The only Chicago figure we can point to in writing is 42 inches. The Department of Buildings' 2011 Porch Design and Construction Guidelines say a footing's bottom must be at least 42 inches below grade. That guide cites code chapters from before 2019, so treat 42 inches as 2011 City guidance, not today's code. For a real job, confirm the required depth with the Department of Buildings before anyone digs.
Most people who search for the frost line are about to dig for a footing: a porch, steps, a deck, a garage slab edge. So the real question is how deep the footing goes. The one Chicago number we can cite is 42 inches, and it comes with a catch. This page sits inside our excavation and foundation guide.
What is the one Chicago figure we can cite?
The Department of Buildings published Porch Design and Construction Guidelines in May 2011. That guide says footings must have the bottom at least 42 inches below grade (City of Chicago).
The catch matters. The guide cites code chapters that predate the 2019 code rewrite, so the City's own text is from the 2011 era and has to be checked against the current Building Code (City of Chicago). We are reporting what the City guidance said, not promising it is today's rule for every structure.
Is 42 inches the frost line?
We would not say that. The City guide gives a footing depth. It does not state a frost depth, and we have not found a Chicago source that does. Frost depth is how far the ground freezes. Footing rules are generally built around it, but the number that governs your project is the one in the code and on your permit, not a number from a web page.
Two things follow:
- Do not size a footing from this page. A porch, a retaining wall and a house addition can each have different requirements.
- Ask for the depth in writing. If a contractor quotes you a footing depth, ask which code section or plan note it comes from.
What do you still have to do before you dig a footing?
Digging a footing is excavation, so the ticket comes first. Call 811 Chicago (DIGGER) at least two business days ahead, and note that it applies to homeowners on private property too (City of Chicago, City of Chicago). Our DIGGER 811 explainer walks through it.
A 42 inch footing is well under the depth where the City's no permit rule for grading and excavation applies, which is 5 feet below grade on private property, with a 10 foot limit when the dig is not within 5 feet of a structure or the public way (City of Chicago). That rule covers grading and excavation only. Building a porch or steps on that footing can still need its own permit. See excavation permit depth rules, and check with the Department of Buildings for your job.
Planning a new footing or a rebuild? Our excavation crew can confirm the required depth with the City and dig it to what the plans say.
Common questions
The Department of Buildings' 2011 Porch Design and Construction Guidelines say the bottom of a footing must be at least 42 inches below grade. That guide predates the 2019 code, so confirm the current requirement for your structure with the Department of Buildings or your plans.
We cannot say that. The 42 inch figure is a footing depth from 2011 City guidance, not a published frost depth. Check the current Building Code or ask the Department of Buildings what applies to your project.
Grading and excavation no deeper than 5 feet below grade on private property has no building permit requirement, but you still need an 811 Chicago ticket. The structure you build on the footing may need its own permit. Check with the Department of Buildings before starting.
Sources
- City of Chicago, Department of Buildings: Porch Design and Construction Guidelines (PDF, May 2011)
- City of Chicago: Guide to Building Permits: General Contractor License FAQ
- City of Chicago: 811 Chicago (DIGGER)
Facts last checked: October 7, 2026.
This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.