ADU Construction Cost Calculator

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Last reviewed: April 2026

ADU Cost Guide 2026 — Detached, Attached, Garage Conversion & More

An accessory dwelling unit (ADU) — also called a granny flat, in-law suite, backyard cottage, or secondary dwelling unit — is one of the most popular home improvement investments of 2026. Rising housing costs, rental income potential, and zoning reform in most US states have made ADUs an increasingly attractive option for homeowners. But ADU costs vary enormously: from $25,000 for a simple JADU to $400,000+ for a premium detached unit in a high-cost city. This guide breaks down every cost factor, type by type.

ADU Cost by Type — 2026 National Averages

Hard Costs vs. Soft Costs — The ADU Budget Most People Miss

Most online ADU cost guides only quote hard costs — actual construction. But soft costs add 15 to 25 percent on top of hard costs and are frequently underbudgeted. Every ADU project must budget for:

• Architectural and design fees: $8,000–$25,000 depending on complexity. Some design-build firms include this in their project price.
• Structural engineering: $3,000–$8,000. Required for detached ADUs and any project with significant structural changes.
• Permit fees: $5,000–$25,000 depending on ADU type, size, and jurisdiction. California cities have streamlined ADU permits significantly since 2020 under AB 68 and related legislation.
• Utility connections: $5,000–$20,000 for water, sewer, and electrical service. Detached ADUs almost always need separate or sub-metered utility connections.
• Survey and geotechnical: $2,000–$6,000 for sloped lots or projects requiring soil testing.
• Landscaping and site restoration: $2,000–$10,000 after construction is complete.

ADU Cost by Location — Regional Multipliers

Garage Conversion vs. Detached ADU — Which Makes More Sense?

The choice between converting a garage and building a new detached ADU is the most important ADU decision most homeowners face. Garage conversion wins on cost: it reuses an existing foundation, framing, and roof, eliminating the most expensive construction elements. Converting a two-car garage typically costs 30 to 50 percent less than a comparable new detached ADU.

Detached ADU wins on flexibility and privacy. A new structure gives you complete layout control, better sound separation between tenant and homeowner, and the ability to leave your garage functional. If your property has space and your budget allows, a detached ADU also typically commands higher rents and adds more property value.

Prefab ADUs are increasingly popular as a middle ground — factory-built units can be installed in as little as 4 to 12 weeks once permitted, significantly faster than site-built construction. Total costs including foundation, delivery, and utility connections typically run $130,000 to $200,000 for a standard 1-bedroom unit.

ADU Rental Income & ROI

In high-rent markets, ADUs can provide exceptional returns. A $200,000 detached ADU generating $2,500 per month in rent produces $30,000 per year in gross rental income — a 15 percent gross yield. After vacancy, maintenance, and property management (typically 25 to 30 percent of gross rents), net yield is 10 to 11 percent, with a payback period of 9 to 10 years.

ADUs also add significant property value: $150,000 to $300,000 in urban California markets, $75,000 to $150,000 in other major metros, and $40,000 to $80,000 in moderate markets. This immediate equity creation means the real ROI is often much higher than rental income alone suggests.

How to Reduce ADU Construction Costs

• Use pre-approved ADU plans. Many cities now offer free or low-cost pre-approved ADU plan sets that skip expensive custom design and can cut months off permit timelines.
• Build larger rather than smaller. Per-square-foot costs decrease significantly for larger ADUs because fixed costs (foundation, utility connections, design fees) are spread over more space. A 600 sq ft ADU costs significantly less per square foot than a 300 sq ft unit.
• Consider prefab or modular. Factory-built ADUs can save 15 to 25 percent on construction costs and dramatically reduce build time.
• Start with a feasibility assessment. Many cities have online ADU feasibility checkers. Knowing your setback, height, and size limits before hiring an architect saves thousands in redesign fees.
• Get at least three contractor bids. ADU contractor pricing varies 30 to 50 percent for identical projects. Always compare full-scope apples-to-apples quotes.