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Guide to Planning Your Modular Prefab Home

Is a modular home right for your project? Here we will walk through the feasibility, the design and planning requirements, and the deployment options for modular homes.


If you are thinking about a custom prefab or modular home, we want you to be excited about the benefits, but we also want you to have a balanced understanding of what the prefab choice means for your project. This introduction will help you cut through the prefab-washing and give you an intelligent starting point for a serious conversation with a professional.

types of prefab

A prefab home uses off-site construction to minimize the impact of construction activities on-site. The dream of prefab is for homes to be manufactured in a clean and dry environment, and erected rapidly on-site, faster, at lower cost, and with factory-level quality control.

There are several different ways to prefab a house:

First, a panelized home (aka “panel-modular”) utilizes wall panels along with floor and roof trays, to rapidly erect the home in hours or days. These panels may be structural only, including framing and sheathing. Alternatively, structural insulation panels (SIPS) have an inner and outer layer of sheathing is bonded to an insulated core of solid rigid foam. There are also other panel providers that install windows and doors. There are very few constraints on the design of a house built this way, and once a house goes up, you would generally never know that it was panelized.

Second, a modular home (aka “box-modular”) is delivered as a box, up to about 15 feet wide and up to 60 feet long, that ships from the factory as complete as possible, with interior and exterior finishes. Like panel-modular, box-modules may be custom designed and engineered to play their role in an overall design in which modules are stacked or placed side-by-side on a traditional foundation. Historically, modular prefab has been largely constrained to suburban rambler-style developments, but it is gaining popularity in cities and among custom clientele for high-end projects, many of whom value the sometimes boxy look of a custom modular home.

Modular homes are often confused with manufactured homes, which are homes permitted using the HUD code and can be delivered easily across state lines. If you are trying to permit a project in a rural jurisdiction, it is important to stress the difference. Manufactured homes are among the cheapest available, they are factory produced in high production, and tightly constrained in their overall design. Many housing communities prohibit them. Modular homes, by contrast, are much more conventional looking, and are engineered according to local building codes.

site access

If you are thinking about going modular, you must have appropriate site access. If you are thinking about using wall panels, there is often more flexibility in delivery, but it is still a serious consideration for difficult sites.

A modular home should be accessible to a large crane, which must be able to swing to pick up the parked module and then set it directly at the final building location. Overhead power lines and excessive tree canopy can be obstacles. Alternatively, a modular home may also be roll (or slide) set, or drop set directly from a truck with the assistance of hydrolic jacks. All of this depends on the layout of the foundation and the site access.

If your site is far removed from the street, a larger crane may do the trick. I have seen a backyard cottage set by a crane that reached over the top of neighboring homes, but it filled me with no small amount of terror.

Is your site in a region with difficult access from major highways? You may need the truck driver to test the route and make recommendations for module length and height clearance.

Wall panels are also delivered by truck, but will have a standard width truck bed. Even so, a small crane or telehandler will be sufficient to set the panels, and this is simplest when it has access directly from the truck to the building site. As the last resort, many panels are two-man panels and may be carried to the job site.

why prefab?

Historically, modular prefab has been constrained to a 5% market share, in rambler-style developments. The push to open higher-end modular builds to a more sophisticated urban clientele has been largely driven by industry outsiders who are asking, “why not?” They point to the fact that the construction industry has not seen significant increases in productivity that could be made possible by the assembly line process or automation. In fact, productivity is declining.

To outsiders, the way construction is structured - with subcontractors performing specialized parts of the work, often procuring and marking up their own materials - seems insane. This is the motivation for some very large investment in vertically integrated prefabrication.

But prefab comes with its own array of risks. Factory spaces, equipment, and automation contribute to a massive increase in overhead expense. Factories don’t scale well, and can fail when business slows. Transportation and delivery adds fixed expenditures to every project.

There is no doubt in my mind that prefab is the future of construction. For it to be the ‘now,’ you should still consider yourself an early adopter who believes that the experience of building a modular home outweighs the costs in the marketplace that is available in your region. Let’s look at the potential benefits.

Reason 1: Achieving an accelerated schedule.

After permits have been obtained, prefab construction may proceed simultaneously with site work, saving time on the overall construction schedule. A full modular build can represent the most time saved if the fabricator’s schedule coincides with the site schedule, and the modules can be fabricated in a time-frame equivalent to the site preparation.

A high-production factory completes a module in about 5 days. A custom modular factory should be able to ship out a four or five module home in about eight weeks, plus lead time. If your project will take significantly longer on the factory floor, there is a good chance that the rent expense associated it being there is eating up the project budget, whether you are aware of it or not.

