Detached, attached, or conversion: choosing the right ADU type

Detached new construction, attached construction and conversion are starting paths—not parcel eligibility results. Compare them only after confirming jurisdiction, existing permitted structures, setbacks and height, lot coverage or FAR, easements, access, mapped constraints, utilities and responsible agencies. Use the ADU comparison and project brief, then pair the selected path with the cost workflow and permit directory.

Detached ADU

A standalone structure separated from the main home. It can provide physical separation and a purpose-designed layout, but requires a new footprint, foundation, envelope and utility strategy. State law establishes protections for certain size, side and rear setback, and height conditions; the reviewing agency still applies parcel facts and objective local standards.

• —Investigate when: a separate unit matches the household plan and a documented site layout preserves required access and constraints.
• —Resolve first: surveyed boundaries, easements, setbacks, height, lot coverage or FAR, emergency access and utility routes.
• —Scope driver: new foundation, envelope and utility work; timing depends on design, agency review, corrections, clearances and construction conditions.

Attached ADU

Shares one or more walls with the primary home. A connection may reduce some new exterior-envelope or utility distance, but it also brings the home's structure, roof, fire separation, circulation and occupied areas into scope. No cost or schedule advantage should be assumed before those conditions are documented.

• —Investigate when: connection to the house supports the intended use and a detached footprint conflicts with known site constraints.
• —Resolve first: existing plans and permit history, structural tie-in, egress, fire separation, roof drainage and utility capacity.
• —Scope driver: existing-building condition and connection complexity; timing remains property- and agency-specific.

Garage / accessory conversion

A qualifying existing-space conversion may use the statewide ministerial path in Government Code § 66323. That does not establish that the enclosure is permitted, structurally reusable or ready for habitation. Fire and safety setbacks, exterior access, foundation and moisture conditions, ceiling height, openings, energy work and utility routes still need evidence and agency review. Parking exemptions are situation-specific under § 66322, not a blanket waiver of every site rule.

• —Investigate when: an enclosed structure exists and its permit history and physical condition can be documented.
• —Resolve first: lawful existing footprint, structural and moisture assessment, residential-code upgrades, access and utility connections.
• —Scope driver: how much work can actually be retained after investigation; an existing shell alone does not establish cost or duration.

How to choose

1. 01Confirm the land-use authority; a postal city is not jurisdiction.
2. 02Document the house and accessory structures with records, measurements and condition observations before assuming reuse.
3. 03Plot property lines, setbacks, height, coverage or FAR, easements, access, overlays and utilities for each possible footprint.
4. 04Ask the responsible planning, building, fire, public-works and utility reviewers which standards and clearances apply.
5. 05Compare only paths that remain plausible, using documented scope and allowances rather than promised rent, ROI, cost or schedule.