When we started the process of getting a modular home building permit in California, we assumed the hardest part was was behind us. In reality, the building permit turned out to be its own maze – with different players, requirements, and timelines than the planning phase.
The permitting process for a modular home in California is different from traditional stick-built construction because it combines state-approved factory plans with local building codes for everything that happens on-site.
This phase is where the project shifts from what you are allowed to build to how it will actually be constructed.
Even with state-approved modular plans, you still need to maintain compliance with all local zoning and planning requirements – such as height, setbacks, parking, and use – just as you would with a traditional site-built home. Planning addressed those requirements for us; the building department handled how the home would be installed and connected on the property.
If you’re planning a modular home, this post walks through how the modular home building permit in California process worked for us, what surprised us, and what we’d do differently next time.
What a Modular Home Building Permit in California Covers
In our city, planning approval determined what we were allowed to build and where it could sit on the lot. The building permit focused on how the project would be constructed – specifically the foundation, utilities, and all on-site work required to install the modular home.
For our project, the permit covered the on-site scope, including:
- All on-site plans (permanent foundation, grading, utilities, and access)
- Local life-safety requirements, such as fire access and sprinkler systems
- Deferred submittals, including solar, sprinklers, and encroachment permits
The factory-approved modular plans handled the structure and internal systems of the home, but the local building department still had authority over everything beneath and around it.
Modular Construction Plans vs. Local Requirements
Modular homes introduce an extra layer of complexity in the permitting process.
The modules are reviewed and approved at the state level by the California Department of Housing and Community Development (HCD), including the structural, electrical, and plumbing components within the modules. However, your local building department still controls everything that connects those modules to your site and must issue a building permit for the foundation, installation, and utilities.
In practice, this means you are coordinating two separate systems:
- State-approved modular plans (handled through HCD and your modular builder)
- Local building requirements (handled by your city or county building department)
Even when the modular plans appear complete, local departments may still require additional clarification, revisions, or documentation to meet their standards for foundations, utilities, fire access, and site conditions.
For modular builders and homeowners, one of the biggest benefits of understanding this split is knowing who can change what at each level of review. Local reviewers cannot modify state-approved factory drawings, but they can – and often do – require changes to foundations, site plans, and how the modular home connects to local services.
How to Prepare for the Building Permit Process
Before submitting for our permit, we spent time digging through local codes and regulations, building department handouts, and available checklists. I pulled everything into a single spreadsheet that included:
- Links to municipal codes and city requirements
- Required submittals and checklists
- Notes specific to modular construction
Abbreviated sample of the task list we used to track city requirements while preparing for our modular home building permit in California
Because modular homes were relatively new to our city, there was not much written guidance. That spreadsheet became our working checklist and reference point throughout the process as we went through multiple review cycles.
This step turned out to be more important than expected. The permit process builds on prior corrections, and staying organized makes each round easier to manage and respond to.
Even though our modular plans were state-approved, local requirements for foundations, utilities, fire access, and site work still applied – and drove many of the checklist items we tracked.
Finding Contractors for On-Site Work
For modular builders and homeowners, one of the biggest early realizations is that the on-site scope is separate from the modular contract. In practice, this means you may need different firms for civil engineering, surveying, trenching, sewer, and electrical work.
Before we could submit for a building permit, we needed contractors for key on-site work – and this was more challenging than expected.
We initially planned to line up a few contractors and compare bids, but most declined because the project was too small. Before long, we shifted from comparing bids to simply trying to find someone willing to take the job – something that can be especially challenging in high-demand markets like the Bay Area.
This applied to:
- Utility contractors
- Civil engineers
- Surveyors
As an example, we hired the civil engineer through a referral, but costs were higher than expected due to our limited options.
We also ran into delays where contractors who initially expressed interest stopped responding after schedule delays, forcing us to restart the search.
A few additional constraints:
- For sewer work, the sewer district required us to use contractors from a pre-approved list authorized to work in the public right-of-way and on sewer connections. Your city or local utility district may have similar requirements for sewer, water, or street work.
- Utility connection timelines varied significantly
Pro Tip: If you’re not using a general contractor, try to find specialists for each scope so you’re not paying an added markup. If you do use a general contractor, part of what you’re paying for is coordination and scheduling across those trades.
Let the City Tell You What’s Missing
It’s easy to get stuck trying to perfect your application and plan submittal, but at some point, you need to submit.
Once submitted, the building permit process becomes reactive – the city defines the next steps through their correction comments.
In our city, every review cycle came with a new plan review fee, and the permit expiration clock kept running during corrections. Each additional round cost both time and money. Other jurisdictions may bundle some re-reviews into the initial fee, so it’s worth checking your local fee schedule early.
This is a standard part of permitting in California and can repeat across multiple review cycles.
Pro Tip: Submit your plans and let the building department identify what’s missing instead of trying to guess.
How Plan Check Corrections Actually Work
This was one of the biggest surprises once we were in plan review.
We assumed early conversations with the building inspector would carry through the review. In reality:
- Verbal guidance can change during plan review
- Different reviewers may interpret plans differently
- Only written correction comments truly “count”
The most effective way we found to respond was to work directly on the reviewer’s correction list:
- Provide a clear written response for each item
- Include marked-up drawings or images to document the correction
“Excerpt from our comment‑response sheet showing how we answered the reviewer’s plan check comments and referenced images for our modular home building permit in California.”
We also learned:
- Administrative staff instructions may not match what the reviewer expects. In our case, drawings that had passed earlier review cycles were removed at staff direction, then later flagged as missing by the reviewer.
- Missing the intent of a comment can cost an entire review cycle
Pro Tip: Don’t hesitate to ask the reviewer to clarify their comments. Providing an incomplete or incorrect response can result in failing the review and starting another cycle.