For the timeline to work for you, site activity needs to be moving forward as rapidly as the fabrication. If the site work lags, there is a risk that the modules will need to be temporarily stored before being delivered, generating extra costs. Even if the builder has provided the minimum level of completion required to set the modules - the foundation or basement framing - this is still a failure for the project schedule and removes the competitive advantage of modular. A site builder should be providing the maximum level of completion suitable for the project at the time of delivery.

At AirMod, our goal is to design a project such that it is feasible for a small crew to make the project move-in ready within a week after delivery of the modules.

Reason 2: Less on-site construction.

Construction activity has the potential to disturb wildlife, neighbors, and occupants already living on site. In Seattle, there are restrictions in some areas due to heron or eagle nesting which limits the construction season and noise disturbance. Modular takes the bulk of this activity into the factory.

Every year, contractors race to get into the ground in the spring, and be dried-in before the rains come. Excavation and foundation work will always be affected by the build season, but modular homes need not be.

Framing buildings in the rain or wind with numb hands can lead to mistakes. Even in the dry season, an unexpected heavy rain can deposit microorganisms into spaces like insulation cavities, that are about to be sealed up for a hundred years. If the presence of life is coupled with the food for life (such as glues impregnated in our wood materials) and if there are any moisture or vapor-related defect in the construction, the house will certainly suffer damage in the long term. Building in a clean dry environment removes some of this risk.

Reason 3: Cost of Construction

As an advocate of prefabrication, I am about to commit heresy. In our present market for most regions on the West Coast, modular construction will often not directly save you money, for the same level of build.

Where it is most likely to save you money is in a remote location or in an urban core. If you are in a remote location, such as an island in the San Juans, a local workforce might not exist. If you are in an urban core, such as Seattle, your local workforce will be very busy. In both cases, having an out-of-area workforce can help to reduce costs. (The ideal situation for a modular factory is where there is a good local workforce, it is at least an hour or two outside an urban core, but it is not so far that it generates excessive transportation costs, often quoted at about $10/mile.)

Why doesn’t prefab guarantee cost savings? In many regions, there are very few ‘boutique’ modular fabricators who are available to build unique custom homes. They are smaller shops that haven’t yet achieved the economies of scale seen by high production builders, but are still burdened by high overhead. In fact, I have seen modular projects bid at 15% higher cost than a fair site bid.

The indirect costs, however, may be a source of value gained that translates into dollars. If you shorten a construction schedule significantly, your investment in a new property becomes utilized more quickly. In a tear down & rebuild scenario, the time you are displaced may also be dramatically reduced. If you are developing a rental property in a hot market, you get into the market sooner. Almost all the cost savings are schedule related or location related.

To have the best chance of a modular project saving you money, it is important to begin with the right step forward. See our section Designing for Modular below.

At AirMod, we keep a project on a flexible path until the permit & construction drawing stage. At this point, you should make a choice based on either a competitive bid, or a strong relationship with the builder or fabricator. This prevents your project from being stuck in the protective bubble of available modular builders.

Reason 4: The Fun Factor

On delivery day, the sun rises. There is already a parked motorcade of massive, forty-foot long modules lined up down the street. Soon, the site crew arrives with a van of rigging. Then the trucks with crane counterweights start to arrive, and form their own parked motorcade. The builders and architects show up with coffee in hand. When the crane shows up, it is like the diva walking in, and everything starts to happen.

The crane is put in position while the site crew builds the rigging. The counterweights are lifted on to the crane. The neighborhood crowd start to gawk at the operation. Then the counterweight trucks are out of sight, and the modules are moved into position. The plastic is taken off the mods, and they are rigged and lifted. When the mods go up, the trucks are gone, and there is nowhere for the module to go but fly into position on the foundation.

Later in the day, as the last module is being placed on the upper floor, the crew and the crowd is sharing a pizza and snacks. The drone is flying overhead and capturing video of the operation.

Nothing beats the experience of having a home delivered and packaged like a smart phone, and moving in only days later. But once a house hits the ground, it almost doesn’t really matter how it got there.

It is more important that the design is good. And modular homes can be sexy. Instead of pretending to be traditional, many modular homes use the box for visual effect, and use the implicit structural logic of the mods for dramatic cantilevers. It is more than just a cool look; when you look at a modular house, you can see how the parts add up to make the whole. You can take them apart in your mind.

At AirMod we believe in modularity as a design goal linked to a fabrication goal. Architecture is as much about the construction as about the creation.

Like homes by Frank Lloyd Wright, our projects utilize multiple layers of interacting grids. There is a primary grid that respects modular fabrication & transportation dimensions; there is a secondary uniform grid that determines floor and wall panel fabrication dimensions. Our goal is to create the integrity of a coherent order that makes homes easier to design, easier to build, and even more exciting to inhabit.

prefab design guide

For modular to represent a competitive advantage to your project, you should begin with the right approach. The most important rule is to design for modular, and consult closely with the modular fabricator for their preferred construction system and specifications. Further - and very important - reduce or eliminate any unnecessary site work.

