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Pre-Fab: How To Make It Work

Updated: 3 days ago

Breeze 3 bedroom single family residence - online plans coming soon

We will often get phone calls from potential clients who are very excited by the idea of a pre-fabricated structure (Home/ADU/Cabin, etc.) and want to know more about how to put one on their site. While pre-fab structures are an excellent option, there are quite a few things about them to take note of before assuming that is the right path for your project. We've put together this post to help answer some basic questions and determine the reality of a potential idea you may have. If you have more questions, or are sure this is the right path for you and want to get started - send us an email!

What we will cover in this blog post:

  1. What is Pre-fab when it comes to residential projects

  2. Where do architects come into the picture?

  3. What to look for in a Pre-fab unit or manufacturer for our region

  4. Why you might choose a Pre-fab dwelling

  5. Logistics - Where & How

  6. The Process

  7. Alternative Options

  8. Questions for Potential Manufacturers

What is Pre-fab when it comes to residential projects

Prefab, pre-fabricated, manufactured, modular, these are just some of the terms people use to describe a unit (home/ADU/etc.) that is produced in a factory and then shipped to it's site where it is placed on top of a foundation - for simplicity's sake, we are just going to use the term pre-fab.

There are generally (2) versions of pre-fab that are most often seen in residential projects:

  1. Modules - Where a semi-truck-bed sized component (module) is shipped to the site as a finished or nearly finished unit and is then craned into place atop a site-built foundation. This module might be the whole dwelling, or it may be just one component that will be site-connected to one or more other modules. The foundation will be designed by a local architect, engineer or designer, and built by your contractor. Then the contractor will oversee the delivery and installation of the module(s)

  2. Kit or Panelized - When you order a kit home or ADU, you will receive a large bundle of pre-fabricated components (walls, floors, roofs) that will need to be assembled on-site by a contractor or very ambitious home-owner, like a giant puzzle. Your foundation will still need to be designed by a local architect, engineer or designer, and built by your contractor.

It is important to note that pre-fab architecture is not new. It has been around, in various re-incarnations for at least a hundred years. Think Sears Catalog homes as early as 1908 (Kit homes), or perhaps the Eames House that was built in 1949 (trying to redefine the kit of parts for a Kit Home)...and everyone's least favorite topic, the ever-present 'mobile home', which technically hasn't been 'mobile' since the 1970's when standardized certification under HUD came into effect. 'Mobile homes' have since been re-termed 'manufactured housing', even if most people still refer to them as mobile homes or trailers. Since at least the early 2000's there has been a slow, quiet reincarnation of manufactured housing, and a push to use the term 'pre-fab' when describing the modern-styled and meticulously crafted cousin to manufactured homes that are found advertised in the back of every Dwell magazine.

So, what is the difference between a pre-fab/modular home and a manufactured home?

  • A manufactured home (mobile home/trailer) is regulated by HUD with federal certifications and compliance requirements. If you are installing a manufactured home, a building permit submittal is usually just the mfg. provided specification and certification package plus a site plan. We will not be addressing manufactured homes further in this post.

  • A pre-fab or modular home is regulated by the same state and local building codes that a stick-built (site-built) home is subject to. This means a permit submittal package for a pre-fab unit must be just as complete as one for a stick-built home. You will need to submit the same plans, elevations, details, structural design/calculations, and energy calculations. This service may be produced by the manufacturer, or it may be a collaboration between the manufacturer and a local architect or engineer.

    • If you are shopping for a pre-fab unit, please ensure the company you are looking at provides a permit submittal package and plan review assistance FOR CALIFORNIA BUILDING CODES & REQUIREMENTS. We cannot emphasize this enough.

Where do architects come into the picture?

There are two potential paths you can take on a pre-fab project:

  • Architect-Led Pre-fab - Where you work with an architect to design a unit for your needs. The architect will, in turn, work with the selected manufacturer to ensure the design fits within the manufacturing parameters. The architect will collaborate with with manufacturer to produce and coordinate all the drawings and calculations required for permitting, and then hand the design off to the mfg. for construction once a permit has been approved. In this scenario, the architect directs the whole process, starting with the design, collaborating with the mfg, coordinating the plan review submittal and working with any other consultants that may be required during the process.

