Windows for a California ADU: Energy, Egress and Property-Line Rules

Accessory dwelling units are built on the edges of lots, at small scale, under state rules meant to make them easy to approve. Their windows are still governed by the same building codes as any house, and on an ADU those rules are tighter in practice because everything is closer together.

This page is part of our California window guide. It is general information, not legal or design advice. Section numbers are from the 2025 California Residential Code and 2025 Energy Code, which apply to permit applications submitted on or after 1 January 2026. Your local building department has the final word, and local amendments are common.

First: what kind of ADU is it?

Almost every window rule depends on this, so settle it before you draw anything.

This classification comes from the California Energy Commission. It says an ADU is a newly constructed building if it is a new, detached structure. Attached ADUs, and conversions of unconditioned spaces like garages into conditioned space, are additions. Converting existing conditioned floor area is an alteration, because the conditioned floor area does not increase.

A junior ADU is defined in the Energy Code as a unit of no more than 500 square feet contained entirely within a single-family building.

Energy code: the window numbers

New detached ADU

A new detached ADU has to meet the single-family new construction requirements. On the prescriptive path, the Energy Commission's 2025 summary of Table 150.1-A gives:

• Maximum U-factor 0.27 in climate zones 1 to 5, 11 to 14 and 16.
• Maximum U-factor 0.30 in climate zones 6 to 10 and 15.
• Maximum SHGC 0.23 in climate zones 2, 4 and 6 to 14, and 0.20 in zone 15. No SHGC requirement in zones 1, 3, 5 and 16.
• Total glazing area of no more than 20 percent of conditioned floor area.
• West-facing glazing of no more than 5 percent of conditioned floor area in climate zones 2, 4 and 6 to 15.

For our service area, that is U-factor 0.27 everywhere. Sacramento, Davis, Folsom and Walnut Creek are Zone 12. Roseville is Zone 11 and Napa is Zone 2. All three need SHGC 0.23 or lower. San Francisco is Zone 3 and Truckee and South Lake Tahoe are Zone 16, with no prescriptive SHGC limit.

There is also a mandatory floor that applies on any path. Under section 150.0(q), fenestration must have a maximum weighted average U-factor of 0.40, with limited exceptions.

The area limits bite hard on a small building. Twenty percent of a 500 square foot ADU is 100 square feet of glass in total, including any glazed door. A sliding patio door uses a large share of that. The west-facing limit on the same unit is 25 square feet, which is about one ordinary window.

The Energy Commission also notes that a newly constructed detached ADU must have a newly installed solar photovoltaic system unless an exception applies. An ADU that counts as an addition does not trigger that requirement.

Attached ADUs and garage conversions

For additions, the Energy Commission's 2025 ADU guidance says new windows, skylights and glazed doors must meet the same Table 150.1-A U-factor and SHGC figures. The area limits are different:

A converted two-car garage usually lands in the first or second row. Those limits are more generous than the new-construction 20 percent. This is one way a conversion is easier than a new build.

Converting conditioned space

Where the ADU is carved out of space that was already heated and cooled, the project is an alteration. Windows you replace follow the replacement rules, including the exception for 75 square feet or less. See what Title 24 requires of replacement windows.

The performance path

All of the above is the prescriptive path. The performance path models the whole building in approved software and allows trade-offs, so more glass in one place can be offset elsewhere. Most architect-designed ADUs with large glazed walls go this way. Mandatory requirements still apply. The U-factor and SHGC you rely on come from the NFRC label. See how to read an NFRC label.

Egress: every sleeping room needs a way out

In the 2025 California Residential Code, emergency escape and rescue openings are in Section R319 (formerly R310). Section R319.1 requires at least one operable emergency escape and rescue opening in every sleeping room, in basements, and in habitable attics. The opening must lead directly to a public way, or to a yard or court that opens to a public way.

The minimum dimensions in R319.2:

• Net clear opening: 5.7 square feet. Grade-floor openings may be 5.0 square feet.
• Net clear height: 24 inches.
• Net clear width: 20 inches.
• Sill height: the bottom of the clear opening no more than 44 inches above the floor.

The dimensions are those obtained by normal operation of the window. It has to open from inside without keys, tools or special knowledge.

What this means for an ADU:

• Style matters more than size. A casement opens its whole sash. A slider or single-hung opens half. A slider has to be much larger than a casement to give the same clear opening. In a small bedroom wall, a casement often solves it.
• A door can do the job. A sleeping room with an exit door opening directly to the outside does not need a separate escape window. In a studio ADU the entry door may already satisfy this, and your plan checker will confirm.
• The path outside matters. The opening has to lead to a yard or court that opens to a public way. A bedroom window facing a narrow fenced side yard deserves a question to the plan checker.
• Bars and grilles over an escape opening must release from inside without a key or tool (R319.4.4).

