When a basement needs one
Pennsylvania's adopted residential code requires at least one operable escape opening in basements, habitable attics and every sleeping room, and each basement bedroom needs its own. There are limited exceptions, such as a basement used only for mechanical equipment and no bigger than 200 square feet, or a fully sprinklered house with specific exit arrangements.
Finishing a basement without adding a bedroom is treated differently. Philadelphia's 2021 residential-code training says basement alterations and repairs that don't create a new sleeping room don't trigger the escape-opening requirement, and existing openings can use reduced sizes based on the largest standard window that fits the existing rough opening. So the first question for any basement plan is simple: will anyone sleep down there?
Measure the open hole, not the window
A clear opening of exactly 20 by 24 inches is only about 3.3 square feet, well short of 5.7, so a window can meet both minimum dimensions and still fail. To hit 5.7 square feet with a 20-inch-wide opening, the clear height has to be about 41 inches; with a 24-inch-high opening, the width has to be about 34 inches.
Measure the unobstructed space when the window is open the normal way, not the glass, the sash or the outside of the frame. On a slider or single-hung window, the sash that stays put takes up a big share of the frame. Ask the supplier for the published net clear opening of the exact model and size, not the rough-opening dimensions.
The handout allows 5.0 square feet for a grade-floor opening. Philadelphia's training defines that as an opening whose bottom is no more than 44 inches above or below the finished ground outside, so it rarely applies to a window sitting in a deep basement well. Also ask the code office for the maximum sill height above the finished floor inside; a basement window high on the wall can be big enough and still be too high to climb out of.
The route outside counts too
Pennsylvania's adopted language says the opening has to lead directly to a public way, or to a yard or court at least 36 inches wide that opens to one. If the yard doesn't reach the street directly, the path to it also has to be at least 36 inches wide. That matters on rowhome lots, where a back window may open into a small yard boxed in by fences. The state version adds a narrow exception for sprinklered in-fill lots with at least 80 square feet of clear yard, which can use a shared easement as narrow as 30 inches.
For a below-grade window, the well outside has to leave room for the window to open and for someone to climb out. Ask the designer to show the well size, any ladder or steps, drainage and how a cover opens from inside on the plans. Don't climb down into a deep or wet well to measure it yourself.
Bars and grates are a common failure in the city. Philadelphia's fire guidance says security gratings or burglar bars on windows need a fire safety opening feature and must open easily from inside. Check that the window isn't painted or nailed shut and that a dresser or shelving isn't blocking it. Working alarms matter just as much; how often smoke and CO alarms should be replaced covers those.
Check which code your project falls under
Philadelphia adopted the 2021 international codes with local amendments starting July 1, 2026, according to the city's applicable-codes page, and the EZ permit handout with the 5.7-square-foot figure predates that switch. Before you order a window or cut the foundation, ask Licenses and Inspections which code applies to your project and whether an existing-building exception helps. Tracking a Philadelphia permit application or inspection explains how to follow it once you apply.
In Bucks and Montgomery counties, the state says more than 90 percent of Pennsylvania municipalities enforce the Uniform Construction Code themselves or through certified third-party agencies, and in opt-out municipalities the owner or contractor hires a certified third-party agency for residential work. Call the township building office first; why permit rules differ from one Bucks County township to the next shows what to ask.
What to ask before cutting a foundation
- Bring a sketch showing room uses, window locations, finished floor height, outside ground level, the well and the path to the street or yard.
- Ask the code office which permits, plans and inspections are needed, and which inspection must happen before drywall covers the work.
- Have a structural engineer or architect design any new or enlarged foundation opening; cutting a wider hole in a bearing wall needs a header sized for the load.
- Ask the contractor's proposal to name the window model and its net clear opening, who designs the opening, how the well drains, and how the window is flashed. What a window replacement quote should say about flashing lists the details to look for.
- Keep the final inspection record, not just the contractor's word that it passed.