    • On this path, you have one point of contact - the architect.

  • Off The Shelf Pre-Fab - This path starts with you selecting a pre-fab unit by a manufacturer that works with clients in California. Many have a catalog of designs to choose from, some are custom builders - the options and price ranges are endless. You will then work with the manufacturer to prepare the required drawings and calculations for plan review. Depending on the capabilities of the manufacturer, they may be able to produce the foundation design and drawings, or you may have to find a local architect to produce the site-specific portion of the project (foundation design and drawings, site plans, etc.).

    • This path is a little more onerous for you, the owner, as you have to figure out the whole process on your own and coordinate all the players (unit manufacturer, contractor, local engineer, plan review, etc.)

What to look for in a Pre-fab manufacturer for our region

If you are building in California, the easiest path forward is to choose a manufacturer based in California or familiar with working in California. There really is no simpler way to do this.

California has a very specific set of requirements for building that are different than other states and/or countries. We have strict fire codes (WUI) and Energy Codes (colloquially referred to as Title 24, although technically Title 24 is the entire California Building Code as it is the 24th Title in the California Code of Regulations and the energy code is just one volume of the building code... but that is a fight for another day) that are not taken into account anywhere else. Most non-CA based pre-fab manufacturers don't want to or just won't bother to help incorporate WUI and energy compliance into their design and documents.

So, if you are going to build (install) a pre-fab unit, SHOP LOCAL! First, see if there are any local manufacturers - here in the Redding area I know of Forever Tiny Homes and US Offsite, but if you have more please send their info my way!

I know what you're thinking... but everything is so much more expensive in California, I want to buy from a cheaper state and import. I wish you the best luck - the manufacturers may or may not be able to provide the proper documentation and calculations and you may be required to do a fair amount of re-construction on site to adjust exterior materials and insulation values, etc. to get the structure approved.

Why you might choose an Pre-Fab unit over a site-built dwelling

There are many situations where a pre-fab unit is a good option. I've outlined a few below, and I'm sure there are many more.

  • You have a shortened building window - whether due to scheduling demands, or perhaps a harsh climate that makes building on-site challenging for much of the year, pre-fab dwellings are much faster to install than a site-built dwelling is to build. Just keep in mind that permitting won't be any quicker than a site-built home, and there may be a significant lead-time (factory build time) along with transportation time that needs to be considered.

  • You live in an area with limited construction materials or resources available - If you live in a very rural area where building materials and contractors/sub-contractors would have to be driven long distances for a site-built home, that can significantly increase the cost of construction. A pre-fab unit would come altogether and only require a few people on-site, drastically reducing the transportation costs for labor and materials.

  • You are looking for rigidly controlled construction in a controlled environment. If you are looking for perfection, site-built homes might make you a bit queasy... however, pre-fab homes are built in controlled thermal environments with a high level of detailing and planning in each and every component. This appeals to those looking for meticulous construction and those with significant health concerns like mold allergies, etc.

Reasons why you would not choose a pre-fab unit:

  • Cost savings. There may be cost savings in going the pre-fab route, but it is not in materials or labor as those are all still required, just in a factory setting. The cost savings is in time. The price tag on a pre-fab unit will usually be fairly comparable to stick-built homes once you include transportation and the added foundation expenses.

  • Flexibility. Pre-fab homes are likely going to be more challenging to modify down the road as there will be more complexity in their construction for fabrication and transportation efficiencies.

  • Higher Design Fee for Coordination. While the architect may not be producing the construction documents (plans, elevations, sections, etc.) they will still have to coordinate your design requirements with the manufacturer's constraints and the site information. So you are saving hours on drawing time, but increasing the hours required for coordination.

