Guest House Conversion Title 24 Requirements
Guest house is not an Energy Code term, so the project is classified by what the building is today. A conditioned guest house converted to an ADU is an alteration, where only altered components must comply and verified existing insulation can earn credit. A never-conditioned one follows the addition route.
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What the Commission says about guest houses
The Commission lists an existing conditioned detached structure, such as a pool house, studio or guest house, repurposed as an ADU as an alteration, because it adds no conditioned floor area or volume. Section 100.1 defines an alteration as a change to water heating, space conditioning, lighting, electrical distribution or the envelope that is not an addition.
What an alteration triggers
- A new water heater, mini-split or lighting must meet the rules for that component. A mini-split may need ECC-Rater testing, with refrigerant charge on the list.
- Added exterior door area meets U-0.20. Up to 16 square feet of added glass is exempt from the total and west-facing area limits, though U-factor and SHGC still apply.
- More than 25 feet of new or replacement duct triggers sealing and testing.
- Credit for insulation depends on proof. On the performance path the baseline for altered insulation is the mandatory level of Section 150.0 unless an ECC-Rater verifies the existing R-value, which then becomes the baseline.
Where the answer changes
If the guest house was never heated or cooled, converting it is an addition under Section 150.2(a), and existing walls count as new. If it was built without a permit, the building department decides how to demonstrate compliance and whether it is treated as new construction or an addition.
For an unpermitted ADU built before 1 January 2020, state law bars a local agency from denying the permit for building standards violations unless correction is needed for health and safety or the building is substandard (Government Code 66311.7). A building department still decides what corrections apply.
See alteration versus addition, unpermitted work and accessory structures. Similar cases are sheds and pool houses. Our ADU Title 24 reports start from your drawings.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
No. It is an alteration only if the space is already conditioned. A guest house that was never heated or cooled is converted unconditioned space, which is an addition.
For altered components, yes, if an ECC-Rater verifies the existing R-value before construction. Without that verification the baseline is the mandatory insulation level.
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View all Additions and Remodels guides → · Permits and Compliance · Title 24 guide