ADU Window Requirements: Egress, Size, and Energy Codes Explained

Understanding ADU Window Requirements

Windows do more than let in light and views—they're critical safety features, energy components, and code requirements that must be carefully planned in your Accessory Dwelling Unit. Get them wrong, and you'll face permit rejections, failed inspections, or costly redesigns.

California has specific requirements for window size, placement, egress, glazing, and energy efficiency. This guide covers everything you need to know about ADU windows, from safety codes to smart design choices.

Emergency Egress Window Requirements

Egress windows are your escape route in an emergency. California Building Code (based on IRC) mandates egress windows in every sleeping room—and that includes bedrooms in ADUs.

Egress Window Minimum Dimensions

Egress-Compatible Window Types

According to the International Residential Code, egress windows must be operational from inside without keys, tools, or special knowledge.

Natural Light and Ventilation Requirements

California Building Code requires minimum glazing and ventilation for habitable rooms:

Natural Light Requirement

• Minimum glazing area: 8% of floor area
• Example: 100 sq ft room requires 8 sq ft of window glass
• Can be reduced to 4% with approved artificial lighting design

Natural Ventilation Requirement

• Minimum openable area: 4% of floor area
• Example: 100 sq ft room requires 4 sq ft of openable window
• Can be replaced entirely by mechanical ventilation (required in bathrooms)

Title 24 Energy Efficiency Requirements

California's Title 24 Building Energy Efficiency Standards set strict requirements for window performance.

U-Factor Requirements by Climate Zone

U-Factor measures heat transfer—lower is better. Most dual-pane Low-E windows meet the 0.30 requirement.

Solar Heat Gain Coefficient (SHGC)

SHGC measures how much solar heat passes through the window. Requirements vary by orientation:

Choosing the Right Window Types

Different window styles suit different ADU situations:

Casement Windows

• Best for: Egress requirements, maximum ventilation
• Pros: Opens 100%, easy egress, tight seal
• Cons: Crank mechanism can fail, screens inside only

Double-Hung Windows

• Best for: Traditional aesthetics, easy cleaning
• Pros: Tilt-in for cleaning, familiar operation
• Cons: Only 50% ventilation, harder to meet egress

Slider Windows

• Best for: Wide openings, easy operation
• Pros: Simple operation, no exterior swing
• Cons: Only 50% ventilation, tracks collect debris

Awning Windows

• Best for: Rain protection, privacy, bathrooms
• Pros: Can remain open in rain, pairs with fixed glass
• Cons: Limited opening, rarely meets egress

ADU Window Costs

For a typical 600 sq ft ADU with 8-10 windows, budget $4,000-8,000 for quality vinyl or $7,000-15,000 for premium fiberglass or wood-clad windows.

Strategic Window Placement

Beyond code requirements, thoughtful window placement improves livability:

Maximize Natural Light

• Position the largest windows on the south side (in California)
• Use higher windows to get light deeper into the space
• Consider transom windows above doors
• Light-colored interior finishes multiply available light

Cross Ventilation

• Place operable windows on opposite walls
• Low windows on the cool (north) side, high on the warm (south) side
• Casement windows can direct breezes into the room

Privacy Considerations

• Avoid windows directly facing the main home
• Use obscured glass in bathrooms
• High windows provide light without sightlines
• Consider landscaping or fencing for ground-level windows

Common Window Mistakes to Avoid

• Wrong egress dimensions: Verify net clear opening, not rough opening
• Missing egress in bedrooms: Every sleeping room needs one
• Ignoring sill height: 44" max from finished floor
• Wrong Low-E coating: Different coatings for different orientations
• Undersized for light: Meet the 8% minimum glazing requirement
• No consideration for furniture: Don't put the egress window behind a bed

Frequently Asked Questions

Does a studio ADU need an egress window?

If the studio has a sleeping area (which most do), yes. The California Building Code considers any room used for sleeping to require egress. Even if you call it a "living room," if a bed could be placed there, plan for egress.

Can I use a door instead of an egress window?

Yes! A door that opens to the exterior and is at least 32" wide and 78" tall qualifies as an egress. This is a common solution in garage conversions.

Are skylights allowed for natural light?

Yes, skylights count toward the 8% glazing requirement. However, they generally don't count for egress or ventilation unless specifically designed for that purpose.

Do bathroom windows need to meet egress requirements?

No, bathrooms are not sleeping rooms and don't require egress windows. However, they need either a window (natural ventilation) or an exhaust fan (mechanical ventilation).

What about window bars or security screens?

If installed on egress windows, they must be openable from inside without keys, tools, or special knowledge. Quick-release mechanisms are required.

Conclusion

Windows are one of the most important elements of your ADU design—affecting safety, comfort, energy efficiency, and the feel of the space. Getting them right from the start avoids costly changes during permitting or construction.

Key takeaways:

• Every bedroom needs an egress window (5.7 sq ft net clear opening)
• Natural light requires 8% glazing area of floor space
• Title 24 sets U-Factor and SHGC requirements by climate zone
• Casement windows are easiest for meeting egress requirements
• Strategic placement improves light, ventilation, and privacy

Ready to plan your ADU windows? Our team can help you navigate code requirements and design a bright, comfortable space.