  • Lower Labor Cost without Sub-Contractors (If your GC uses Sub-contractors). As stated above, a pre-fab unit requires all the same parts and pieces to be fit together as a site-built structure, it is just done in a factory setting.

Logistics - Where & How

Where is the pre-fab structure coming from and how will it be delivered and then put in-place on site? These are critical questions as modular units need to be lifted off the truck and set down as a single unit. This may require a very large and powerful crane as well as space to operate that crane. Will it have to lift the module over your home? Are the streets leading to your lot accessible to a semi-truck? The logistics of getting the unit to your lot and into place often significant planning and forethought. Dense urban areas and remote lots up narrow winding roads may not be suitable for pre-fab. Discuss the delivery and installation logistics with potential manufacturers and contractors up front.

The Pacific Crest Cottage ADU for Shasta County

The Process - architect led pre-fab

Each manufacturer will vary in their internal processes, but the permit submittal process will be relatively similar in each case.

  1. Select your architect and put together a conceptual design concept based on your project requirements and the architect's general knowledge of the manufacturing constraints available.

  2. Select a manufacturer who is comfortable working with California's codes and is capable of producing the above conceptual design.

  3. The architect & manufacturer work together to produce the required documents for permitting, fabrication and installation.

  4. Your architect will compile a permit package (construction documents) containing the manufacturer's drawings and documents along with the site plan and foundation design package and help you submit you local plan review agency.

  5. Plan Review comments will be responded to by the design team who put together the construction documents.

  6. Engage a contractor who will handle the foundation build and oversee the installation of the unit

  7. Once you receive word that your permit is approved, your contractor will pick up the approved permit and you can work with the manufacturer to schedule the delivery (allowing for their fabrication and transportation timelines).

  8. Foundation is completed by the contractor

  9. If it is a modular unit, the pre-fab unit is delivered and installed. If it is a kit unit, it will be delivered to the house flat-packed by components and your contractor will then begin piecing it together.

Alternative Options

Looking for efficiency in the process but perhaps pre-fab isn't going to work for you? There are a few alternative paths forward:

Pre-Approved ADUs

If you like the idea of skipping the design phase and just want to pick an existing design and get to building as quickly as possible, the best option may be a pre-approved design. Many AHJs (Redding and Shasta County included) have launched pre-approved ADU programs where they offer a handful of options for ADUs of various sizes over the counter. You skip the whole architect/engineer/designer process and just need a site plan made for the unit you select. The plans are available to residents of the jurisdiction at no cost, just the permit fee. It streamlines the whole building process significantly.

Online Plans

This is a double-edged sword. Please check out our blog post about Online Plans to know what to look for. We are working towards launching online plan options that will include architectural drawings, structural design and calculations, energy calculations, and AHJ required site plans for a fixed cost. These won't be bespoke designs like hiring us to do a custom home, but they will be cost effective options that have a significantly reduced production timeline.

ADU 2 for Shasta County

Questions for Potential Manufacturers

  1. Is the company comfortable building to the California Building Code?

  2. Will the manufacturer handle the permit process altogether? OR

  3. Will the manufacturer work with your architect to providing a permit submittal package?

    1. If so, what are they capable of producing in-house? You will need Plans, Sections, Elevations, Structural Calculations and Details, and Energy Calculations

    2. Will the mfg. respond to questions during plan check?

  4. Will the mfg. work with your selected Architect/Engineer/Designer on the foundation design or do they do that in-house?

  5. Are there any specific requirements of a contractor on-site for the delivery and installation? Or do they have a team that can do that as well?



AHJ - Authority Having Jurisdiction - the government agency or building department that will issue your building permit

Building Permit - Approval document stating that the construction documents have been reviewed by your AHJ and have been found to be in conformance with applicable state and local codes.

WUI - Wildland Urban Interface - The portion of a town or city that borders open space or 'wildland' and is therefore at a higher fire risk

VHFSZ - Very High Fire Severity Zone - Another term used by the building code and Fire Departments/Marshals to indicated where buildings need to be hardened against the risk of wildfire


Links & References


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