adu guide

Custom vs. Prefab ADUs

The two most common methods of building ADUs carry different pros and cons. Learn why Cottage is choosing to focus on custom, stick-built ADUs.

Kevin ChenSeptember 03, 2020

It feels like every day that a new prefab ADU design pops up in our Instagram feed. They often seem pretty appealing, with sleek designs, short timeframes and approachable price tags. While many homeowners start their ADU research process by looking at prefabs, savvy homeowners end up choosing custom designs and site-built construction.

Here are a few factors things homeowners should consider when deciding between custom, prefab or modular ADUs:

How much choice do I get?

Prefab ADUs come in a few different size options, from a compact 400 sq. ft. to the state maximum 1,200 sq. ft.. You also have the ability to rotate the unit and place it anywhere on the property outside of the required setbacks. Some also give a few options on exterior siding types (stucco, wood siding, etc.).

With a custom ADU, you’ll have the ability to make as many or as few design choices as you want. You’ll be able to choose the exact positioning, floor plan, size, interior and exterior finishings. Do you want high ceilings, a vaulted ceiling, a window exactly where you want it? No problem. Do you want your dream, modern farmhouse ADU? We'll take care of that for you.

At Cottage, we believe one size doesn't fit all. Every property and every homeowner are unique and a different ADU design should be tailored to fit.

What is faster?

Prefab ADU companies often make claims that their ADUs can be ‘installed in 2 weeks’. Of course, that doesn’t include key phases of every ADU project that need to be completed whether the unit will be prefab or custom-built. Every ADU project will require city permitting, which often requires a few months to assemble the plans, submit them to the city, and have the corresponding planning and building departments review them.

When it comes to construction, the ‘box’ isn’t the hard part. No matter the type of build, you’ll still need contractors to take care of site work - trenching for utilities tie ins, grading the site, and installing a foundation. In our experience, that’s more than 30% of the on-site construction time, while the actual framing and finishing typically takes 8-12 weeks for our custom site-built ADUs.

Most prefab companies also require manufacturing lead time at their facilities. Manufacturing will often take 2 months instead of '2 weeks' and only can begin once permits are pulled from the city.

Prefab ADUs certainly have the potential to be completed more quickly, but with an efficiently run stick-built process, the difference in delivery time can be less than a month with Cottage's ADUs.

What is cheaper?

Similar to car ads, the prefab ADU ads advertise the ‘base price’ but the photos include all the extras. When comparing costs, it’s important to make an apples-to-apples comparison.

In our experience, it’s common to see the following costs conveniently left out:

  • Specialist consultants: If the city requires a geotechnical report or boundary survey, is that included in my price?
  • City permits: Are plan check, permit costs, or impact fees included?
  • Site prep: Does it include demolition and removal of your existing shed, concrete pad, or other existing site structures?
  • Foundation: Is the foundation included? Does it include a durable, long-lasting slab on grade or perimeter footing foundation, or just a few helical piles?
  • Installation: Does that include a crane for lifting the ADU over your home or trucking to deliver your unit from often out-of-state to the project site?
  • Finishes: What kind of appliances, flooring, doors and windows does the ‘base price’ include? How much is it to upgrade to what’s photographed on the website?

While custom ADUs can be expensive, you may be surprised to learn that Cottage's custom ADUs cost 10%+ less than prefab units of the same size and finish quality.

Will it even work on my property?

Most prefabs ADUs are trucked in on the back of a trailer and craned from the street or driveway. Is your driveway strong enough to handle the load of the crane and ADU, or will it crack the concrete or pavers? Do you have telephone lines or protected trees that might get in the way? Smaller backyards can also be tricky to find the right fit for the limited number of options.

Custom, stick-built ADUs can be configured to be the exact size and shape to work with your property. L-shaped or even U-shaped ADUs are actually common to work with limited backyard space. Construction is completed on site and doesn't rely on cranes. As long as we have a doorway's worth of width to walk through, we can build on your property!

Which type will have a better return on investment?

In the Bay Area, you’re sitting on some of the most valuable land in the country, and you should think about the return on investment for your ADU, both in equity and cash terms.

Equity: If you need to obtain financing, what how will the bank appraise a prefab unit vs. real site-built construction? If you eventually plan to sell the home, how will a buyer value it versus a custom ADU that is architecturally designed to fit your property?

Cash: If you plan on renting out the ADU, how easy will it be to attract and retain great tenants? How will the construction hold up to the wear and tear of renters and move in / move out days?

We’re big believers in potential for innovation in construction, but we’re not yet at a point where prefab or modular are worth the trade offs. Set up a consultation with our team and let us describe what you can expect with a custom Cottage ADU!

Get your free consultation and estimate today!