By Sharon Simonson

So you’ve decided to maximize the useful value of your San José home property, and you’ve determined that your lot can accommodate an accessory dwelling unit by completing the City’s ADU Universal Checklist.

What sort of ADU are you going to build, and how are you going to build it?

Without question, the least difficult approach—the least expensive, the least time-consuming and the least head-crunching—is to select from the City of San José’s list of preapproved ADU builders and designs.

On that list are seven homebuilders who altogether have created 14 ADU designs that meet all California building codes, while also allowing you to select materials and finishes, inside and out, to customize its appearance to your taste.

Using a preapproved ADU design and the City’s dedicated and expedited ADU planning process, you can expect to have a building permit to begin construction within a month after your plans have been submitted, said Andrew Wozencroft, a senior engineer who supervises Building Plan Review for accessory dwelling units.

“Nine times out of ten, these projects are approved the first time they go through Plan Review,” he said.

Preapproved ADU designs and their builders “can help you identify pitfalls early on, such as whether an ADU is economically achievable. They can help ensure you will get an ADU building permit and get through to final construction,” he said.

Preapproved designs also allow homebuilders to predict final price with much greater certainty, which can ease project financing.

Custom-designed accessory dwelling units, while offering you greater choice about your ADU’s configuration and appearance, typically require multiple passes through Plan Review before a building permit is secured, Wozencroft said. Months can pass before all building plans are adequate to allow a project to begin construction, and the final price is relatively uncertain.

Designing an ADU is harder than designing a house or an interior remodel, Wozencroft said. Meeting California’s famous energy conservation standards can prove difficult. In some cases, where smaller primary residences have large lots, the ADU has more living area than the original home. “The ADU uses the resources of the main house as a hub in terms of plumbing, sewage, electrical,” Wozencroft said. “It is a unique challenge to design.”

Conceiving and creating a custom-designed ADU is a time-consuming and decision-intensive process.

Many homeowners assume that ADUs are less expensive to build than a standard new house, but ADUs still must meet California’s rigorous building codes, conceived to protect human life and limb during catastrophes such as fire, earthquake and flood. Final construction cost typically amounts to $400 a square foot to $500 a square foot—the same square foot price range as would be the cost to build a primary residence.

“These structures are intended to last for fifty to a hundred years. There is a certain level of craftsmanship and durability that the code requires,” Wozencroft said.

At current building costs, the base price for an ADU in San José is $180,000 to $200,000.

Once you know that you can add an ADU to your home property, speak with all of the City’s preapproved vendors as part of your due diligence, Wozencroft said. For every potential designer or building contractor, review the ADU projects that each has completed in the City of San José. Ask any prospective vendor to show you a set of drawings for one San José project that has been approved and constructed.

If you want to evaluate custom-designed construction more deeply, ask three contractors for a construction cost estimate for your proposed project, then have each contractor explain the cost factors.

Homeowners who are not professional engineers, architects, or building designers should think deeply before trying to manage an ADU project without professional help. “It’s similar to the concept of, ‘Do you think you could create and design a car?’” Wozencroft said.

And anyone considering an ADU that is attached to the main house or that is a so-called Junior ADU (JADU) where existing square footage in a single-family home is converted to a separate living space must hire a professional designer such as a licensed architect or engineer.

In his experience, many homeowners don’t partner with a building contractor until after they’ve secured a building permit. Outcomes are better and more predictable when your ADU designer and a proposed building contractor work in tandem beginning in the early stages of the process, Wozencroft said.

Regardless of whether you chose a preapproved ADU or custom-design construction, your ADU will constitute a unique addition to your property based on the attributes of your existing home and the contours of your existing lot including the location of trees or other structures, and of course, the attributes of the ADU itself.

“Customization is always there,” Wozencroft said, “and a lot is special to you.”