Does a Room Addition Need Title 24?

Size decides the test

The prescriptive rules for additions split at 700 square feet, and the smaller band splits again at 400. Knowing which band a project sits in before the design is fixed is worth real money.

Over 700 square feet, the addition meets the prescriptive package for new homes, with a few modifications: extended walls can keep their existing depth with R-15 or R-21 cavity insulation, and total glazing is capped at the greater of 175 square feet or 20 per cent of the addition's floor area.

700 square feet or less, the package is modified further. The attic needs R-38 or R-30 depending on the climate zone, zones 2 to 15 need a radiant barrier, and Quality Insulation Installation is not required. Glazing is capped at the greater of 120 square feet or 25 per cent of the floor area between 401 and 700 square feet, and the greater of 75 square feet or 30 per cent at 400 square feet or less. In zones 2, 4 and 6 to 15, west-facing glazing is also held to 60 square feet.

Why the cap matters at design stage

A 900 square foot addition with a wall of glass onto the garden is the classic case. Twenty per cent of 900 is 180 square feet, which is more than 175, so 180 square feet is the cap, and a design drawn with 260 square feet of glazing will not pass prescriptively. It can still comply, but by the performance path, which means trading the glazing against better envelope or equipment elsewhere.

Discovering that after the drawings are finished is expensive. Discovering it at concept stage usually costs nothing, because the glazing can be redistributed or the compliance path chosen deliberately.

The prescriptive and performance routes

The prescriptive route is a checklist: meet each requirement and the addition complies. The performance route models the addition and shows it performs at least as well as a reference design of the same size and shape, which allows a shortfall in one place to be made up in another. Larger additions and glass-heavy designs usually end up on the performance route.

Common Title 24 Questions

1. When is Title 24 required for a home?
2. Does a remodel need Title 24?
3. What plans are needed for a residential Title 24 report?
4. How much does a residential Title 24 report cost?
5. How long does a residential Title 24 report take?
6. What is the CF1R for a house?

Explore Additions and Remodels

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3. Sunroom Conversion Title 24 Requirements
4. Does a Sunroom Need Title 24?
5. Do I Need Title 24 for a Garage Conversion?

View all Additions and Remodels guides → · Permits and Compliance · Title 24 guide