The Illinois residential stair code follows the International Residential Code (IRC), which as of January 1, 2025 serves as the minimum standard for residential structural design in every jurisdiction across the state. Local building departments can enforce stricter rules, but they cannot fall below the IRC baseline. Before you cut a single stringer, confirm which edition of the IRC your municipality has adopted and whether any local amendments apply.
Why the IRC Now Applies Statewide
Illinois historically left building codes to local governments. Public Act 103-0510 changed that in two phases.1Illinois General Assembly. Public Act 103-0510 As of January 1, 2024, new residential construction in any jurisdiction without its own building code must comply with the current edition of the IRC.2Capital Development Board. Building Codes and Regulations As of January 1, 2025, any municipality or county that adopts or amends a building code must make its structural requirements at least as stringent as the IRC.
Chicago is the notable structural exception. The city enforces the Chicago Building Code rather than adopting the IRC directly, though its stair requirements largely parallel the IRC provisions below.3American Legal Publishing Corporation. Municipal Code of Chicago – Building Code If you are building in the city, work from Chicago’s code directly.
Riser Height, Tread Depth, and Uniformity
Every step must fit a narrow dimensional range. The maximum riser height is 7¾ inches.4International Code Council. 2018 International Residential Code – R311.7.5.1 Risers The minimum tread depth is 10 inches, measured horizontally from the leading edge of one tread to the leading edge of the next.5International Code Council. 2018 International Residential Code – R311.7.5.2 Treads
Consistency within the flight matters just as much. The tallest riser cannot exceed the shortest by more than ⅜ of an inch, and the same ⅜-inch tolerance applies to tread depths.5International Code Council. 2018 International Residential Code – R311.7.5.2 Treads Your stride locks onto the rhythm of a staircase after a step or two, so a single off-size riser is the classic cause of a fall. Uniformity is one of the most common reasons a staircase fails inspection.
Nosings
On stairs with solid risers, the nosing (the rounded lip projecting past the riser below) must stick out between ¾ inch and 1¼ inches. The radius at the nosing tip cannot exceed 9/16 of an inch, and any bevel is limited to ½ inch. Nosing projections also follow the ⅜-inch uniformity rule across the flight.
If your treads are at least 11 inches deep, no nosing projection is required. The extra tread depth already gives each step a stable landing surface.
Width and Headroom
A residential stairway needs a minimum clear width of 36 inches above the handrail.6International Code Council. 2021 International Residential Code – R311.7.1 Width At and below handrail height, a handrail may project into the required width by up to 4½ inches on each side, leaving a clear width of at least 31½ inches with one handrail or 27 inches with handrails on both sides.7International Code Council. International Residential Code – R311.7.1 Width These dimensions are sized so furniture, appliances, and stretchers can pass through, and so two people can meet without turning sideways.
Headroom throughout the stairway must be at least 6 feet 8 inches, measured vertically from a sloped line connecting the tread nosings to any ceiling or obstruction above.8International Code Council. 2018 International Residential Code – R311.7.2 Headroom On landings and platforms, measure straight up from the floor. A single limited exception lets a floor opening edge project up to 4¾ inches horizontally into the required headroom where tread nosings extend under that opening at the side of a flight.
Headroom shortfalls are a common surprise in older homes, especially where a staircase passes under a floor joist or ductwork. Fixing it usually means either raising the opening above or lowering the landing below, both structural changes, so catch this measurement early.
Landings
A landing or floor is required at the top and bottom of every stairway. The landing must be at least as wide as the stair it serves, and for a straight-run staircase, the depth in the direction of travel must be at least 36 inches.9International Code Council. International Residential Code – R311.7.6 Landings for Stairways One exception: no landing is required at the top of an interior flight if no door swings over the stairs.
If a door does open onto a landing, the door in its fully open position cannot reduce the required egress width by more than 7 inches. Homeowners who add a door to an existing stairway often miss this detail.
Handrails and Guards
Any portion of an open-sided stairway more than 30 inches above the floor or ground below needs a guard. Guards on the open sides of stairs must be at least 34 inches high, measured vertically from the line connecting the tread nosings. Where the top of the guard also serves as the handrail, it must sit between 34 and 38 inches.
Openings in a guard cannot allow a 4-inch sphere to pass through, which keeps small children from slipping between balusters.10International Code Council. 2021 International Residential Code – R312.1.3 Opening Limitations The triangular opening formed where a tread, riser, and the bottom rail of a guard meet uses a separate 6-inch sphere test, because that geometry is harder for a child to pass through.
Handrail Shape and Continuity
Handrails must run continuously from directly above the top riser to directly above the bottom riser. At each end, the rail must return to a wall, continue into a guard surface, or terminate at a post. An exposed end that juts out catches clothing, bags, and belt loops.
A round handrail needs an outside diameter between 1¼ and 2 inches so an average hand can close around it. Non-circular profiles are allowed if the perimeter falls between 4 and 6¼ inches and the cross section is no more than 2¼ inches at its widest point. Decorative flat-topped rails that cannot be gripped firmly will fail inspection, no matter how good they look.
Spiral and Winder Stairs
Spiral and winder designs follow tighter rules because the pie-shaped treads create uneven footing.
Spirals
A spiral staircase needs a clear width of at least 26 inches at and below the handrail. Each tread needs a minimum depth of 6¾ inches at the walkline, and every tread in the spiral must be identical. The maximum riser height is 9½ inches, taller than the 7¾-inch limit on conventional stairs. Headroom drops to 6 feet 6 inches.11International Code Council. International Residential Code – R311.7.10.1 Spiral Stairways
Winders
Winders are fan-shaped treads used where a staircase turns without a landing. Tread depth must be at least 10 inches at the walkline, measured 12 inches in from the narrow side of the tread. At the narrowest point within the clear width of the stair, the tread cannot be less than 6 inches deep.12International Code Council. 2021 International Residential Code – R311.7.5.2.1 Winder Treads Winders follow the same riser height and uniformity rules as conventional stairs.
Enclosed Space Under the Stairs
If the space beneath a stairway is accessible through a door or access panel, the walls, the underside of the stair surface, and any soffits on the enclosed side must be covered with ½-inch gypsum board.13International Code Council. International Residential Code – R302.7 Under-Stair Protection Standard drywall satisfies this; fire-rated board is not required. Leave the space open and unenclosed and the requirement doesn’t apply, which is why many builders use the area as an open nook rather than a closet.
Lighting and Switches
Every interior stairway needs an artificial light source illuminating the treads and landings to at least 1 foot-candle, measured at the center of each tread and landing.14International Code Council. 2018 International Residential Code – R303.7 Interior Stairway Illumination The code sets a minimum light level, not a fixture type, so recessed cans, sconces, and overhead fixtures all qualify.
Stairways with six or more risers need a wall switch at each floor level the stairs serve.14International Code Council. 2018 International Residential Code – R303.7 Interior Stairway Illumination Shorter interior flights of fewer than six risers can use a single switch, a remote switch, or an automatic or continuously operating light source.
Exterior stairways need a light at the top landing. If the stair provides access to a basement from grade level, a second light is required at the bottom landing.15International Code Council. International Residential Code – R303.8 Exterior Stairway Illumination Exterior stairs serving deck exits or upper-story entries only need the top-landing fixture.