, drivers give up on the tight rows, and the marking gets scuffed out within a season. We will show you both counts and let you decide.

We design layouts for retail centers, office parks, industrial and distribution buildings, medical offices, schools, churches, and multifamily and association properties across the suburbs, including Naperville, Wheaton, Elmhurst, Lombard, Glen Ellyn, Carol Stream, Addison, Aurora, and Bolingbrook.