Pro Tip: Ask your modular builder how they handle plan modifications and in what manner revisions are prioritized. In our case, the builder initially worked on a first-in, first-out basis. With the building permit timeline running, we didn’t have the luxury of waiting while they finished plans for customers still in the design stage.
Timelines and Expiration in California
Building permit timelines can vary significantly by city, but review cycles of several weeks are common in California.
In our case, the building permit was valid for one year after issuance. Extensions were not allowed, which we believe may have been related to how our city handled administratively approved planning applications, potentially including those submitted under SB 330. In many California jurisdictions, building permits can be extended in six-month increments or reinstated once construction activity has started, so it’s important to confirm how your own city or county handles permit extensions and abandonment.
This meant we had to carefully manage:
- When we submitted the building permit
- How long each plan check review cycle took
- Contractor availability and scheduling
Unlike planning approvals, building permits often have strict expiration timelines that continue running during plan check review cycles.
Pro Tip: Ask early:
- Verify how long the building permit is valid
- What counts as “activity” to keep the permit active – this may include scheduling or passing a progress inspection
- Whether building permit extensions are allowed
Modular-Specific Challenges
Even with a complete modular plan set, our local departments still had questions during the review process.
In our case, the fire department was unfamiliar with a modular sprinkler system being pre-installed at the factory. Existing structural drawings also required modification during plan check review, and some key information was incorrect or missing from the modular plan cover sheet.
To address this, we created a local addendum cover sheet that included local codes, utility providers, deferred submittals, and clarification of missing details in the modular plans – items that were not included on the modular builder’s cover sheet but were required by the city.
From that experience, a few modular-specific lessons stood out:
- Local reviewers may be less familiar with modular or factory-built systems, which can lead to more questions about sprinklers, temporary shoring, and how systems connect to local utilities
- Even though the modules are state-approved, local building departments still expect the same level of clarity for site plans, foundations, and life-safety requirements as they do for site-built homes
- The handoff between your modular builder and your local design team (civil, structural, soils engineer) is critical – missing information at that interface is where many issues arise during the building permit review process
Pro Tip: For us, the fire department required a larger water meter to support the sprinkler system. In our case, a 1-inch meter was required instead of the standard 1/2-inch. Requirements vary by city and water provider, so it’s worth confirming meter and service sizing early – otherwise, you may need to pay for a meter upgrade later.
Pro Tip: When you receive your plans from the builder, confirm they include all required local codes and regulations. If they are already stamped by the DAA, ask your city whether you can create a cover sheet as a form to document missing items in an efficient, compliant way. Missing information can result in permit delays.
Pro Tip: Coordinate early with your geotechnical (soils) engineer on foundation design assumptions. In our case, the basement foundation design was significantly more robust than what multiple structural and shoring engineers expected. We later learned that more conservative design approaches have become common in some situations, which can materially impact cost. This ended up increasing our foundation costs far beyond what we anticipated – ultimately exceeding the cost of the modular home itself.
What to Double-Check Before You Submit
A few items that can help avoid an extra plan check review cycle:
- Deferred submittals clearly listed. Make sure anything submitted later – such as sprinkler system permits, solar system permits (as required by the State of California), or encroachment permits – is identified on the cover sheet.
- Site plan includes all required local annotations. This typically includes setbacks, easements, utility locations, fire access, and other jurisdiction-specific requirements reviewed during plan check.
- Consistency across drawings. Key values – such as square footage, elevations, wind speed, and structural notes – should match across all plan sheets to avoid building permit corrections. In our case, the wind speed listed on the modular plans did not match the structural drawings (both provided by the modular builder), which triggered a correction.
- Foundation calculations aligned with local seismic and soil requirements. Even with state-approved modular plans, the local building department will closely review the soils report, foundation design and supporting calculations as part of the their review.
Catching these early can save time, reduce additional plan check review cycles, and minimize added building permit fees.
A few things stood out once we were in the building permit process for our modular home:
- The reviewer’s correction list is what matters. Once it is issued, treat it as the source of truth and clarify any prior verbal guidance against it during the building permit review.
- The permit clock keeps running during plan check review cycles. Time does not pause while corrections are being addressed, so delays can directly impact your building permit validity.
- Modular plans are not “plug and play” at the local level. Even with state-approved plans, local building departments may still require additional details, clarifications, or revisions.
- Clearly label state-approved vs. local drawings. Make sure it is obvious which sheets are HCD/state-approved factory drawings and which are local site or foundation sheets so reviewers understand what they can and cannot require you to change.
Final Thoughts: What We’d Do Differently
The building permit phase turned out to be a full project in itself.
For a modular home in California, the building permit process requires coordinating two systems – state-approved modular plans and local building department requirements – and the gaps between them often fall on the homeowner or project team to resolve.
If we were starting over, we would:
- Create an addendum cover sheet from the beginning
- Ask detailed questions about timelines and expiration
- Treat plan check correction lists as the primary task list and respond to each item carefully
For modular builders and homeowners, understanding the split between state and local authority can actually work in your favor. A complete, HCD-approved modular plan set can reduce structural questions, while a strong local cover sheet and clearly labeled site and foundation drawings help local reviewers focus on their portion of the building permit review instead of trying to reinterpret factory plans.
For anyone going through the modular home permit in California, understanding how these pieces fit together can make a significant difference in both timeline and cost.
The building permit process isn’t simple, but once you understand how it works, it becomes far more manageable – and gives you a much better chance of keeping your project on track.
Discover more from The Modularist - Building a Modular Home
Subscribe to get the latest posts sent to your email.