An accessory dwelling unit can house family, produce rental income or add long-term value to a lot. California has pushed cities and counties to approve them, but a buildable ADU still depends on details that should be checked before anyone pays for plans.
What state law already settles
- Review timeline. When there is an existing home on the lot, the permitting agency must approve or deny a complete ADU or JADU application within 60 days, according to the California HCD ADU Handbook.
- Impact fees. Local agencies cannot charge impact fees on ADUs under 750 square feet or on JADUs, and larger ADUs can only be charged proportionately, per HCD's housing law fact sheet. Utility connection and plan-check fees are separate.
- Types. Detached ADUs, attached ADUs, garage conversions and junior ADUs inside the existing home each have different cost and permitting profiles.
What decides whether your lot works
Utilities. The first real cost question is sewer or septic, water, and electrical capacity. An older 100-amp panel often needs an upgrade. On foothill lots with septic, capacity and setbacks can decide the whole plan.
Access and placement. Equipment access for foundation work, setbacks from property lines and the existing house, and drainage all shape where the unit can sit.
Structure type. A garage conversion reuses a slab and walls but often needs foundation, insulation and moisture corrections. A detached new build costs more but gives a cleaner result.
Sequence that keeps an ADU on budget
- Site review: utilities, access, setbacks, existing structures.
- Feasibility and budget range before full design.
- Plans, engineering and Title 24 energy compliance.
- Permit submission and plan-check responses.
- Construction under a written scope with documented change orders.
Rules vary by city and change often. Confirm current requirements with your local building department before design. We work across Modesto, Oakdale, Turlock, Riverbank, Ceres and the surrounding counties.
Common questions
- Do I need to live on the property to build an ADU?
- Current California law generally does not require owner occupancy for a standard ADU, while junior ADUs have their own occupancy rules. Confirm with your local building department before you design.
- Can I convert my garage into an ADU?
- Often yes. Garage conversions usually need foundation, insulation, moisture and egress corrections, which a site review identifies before plans are drawn.
Planning a project like this?
On Point Design Construction Inc. is a California licensed general contractor (CSLB #1105082) serving the Central Valley and Sierra foothills. Send the project details or call and you will get a written scope and an itemized estimate.