Conversions

The code's relaxed rule for replacement windows (R319.5) applies only where the replacement is not part of a change of occupancy. Turning a garage into a dwelling creates a sleeping room where there was none, and R319.1 applies to it. Section R319.7.1 sets reduced minimums for an existing operable window where a change of occupancy would require an escape opening: 4 square feet of net clear opening, 22 inches high and 20 inches wide. Whether your conversion can use it is a question for your building department. Our page on egress window requirements covers the general rules.

Windows near the property line

This is the rule that surprises ADU owners most. It comes from two laws working together.

State law on setbacks

Government Code section 66314(d)(7) limits what a city or county can demand. For a new ADU that is not a conversion, "a setback of no more than four feet from the side and rear lot lines shall be required." For an ADU converted from an existing living area or accessory structure, or rebuilt in the same location and to the same dimensions as an existing structure, no setback may be required.

So a new ADU can sit 4 feet from the fence, and a converted garage can stay where it is, even on the lot line.

The building code on openings

Section 66314(d)(8) keeps local building code requirements for detached dwellings in force. That includes California Residential Code section R302.1, which regulates exterior walls according to fire separation distance, generally the distance to the lot line.

The same tables govern the wall itself. Without sprinklers, a wall that is not fire-resistance rated needs at least 5 feet of separation, so a wall closer than that must be 1-hour rated. With sprinklers throughout, an unrated wall is allowed at 3 feet.

Put the two laws together:

• New ADU at the 4-foot setback, no sprinklers. The wall facing the lot line must be fire-resistance rated, and windows in it are limited to 25 percent of that wall's area. That is enough for a couple of modest windows, not a wall of glass.
• The same ADU moved to 5 feet. The wall can be unrated and the openings are unlimited by this table. One extra foot changes the design.
• The same ADU with sprinklers. Unlimited openings at 3 feet and beyond.
• Converted garage within 3 feet of the line. Openings are not allowed in that wall under the table. Plan the windows on the other walls, and ask the building department how it treats an existing window.

On sprinklers, section 66314(d)(12) says an ADU is not required to have fire sprinklers if they are not required for the primary residence. Many ADUs are therefore unsprinklered, and Table R302.1(1) is the one that applies.

The limits do not apply to walls perpendicular to the line used to measure the distance.

Light and ventilation

Windows on an ADU are also how habitable rooms meet the minimum for daylight and fresh air. Section R325.1 of the 2025 Residential Code requires:

• glazed area of at least 8 percent of the floor area of each habitable room; and
• openable area to the outdoors of at least 4 percent of the floor area for natural ventilation.

There are exceptions where mechanical ventilation and artificial light are provided to the code's standard. A bathroom needs either a window of at least 3 square feet, half of it openable, or artificial light and an exhaust fan, and a bathroom with a tub or shower needs mechanical ventilation regardless.

Safety glazing

Safety glazing is in Section R324 (formerly R308). The hazardous locations in R324.4 include glazing in doors, glazing within 24 inches of a door where the bottom edge is less than 60 inches above the floor, large panes near the floor next to a walking surface, glazing near tubs and showers where the bottom edge is less than 60 inches above the standing surface, and glazing near stairs.

An ADU packs a door, a bathroom and windows into a short run of wall, so more of its glass ends up in these locations than in a typical house. Mark them on the window schedule before ordering. See California requirements for tempered glass.

Wildfire areas

If the lot is in a mapped Fire Hazard Severity Zone or a Wildland-Urban Interface area, a new ADU has to meet the wildfire building standards, which since 1 January 2026 are in the California Wildland-Urban Interface Code (Title 24, Part 7). Those include requirements for exterior glazing. We keep the detail in one place. See wildfire-resistant exteriors. Check your address on the state's fire hazard map before you order windows, because it changes the specification.

Garage conversions: the specific issues

• The garage door opening. It is the largest opening you will have and the natural place for a window wall or a glazed door. Count it against the glazing area limit for an addition, and check whether it faces west.
• Existing small windows. Garage windows are often single-pane, too small or too high for egress. Plan to replace them, and check each against the lot-line table.
• The wall on the property line. Many older garages sit on or near the line. That wall will probably have no windows.
• The sleeping area. Decide where the bed goes early, because that room needs the escape opening, and the wall it can go in may be limited.
• Privacy. A converted garage usually looks at a fence, a driveway or the main house. See obscure and privacy glass, and how to choose a window style once the code limits are mapped.

Sources

• California Energy Commission: 2025 Energy Code Accessory Dwelling Units FAQs
• California Energy Commission: 2025 Energy Code Single-family Accessory Dwelling Units (fenestration requirements for additions and new construction)
• 2025 California Residential Code, Chapter 3 (sections R302.1, R319, R324 and R325)
• Government Code section 66314 (ADU setbacks, building code and sprinklers)

Working with Heritage Exteriors

If you are building an ADU or converting a garage, bring us your designer's window schedule and we will quote from it and tell you if a window looks likely to conflict with the energy, egress or lot-line rules above. Your designer and building department have the final word on the plans. Licensed in California, CSLB #1055919. Call (916) 546-2991.