Egress Window Requirements: Size, Sill, Well and Ladder

An egress window, in code language an emergency escape and rescue opening, has to meet four measurements in the model residential code: a net clear opening of at least 5.7 square feet, a clear height of at least 24 inches, a clear width of at least 20 inches, and a sill no more than 44 inches above the floor. That is how the City of Boise's handout of Section R310 of the International Residential Code (IRC), dated March 2022, states it (Boise handout) ¹. A window below ground also needs a well of at least 9 square feet, and a well deeper than 44 inches needs a ladder or steps ¹.

The IRC is a model code. It applies where a state or city has adopted it, in the edition it chose and with its own amendments, so the locally adopted code governs your house. The two official handouts cited in this article show why that matters: they agree on most numbers and differ on a few. Your building department can tell you which edition applies. For what the work costs, see egress window cost; this article covers only the rules.

Where an egress opening is needed

Boise's handout of R310 says basements, habitable attics and every sleeping room shall have at least one operable emergency escape and rescue opening, and that where a basement contains one or more sleeping rooms, each of those rooms needs its own (Boise handout) ¹. The opening has to lead directly to a public way, or to a yard or court that opens to one ¹. The same handout lists exceptions, such as basements used only to house mechanical equipment with no more than 200 square feet of floor area, and some basement sleeping rooms in a house with an automatic sprinkler system ¹.

Duluth's handout of the Minnesota Residential Code words the scope differently: basements with habitable space and every sleeping room (Duluth handout) ². That is one example of a local version reading differently from the model code.

The opening: area, height, width and sill

Here are the numbers as each handout gives them.

Both handouts say the net clear dimensions are measured as the result of the normal operation of the window from the inside (Boise, Duluth) ¹². That is the clause that trips people up. A window can be large on the outside and still fall short, because what counts is the hole a person can climb through once the sash is open. In a hung window or a slider only part of the frame opens at a time, so the clear opening is a fraction of the window's size; a casement swings out on side hinges and leaves most of the frame open. The types of windows guide shows how each style opens, and casement windows covers that style in detail.

The height and width minimums do not add up to the area minimum on their own: 24 by 20 inches is about 3.3 square feet, so a window has to exceed one or both minimums to reach 5.7 square feet. That is our estimate from the handout's figures ¹.

Duluth's handout defines a grade-floor opening as one whose sill is not more than 44 inches above or below the finished ground next to it ².

Opening from the inside

Both handouts say the opening must work from inside the room without keys or tools; Boise's version adds "special knowledge" (Boise) ¹². Duluth's handout adds that bars, grilles, covers and screens over the opening or its well are allowed only if the clear opening still meets the size rules and the device releases from inside without a key, a tool or more force than the window itself takes to open (Duluth handout) ².

Storm windows count too. ENERGY STAR advises choosing a storm window that matches how the existing window operates, especially in bedrooms and other places where escape and rescue windows may be needed or required by building codes ³. A fixed storm panel over an egress window can defeat it. The storm windows article covers interior and exterior types.

Window wells

When the sill sits below the ground outside, the window needs a well. Both handouts give the same dimensions (Boise, Duluth) ¹²:

• A horizontal area of at least 9 square feet ¹.
• A horizontal projection and width of at least 36 inches each ¹.
• Big enough for the window to open fully ¹².

Boise's handout also says the well must drain, by connection to the foundation drainage system or an approved alternative, with an exception for well-drained soils ¹. Both handouts say an opening under a deck or porch must open fully and leave a path at least 36 inches high to a yard or court ¹².

Ladders and steps

A deep well needs a way out. In both handouts, a well with a vertical depth greater than 44 inches must have a permanently affixed ladder or steps that can be used with the window fully open (Boise, Duluth) ¹². The rungs need:

• An inside width of at least 12 inches ¹.
• A projection of at least 3 inches from the wall ¹.
• Spacing of not more than 18 inches on center, vertically, for the full height of the well ¹.

The ladder may encroach up to 6 inches into the required well dimensions ¹².

Replacement windows

Both handouts exempt a replacement window from the area, height, width and sill rules when it is the manufacturer's largest standard size that fits the existing frame or rough opening, and is the same operating style as the old window or a style with an equal or larger opening (Boise, Duluth) ¹². Boise's version adds that the replacement must not be part of a change of occupancy ¹. Duluth's adds separate minimums for rooms used for foster care or day care under Minnesota's rules ². A new opening cut into a basement wall has no existing frame or opening to fit, so this exemption, as both handouts word it, does not reach it.

Before ordering a replacement for a bedroom window, it helps to know the opening the new window will give. How installers measure for replacement windows explains the measurements, and standard window sizes explains size codes.

Permits and who checks

A new egress opening usually goes through the local building department, which checks it against its adopted code; do you need a permit to replace windows shows how cities differ. Questions worth putting to an installer:

• Which code edition does the building department here enforce, and does it change any of these numbers?
• What net clear opening, in inches and square feet, will the window give when opened normally?
• How far will the sill sit above the finished floor?
• Who pulls the permit and books the inspection?
• Is the installer licensed where your state licenses this work? The state pages, such as Pennsylvania and Oregon, link the lookups.

For cost, see egress window cost and basement window replacement cost; for choosing who does the work, see how to choose a window installer. When you have the plan, you can compare quotes from window installers.

About these figures

• ¹ Official record + Estimate: City of Boise IRC handout #438 (March 2022) reproducing Section R310: scope and exceptions (200 square feet), 5.7 and 5 square foot openings, 24 and 20 inch minimums, 44 inch sill, 9 square foot well with 36 inch projection and width, drainage, 44 inch ladder depth, 12, 3 and 18 inch rung rules, 6 inch encroachment, 36 inch path under decks, and the replacement window exemption; the 3.3 square foot product of the minimums is our arithmetic from these figures (24 times 20 inches is 480 square inches). Checked October 26, 2026.
• ² Official record: City of Duluth handout (Doc 213) reproducing the Minnesota Residential Code's emergency escape and rescue rules: scope, 5.7 and 5 square foot openings, grade-floor definition, 24 and 20 inch minimums, 44 inch sill, operation without keys or tools, bars and covers, well, ladder, deck and porch, and replacement window rules, checked October 26, 2026.
• ³ Official record: ENERGY STAR storm windows page advising a storm that matches the existing window's operation where escape and rescue windows may be needed, checked October 26, 2026.