Building in Phases: How Title 24 Treats a Later Addition to a New or Recently Built House
After phase one is permitted it counts as an existing building, so phase two is judged as an addition or alteration under Section 150.2, against the code in force on the phase-two application date. Solar, ducts and the definition of conditioned space are decided in phase one, so design it with the second phase in mind.
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Phase one becomes the existing building
The compliance manual quotes the building code: an existing building was erected before the current code or has had a legal building permit issued. Phase two is then an addition, meaning any change that increases conditioned floor area and volume (Section 100.1(b)), or an alteration to a regulated system. Newly conditioned space, such as a finished attic, also counts as an addition.
Which code phase two meets
The code that applies is the one in force when the new permit is applied for. If an earlier permit expired with no construction, Section 10-103(d)1 requires a fresh written determination under the code on the new date. A phase two applied for years later may meet a newer edition (expired permits).
How phase two is judged
- Prescriptively, requirement by requirement.
- By performance for the addition alone.
- By performance on existing plus addition plus alterations, with every feature entered as existing, altered or new.
The choice trades effort for flexibility. Addition alone needs little data about the existing house, but improvements to it cannot offset the addition. The third option needs the existing plans, yet lets those improvements count and leaves unaltered components as they are. See prescriptive versus performance for additions.
What phase one decides now
- Solar: PV requirements do not apply to additions or alterations, so the system sized in phase one counts. A second detached building would be a new building.
- Ducts: an existing system extended to an addition must meet duct insulation and leakage rules, with extended ducts sealed, tested and verified.
- Conditioned space: a shell is conditioned if it meets the definitions (heating above 10 Btu/hr-ft², cooling above 5, or indirect conditioning). Conditioning it later is an addition.
When phase two is really new construction
A big enough alteration can be treated as a new building; the manual's example removes all wallboard, insulation and exterior walls. A second stage inside the same permit belongs to that permit's report. See room addition requirements and residential Title 24 reports.
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. Phase two is an addition or alteration and must comply when it is permitted. Planning space, duct runs and equipment access in phase one can make the later work simpler.
No. The edition is set by the date the phase-two permit application is made, so a later edition may apply.
Explore Title 24 Reports
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- How to Find a Copy of Your Title 24 Report
View all Title 24 Reports guides → · Reports and Calculations · Title 24 guide