In this webinar, the ADU Resource Center of San Mateo County and Hello Housing walked through the end-to-end ADU process, starting with early planning and ending at move-in.
SOME TAKEAWAYS
Understand your ADU options.
Common types include JADUs (up to 500 sq ft, typically the lowest-cost option), conversion ADUs (turning existing space like a garage, basement, or attic into an ADU), attached ADUs, and detached backyard units (often the most flexible, but often the most expensive).
Choose a construction method with eyes open.
Your options for ADUs include stick-built, panelized, or modular construction. Prefab/modular can save time, but the “unit price” often excludes major costs like foundation work, delivery and sales tax, so totals can end up similar to stick-built.
Plan for a multi-phase timeline.
A typical project includes planning (often a few months), design (variable), permitting (variable), and construction (often 6–12 months). Permitting timelines can extend if the city issues plan-check comments that require revisions. Weather and contractor availability also affect schedules.
Budget using “all-in” costs, not just construction.
Total ADU cost includes soft costs (design, engineering, permitting, Title 24 energy compliance), infrastructure (utility upgrades), and hard costs (building). Costs vary widely depending on site conditions and project choices.
Site conditions can drive surprises.
Hillsides, soil issues, geohazard zones, flood/coastal zones, creek setbacks, septic systems, easements, and utility upgrades can all increase costs and complexity. Learning your property constraints early is one of the best ways to avoid delays.
State law helps, but local rules still matter.
California ADU laws have reduced barriers by requiring ministerial approvals for ADUs, reducing parking requirements and placing limits on HOA restrictions, among other things. However, each jurisdiction still has its own ordinance and process. It’s smart to talk with planning staff early. Go to your local City or Town’s ADU page to get connected.
Utilities can be a major bottleneck.
Utility providers (especially for new meters or service connections) can have long lead times. Apply as early as you can once permits are approved.
Vet contractors carefully.
Get multiple bids and compare them “apples to apples”. Watch for vague bids, unusually low pricing, pressure tactics, large upfront payments, and poor communication. Verify licenses/insurance and look for local ADU experience. We have a webinar all about this where you can learn more.
If you’ll rent the ADU, prepare to be a landlord.
Know your responsibilities, line up reliable tradespeople, and understand local rental rules before leasing. Our webinar on renting your ADU includes lots of useful tips.