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:
- Hard costs (construction)
- Soft costs (design, permits, engineering)
- Utility and connection fees (metering, trenching, upgrades)
- Contingency (10-20% depending on site complexity)
Get a quote from a local design-build ADU specialist and plug in San Jose-specific numbers based on recent builds.
Mistake 3: Choosing Design That Looks Great But Works Poorly
Why it’s costly:
- A stunning ADU design is worthless if it ends up feeling cramped, poorly ventilated, dark or difficult to rent.
- Many plans focus on aesthetics but overlook liveability, energy codes and future use.
How to fix it:
- Work with a builder/designer who can show completed ADUs in San Jose.
- Ask about daylighting, ventilation, clearances, kitchen/bath layouts and future flexibility.
- Always verify local energy, seismic and fire code requirements; make sure your layout is as functional as it is beautiful.
Mistake 4: Neglecting Site Access, Landscaping & Neighbours
Why it’s costly:
- In dense San Jose neighborhoods, site access, tree protection, utility routing and neighbour impact matter.
- Bad access means crane/truck delays, extra grading, and higher cost.
- Poor neighbour relations can lead to complaints and slowdowns.
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:
- Some push to “get it done fast” and skip thorough code review, quality materials or required inspections.
- This often leads to change orders, re-work, or even permit revocation. Quality shortcuts cost more in the long run.
How to fix it:
- Choose a builder with a robust permitting workflow, experienced with San Jose’s plan-check and inspection process.
- Ask for references, check their portfolio of completed ADUs. Set realistic timeline expectations (e.g., ~8–16 weeks for permits, ~6–9 months construction) and build in quality benchmarks for materials and finishes.
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.