ADU Plans

The all-in cost to build a turn-key detached ADU in California ranges from $375 to $600+ per square foot, depending on size and finishes.

Vertical building construction accounts for roughly 80-85% of total costs, with the remainder going to design, permitting, and site work.

California AB 1332 requires cities to offer pre-approved ADU plans and approve or deny permits using these plans within 30 days.

Under California SB 543, local agencies have exactly 15 days to determine if an ADU application is complete; otherwise, it is automatically deemed complete.

ADUs in California that are 750 square feet or smaller are exempt from all local impact fees, saving homeowners thousands of dollars.

The rest of it, in plain terms

• The 800 square foot state-exempt ADU size limit in California is now measured by interior livable space only, excluding exterior walls.
• Garage conversions are typically the most affordable ADU option, generally costing between $140,000 and $245,000+ depending on the existing structure's condition.
• Basic site work for a level lot, including utility trenching and finish grading, typically costs around $35,000.
• California AB 976 permanently eliminated owner-occupancy requirements for standard ADUs, allowing them to be used purely as rental investments.
• For Junior ADUs (JADUs), owner-occupancy is no longer required if the unit has its own separate bathroom, per California AB 1154.

Figures below are researched market numbers for budgeting and comparison. They are not quotes. Final pricing depends on your site, your jurisdiction, finish selections, foundation and utility distance.

• All-in cost for a detached ADU: $375-$600+ per square foot.
• Garage conversion ADU: $140,000 - $245,000+ (Cali ADU).
• Single-story detached ADU (new construction): $219,000 - $389,000 (Cali ADU).
• Two-story detached ADU (new construction): $339,000 - $459,000 (Cali ADU).
• Construction Drawings (Plans): $2,300 for standard plans, ~$6,000 for semi-custom, $8,500+ for full custom.
• Permits & Fees: Budget $10-$20 per square foot, though ADUs under 750 sq ft are exempt from impact fees in CA.
• Basic Site Work & General Conditions: ~$35,000.
• Vertical Building Construction: $200,000 - $400,000+.

ADU rules are set locally and change often. These are the provisions that most commonly control the outcome. Verify against your own jurisdiction before you design anything.

• California AB 1332: Requires all local agencies to develop a program for pre-approved ADU plans by January 1, 2025, and mandates a 30-day review period for applications using these plans.
• California SB 543: Mandates a 15-day completeness clock for ADU applications, measures the 800 sq ft state-exempt ADU size limit by interior livable space only, and exempts ADUs under 750 sq ft from all impact fees.
• California AB 976: Permanently removes owner-occupancy requirements for standard ADUs, allowing both the primary home and ADU to be rented.
• California AB 1154: Removes owner-occupancy requirements for Junior ADUs (JADUs) if the unit has its own separate bathroom.
• California AB 462: Allows a detached ADU to receive a Certificate of Occupancy before the primary dwelling is rebuilt in areas under a state emergency proclamation (e.g., fire zones).

There are three genuinely different products sold under the word plans, and confusing them costs months. Stock plans are drawings you buy off a shelf. They are inexpensive and they are not permit-ready in your jurisdiction, because they carry no structural engineering for your seismic or wind conditions, no local energy compliance calculations, and no site plan showing your actual parcel.

Pre-approved plans are a different animal and genuinely valuable where they exist. A growing number of cities and several states maintain libraries of ADU designs already reviewed for code compliance, so the plan check that normally consumes months is largely complete before you apply. If your jurisdiction runs a pre-approved program, starting there is almost always the fastest and cheapest path to a permit.

Custom drawings make sense when your lot is constrained, when you want something specific, or when no pre-approved plan fits your setbacks. Expect to pay architectural fees plus structural engineering, plus a separate energy compliance report in states that require one. Budget the full documentation package, not just the architect, because the drawings alone will not get you a permit.

What each plan type actually includes

Most pages on this subject put the cost behind a contact form. The reasoning is understandable and the result is that you cannot budget, cannot compare, and cannot tell whether a bid is reasonable until you have already given up your phone number to three companies.

We publish the figures instead. They are researched market ranges rather than quotes, and site conditions will move them more than any other factor, but a range you can plan against beats a call-back you have to wait for. The constraints that decide feasibility — setbacks, height, utility distance, site access — are stated up front for the same reason. If your project is not viable, that is worth knowing on the first visit rather than the third phone call.

Factory production windows move with queue depth, material lead times and seasonal demand. We quote the current window rather than a fixed promise, because anyone promising a specific delivery date months out without seeing the factory schedule is guessing. Permitting is usually the longer pole: in ministerial states the review clock is defined by statute, while in discretionary jurisdictions it can run considerably longer.

The sequence that controls your schedule is site plan, permit submittal, plan check and corrections, permit issuance, factory slot, delivery and set, then utility connection and final inspection. Factory build time overlaps permitting only if you commit to the slot before permits are issued, which carries its own risk.