By Steven Vance with help from Spencer Blaney, R.A.
This blog post is meant to be a general discussion about the new allowances in the updated Chicago building code. It does not take the place of an architect's code analysis for a specific project with real building and site design parameters, and may not consider all standards and exceptions that could affect a given project.
The updated Chicago building code has several changes that impact – for the better – how small residential buildings can be designed and constructed.
Generally, the updated CBC allows wood frame construction for buildings with 1-3 dwelling units to a taller height and larger envelope than the current CBC. The current Type IVA construction allows two stories above grade plane, while the updated CBC's Type VA construction allows three stories above grade plane. Current CBC's Type IA construction is most like the updated CBC's Type VA construction.
The current CBC allows up to 4,000 s.f. of building area in Type IVA construction of a multi-story residential building (13-48-080); in single-story residential buildings, the limit is 5,000 s.f. The updated CBC's Type VA limits area to 5,000 s.f. in a small residential building without sprinklers regardless of the number of stories.
In the current CBC, a third story is allowed in a small residential building (occupancy class A) of Type IVA construction if it is limited to 800 s.f. in area and the first and second stories are limited to 1,600 s.f. in area (13-48-030, note "h").
This third-story limitation can be seen at all of LUCHA's new Tierra Linda three-flats, designed by Landon Bone Baker (shown above). The updated CBC doesn't have that third-story area limitation, and it allows the Type VA building to have a fourth story if sprinklers are installed (table 504.4).
In some instances, both codes' allowable area factor could increase the 5,000 s.f. limit. Based on the current CBC, many small residential projects cannot take advantage of an increase in allowable area because they don't meet the minimum frontage requirement (13-48-090). In the updated CBC, on the other hand, there are more scenarios where an increase in the allowable area may be possible because frontage is measured differently (506.3.2).
However, a small residential project will most likely run into Chicago zoning code floor area ratio (FAR) limitations on maximum area long before it is affected by building code standards on maximum building area for wood frame construction. For example, the RT-4 zoning district allows up to 3,750 s.f. of building area on a standard Chicago lot (25'x125', 3,125 s.f. lot area with 1.2 FAR).
Also in the updated CBC, including sprinklers in Type VA construction allows a three-story small residential building to have four stories. A basement is allowed in both the current and updated Chicago building codes.
Additionally, a wood frame building can be built up to the lot line with compliant fire-retardant treated wood framing that has a 2-hour fire rating, and a noncombustible exterior wall covering (table 602, note j, and 2302.2).
Additional code changes impacting small residential buildings
The requirement for egress (exit) windows was not adopted in the updated CBC (section 1030 in IBC 2018).
The updated CBC is keeping the current CBC's standard for steeper stairs, tighter winders, and narrower spiral stairs – these are less stringent than what the IBC requires. The updated CBC will adopt the IBC's minimum ceiling height of 7'-0", which is a reduction from the current CBC's minimum ceiling height of 7'-6".
The updated CBC has new provisions for lofts. In the current CBC, lofts are considered mezzanines, and follow mezzanine standards. In the updated CBC, loft becomes its own defined term, and has loft-specific standards found in section 1207.5. The difference between lofts and mezzanines in the updated CBC is that loft standards apply only in dwelling and sleeping units, and mezzanine standards apply in all other uses.
In the updated CBC, lofts are limited to a maximum area of 150 s.f. The minimum dimension is 5 feet in any horizontal direction, and access can be provided by steep stairs or a permanent ladder. A portion of every loft must have a ceiling height of at least 5 feet.
Builders can choose to start using the updated CBC starting with building permit applications submitted on or after December 1, 2019. The updated CBC will be mandatory for building permit applications submitted on and after August 1, 2020.