5 Costly ADU Mistakes San Jose Homeowners Make and How to Fix Them
Accessory Dwelling Units (ADUs) are a booming opportunity in San Jose. With limited space, high home values, and strong rental demand, building an ADU can add income, equity, and flexibility. But many homeowners get tripped up by avoidable mistakes that cost time, money, and peace of mind. Below are the five biggest pitfalls we see in the Bay Area, along with tips on how to avoid them. Mistake 1: Assuming All Cities Follow the Same Rules Why it’s costly: Even though California has strong ADU-friendly legislation, each city still enforces its own zoning, setbacks, parking exemptions and plan-check process. San Jose, Campbell, Los Gatos and other jurisdictions each have unique standards. How to fix it: Start by visiting the official planning department website for your specific city (e.g., City of San Jose). Download the current ADU guide, check the setback/height limits, parking exemptions and whether ministerial (automatic) approval applies. Make this a written part of your budget and timeline plan. Mistake 2: Underestimating Total Project Budget (Soft Costs + Utilities) Why it’s costly: When homeowners only budget construction (framing, roofing, finishes) and ignore architecture/engineering, soils reports, permit fees, utility upgrades and contingency, they end up scrambling and increasing debt or compromising quality. How to fix it: Create a full-scope budget broken into: Mistake 3: Choosing Design That Looks Great But Works Poorly Why it’s costly: How to fix it: Mistake 4: Neglecting Site Access, Landscaping & Neighbours Why it’s costly: How to fix it: During the site visit ask about: equipment access, staging zones, tree protection, drainage, screening for neighbours, and landscaping plan. Build a neighbour-friendly plan from the start, include access logistics in the contract, and factor in any additional cost for tree roots, retaining walls or tricky grading. Mistake 5: Rushing Permits & Skipping Quality Standards Why it’s costly: How to fix it: Final Thoughts Building an ADU in San Jose is one of the smartest moves a homeowner can make — for rental income, multigenerational living or increasing property value. But like any big investment, the difference between success and stress comes down to avoiding risks and setting your process up correctly. Start with verifying local rules, build a full budget including all costs, pay attention to design and access, and choose a builder who knows the San Jose market inside-out. If you do that, you’ll be on your way to a valuable, well-used ADU rather than a costly DIY headache.