You may be thinking about adding an ADU to your property and considering the best option to achieve your goals. There are three primary ways to deliver an ADU project: Prefabricated ADUs, Pre Designed ADUs, and Custom Designed ADUs. All have their strengths and weaknesses which we discuss below.

Prefab ADU

Before we discuss the strengths and weaknesses of this approach lets quickly touch on the definition of Prefab vs Modular and other common terms. Prefabricated typically refers to projects where all, or parts of the building are manufactured in a factory and shipped to the site to be assembled. This could be wall panels, roof trusses, and other specific building components, or could be the whole unit itself. Modular construction is typically referring to projects where the whole unit is made off site and shipped as a single object and the placed on a foundation. Sometimes multiple modules are combined together to complete the structure. For example, when you are driving on the highway and see those extra wide load trucks with homes on the back, that would be a modular home. If you see a truck that is flat packed with wall panels, that would be prefabricated. For this article we lump these two approaches together in this prefab ADU category.

The main advantage to prefab is a potentially faster schedule, and that build quality could be better within a factory where they don’t have to deal with weather and other site conditions and where machines and better tools could allow for more accurate construction. Many prefab companies will state that their builds are higher quality with smaller tolerances when compared to site build ADUs.

The other advantage is that often these can be built ahead of time and sold as a product, meaning the length of time for the construction can be dramatically cut down. If the unit is already built and just needs to be shipped to site, all that would be needed is site specific work like utilities, foundations, etc. at which point the finished unit could be craned into place and installed in a couple of days.

While the time argument is compelling, not many of the prefab companies have inventory of units sitting around waiting to be sold. Most build to order and have long lead times as demand increases and labor is hard to come by. Often it takes just as long to order and receive a prefabricated ADU as it does to custom build one on site. Hopefully this is a short term challenge and over time prefabricated units will become available in a shorter timeframe. When this is the case it will be a very compelling argument to choose a prefabricated ADU.

There are a couple downsides to Prefabricated ADUs. The primary one is that your choices for the design is limited. These ADU projects are predesigned and manufactured so you don’t have the ability to adjust the design, select different materials, and make other revisions that you may get to make if you were going a more custom route. The other challenge is that these projects show up to site as large objects that need to be moved into place, meaning often you need a crane and other large equipment to help assemble the ADU. The drawback here is that there could be site conditions that prevent prefabricated units from being viable at all. Overhead power lines, trees, steeply sloped sites, and other conditions may make your property unable to accommodate a prefabricated unit.

It is important to check with the company and share details about your property to verify if they can actually install their product on your property, and what additional installation costs may be required.

The other issue to look carefully at before moving in this direction is the cost. It is important to understand the base cost of the prefabricated parts of the project, with the additional cost required for the site specific work. You may see a prefab ADU being advertised for say $200,000 but typically that wouldn’t include things like excavation, concrete foundation work, utility installation, complications with access to the site, landscaping and other costs that you would need to incur. When comparing prices make sure you are comparing the total project costs and not just the cost of the prefabricated ADU itself. These additional costs could add thousands of dollars and maybe even $100,000 or more depending on the specifics of your property. I’m not trying to scare anyone away from exploring prefabricated accessory dwelling unit options, but know that the sticker price on a website may not be the full project cost to you.

Pre Designed ADU

Where Prefab ADUs are a fully built unit shipped and installed on site, Pre-designed ADUs are where you are simply purchasing the design drawings for an ADU project. Typically this could be a PDF file, a CAD file, or perhaps a Sketchup or BIM file like Revit or ArchiCAD. Most likely it would be a PDF of the ADU plans that you could then print, share with a local designer or engineer to make revisions, and submit for permit review at your local building department.

There are many advantages to starting your ADU project with a pre-designed plan set. It can save a significant amount of time when compared to going with a custom designed ADU project. Instead of working with a designer to start a new design from scratch you can pick a design that you like and meets your project goals and then just make some small revisions to get it ready for permit approval. We find that starting with ADU plans can save approximately 4-5 months of time.

When you save time, you also save money. Hiring a designer or architect to design a custom ADU should cost about 10% of the project budget. For a nice $300,000 ADU that would mean design fees of around $30,000. Although, this can be great value when the custom designed ADU is specific to your property, aesthetic taste, and project goals, for those who can’t afford this premium price, starting with a pre-design accessory dwelling unit can cut this fee significantly. We sell our plans for around $1,000 and often make small revisions for another $1,000 to $2,000 for our clients. You can then take the drawings to a structural engineer to complete their work for another few thousand dollars. Alltogether you can probably get a permit ready set of drawings for close to $5,000 or less. This is much more affordable for most people than going with the custom route. All of our plans are designed by talented architects and designers who specialize in ADUs, and many are award winning designs, meaning you are getting a high-quality design for a fraction of the cost of a custom ADU.

Another advantage to a pre-designed ADU is that you can get construction cost estimates done pretty early in the project. If you have engaged a general contractor already, they can probably give a good estimate of what a design would cost by analyzing the ADU design drawings and/or seeing images of other built projects with the same design. You can also ask the company selling the plans about cost estimates from other builds. Having this cost information up front can help you develop a project budget and know whether the ADU will be a good investment.

visit our adu store to pick out the ideal plan for you

This all being said, there are many reasons why a custom ADU design could be the best approach for your project. We will explore that in the next section.

Custom Design and Built ADU

To be honest, if money wasn’t a barrier, I would recommend that everyone go with a custom designed ADU project. Hiring a skillful designer to craft an ADU design that is specific to your goals, taste, and property can give you the best result. However, it is never the case that money isn’t a factor in people’s decisions.

Custom ADU design is definitely the most expensive option of the three. It also probably takes the longest as you have to work with the designer to develop design ideas, work through iterations, and ultimately land on the best solution for the project. We typically estimate that the design process takes about 5-6 months for most of our clients. This could be longer or shorter depending on your ability to make decisions, the size and complexity of the project, and the local jurisdiction and their rules and regulations. Often there are city requirements that require additional work or bringing on other consultants that can add time and cost to the design phase. .

The other aspect of custom designed ADUs is that often people who go this route want something more unique or higher end. Even when they say they are on a tight budget, in our experience people tend to make decisions or select designs or materials that can push up the cost. My opinion is that you get what you pay for and often these decisions lead to better quality projects that are beautiful spaces to live. But it is important to consider that once you start designing, the sky is the limit and you may talk yourself into doing something higher end than what you started out imagining. This can be both a pro and a con to going with a custom designed accessory dwelling unit.

Advantages start with designing something specific to your property. Taking cues from the main house, the site topography, solar orientation, views, landscape features, neighborhood context, privacy concerns, and the taste of the owner means that a custom design project can really perform better than most other options. It allows you to make adjustments that mean the ADU can fit seamlessly into the surroundings and compliment what is already there. You can’t really get this level of site-specific advantages when going with a prefabricated ADU or a pre-designed ADU. This is also why we tend to not recommend garage conversions very often. They just limit what is possible by too much, meaning the end result is not ideal for what you could get with a fully custom, ground up construction ADU.

If you can afford it, we would highly recommend this approach. If budget is tight, that is when you should definitely start exploring the other options described above.

If you want to learn more about our custom design services, please contact us and tell us more about your project goals.