The most important consideration are dimensional limitations on the mods:

Width: Modules should be a maximum of 16 feet wide, including all exterior siding and trim boards. It is usually better to stay slightly under this width early in the project. If you are measuring at the eave it is safe to provide a full 16 feet to the eave fascia.

Length: The longest module we have designed is 65 feet, but it was too long. Long modules can be stressful on the mods themselves and their structural rigging, and their weight determines the size of the crane. 40 feet is closer to an ideal size for a module.

The looming threat of site work is a constant struggle in modular design. If there is an important structural connection that needs to occur in a finished interior space, it can mean accessing the cavity of a wall, installing drywall, and repainting the entire room. At AirMod we are dedicated to the art of the modular house, and this means preserving a completely intact interior finished space. Structural connections occur only at the exterior, to be covered by trim boards. Marriage lines between modules are made with metal or wood trims.

Here are some of the most crippling potential problems in modular design:

In a conventional house, wide open rooms can use continuous beams or open web trusses to span between exterior walls. In modular construction, long open spans between adjacent modules create a structural problem that may require tens of thousands of dollars in design, engineering, logistics, and construction to accommodate. Sometimes these workarounds can be avoided with a single wood column in the right place. Be sure to limit long spans to what can be accommodated within the depth of the structural framing, often 15-20 feet.

While flat-roof modules may be easy to transport, sloped roofs with eaves are a durable, time-tested strategy for waterproofing a building and controlling solar heating. We have experimented with a wide variety of ways of constructing eaves on modular homes. Our preferred strategy is to accommodate an eave within the overall width or length of the factory-built module. This has a big impact on the orientation of the modules on site, and can be a determining factor in the overall design. By contrast, nearly any solution for a site-built eave requires the entire roof structure to be installed on-site, and is often simpler if approached from this point of view.

  • The “eave” problem.

  1. Design with the system and specifications in place by that fabricator.

    • Know the technical capabilities of the shop. Do they have a crane in house?

    • Can they accommodate spray-foam insulation?

  2. Design to minimize site-work.

    • Provide wall panels for any non modular components.

  3. Design to minimize mechanical, electrical, and plumbing crossovers.

Best case scenario, engage a site contractor who is willing to subcontract the work to a modular fabricator if the price is right. The contractor may decide that the profit gained over a shorter time frame allows them to increase their throughput.

In the last few years I have been involved with sixteen custom modular homes. Each project was a

contained to a 5% market share (in rambler-style developments or manufactured homes with limited options). Panel prefab has a larger reach. In some regions around the world, panel prefab has already dominated timber frame building construction, with 95% market share.

I have designed or permitted over a dozen prefab homes in the Seattle region, both modular and panelized, and feel they are an exciting area of design innovation and an aesthetic opportunity.

Prefab homes require more coordination and up-front planning, and they require the designer to respect the natural limitations of the system. But I believe there is untapped beauty and opportunity in prefab.

with whom do I contract?

Should I contract with an architect, design-builder, or design-fabricator?

I advise clients to contract directly with an architect. In this relationship, the client has an unbiased advocate. The architect can educate the client and help them to document, in the contract, expectations regarding the schedule or specifications of the project.

On the other hand, no one likes competitive bidding as it requires a tremendous amount of work on the part of the builder, and subtleties may be easily concealed in estimates. The best situation is when a builder is involved from an early point in the design process and can participate in decision making while estimating the project.

Should I contract with a general contractor or a prefabricator?

Some shops will offer full-service prefabrication and site construction. In my experience, these shops are experts in prefabrication, and subcontract the site work to a site general. In my opinion, this is backwards. A client should hold a contract with whoever is the Last Person On Site (LPOS).

Every mistake, omission, or code violation that occurs throughout the life of the project is the responsibility of the LPOS to correct. Fabricators who specialize in one part of the production should be subcontracted to the LPOS, and this helps to maintain the flexible path. Severe schedule violations can result in a project derailing into another means of fabrication, or even a stick-built solution if needed. At the end of the day the LPOS has all options at their disposal for deploying the project, and contracts with prefabricators are business-to-business. This maintains a flexible path through deployment of the project.

What does airmod mean by “flexible path?”

Our approach to prefabrication uses a flexible path. The project is designed from conception to be friendly to prefabrication, often both panelized and modular. When the project is nearing the end of design, a choice can be made based on a number of factors: how the design has evolved, a trusting relationship with a builder or fabricator; or a competitive bid between two options, such as a modular fabricator and a SIPs builder.