The Complete ADU Building Guide

Accessory Dwelling Units (ADUs) have become one of the most popular home improvement projects in California. Whether you're looking to house aging parents, create rental income, or add a home office, an ADU can significantly enhance your property's value and functionality. This comprehensive guide covers everything you need to know about building an ADU.

What is an ADU?

An Accessory Dwelling Unit is a secondary housing unit on a single-family residential lot. ADUs can be attached to the primary residence, converted from existing space (like a garage), or built as a detached structure. Also known as granny flats, in-law units, or backyard cottages, ADUs provide complete independent living facilities including a kitchen, bathroom, and sleeping area.

Types of ADUs

Understanding the different types of ADUs helps you determine which option best suits your property and needs:

• Detached ADU: A separate structure built in your backyard, offering maximum privacy for both units
• Attached ADU: Built as an addition to your existing home, sharing at least one wall
• Garage Conversion: Transforming an existing garage into living space, often the most affordable option
• Above-Garage ADU: A new living unit built on top of an existing or new garage
• Interior Conversion: Converting existing space within your home, such as a basement or attic
• Junior ADU (JADU): A smaller unit (up to 500 sq ft) created within the existing footprint of the main house

California ADU Regulations

California has some of the most ADU-friendly laws in the country, making it easier than ever to add a secondary unit. Key regulations include:

• Maximum size of 1,200 square feet for detached ADUs in most jurisdictions
• No minimum lot size requirements for most ADU types
• Reduced setback requirements (4 feet side and rear for detached ADUs)
• No parking requirements if within half mile of public transit
• Streamlined permitting process with 60-day approval timeline
• No owner-occupancy requirements for many ADU types
• Impact fees waived for ADUs under 750 square feet

The ADU Building Process

Building an ADU involves several key phases:

1. Feasibility Assessment

Before investing in design, evaluate your property's potential. Consider lot size, existing structures, utility access, and local zoning requirements. A professional contractor can help identify any potential challenges early.

2. Design and Planning

Work with an architect or designer to create plans that meet your needs while complying with building codes. Consider the intended use, privacy requirements, and how the ADU will complement your existing property.

3. Permitting

Submit plans to your local building department for review. California law requires cities to approve or deny ADU applications within 60 days. A reputable contractor will manage this process and address any required revisions.

4. Construction

Construction typically takes 4-8 months depending on the project scope. Key phases include site preparation, foundation, framing, roofing, mechanical systems (plumbing, electrical, HVAC), and interior finishing.

5. Final Inspection and Occupancy

Once construction is complete, the building department conducts final inspections. Upon approval, you'll receive a Certificate of Occupancy, allowing the unit to be legally occupied.

ADU Costs in 2025

ADU construction costs vary significantly based on type, size, and finishes. Here are typical ranges:

• Garage Conversion: $100,000 - $180,000
• Detached ADU (400-600 sq ft): $180,000 - $280,000
• Detached ADU (800-1,200 sq ft): $280,000 - $450,000+
• Attached ADU: $150,000 - $350,000
• Above-Garage ADU: $200,000 - $400,000

These costs include design, permits, construction, and standard finishes. Premium materials, complex sites, or high-end appliances will increase costs.

Financing Your ADU

Several financing options are available for ADU construction:

• Home Equity Line of Credit (HELOC): Leverage your home's equity with flexible draw periods
• Cash-Out Refinance: Replace your existing mortgage with a larger one and use the difference
• Construction Loan: Short-term financing that converts to a permanent mortgage
• ADU-Specific Loans: Some lenders offer products designed specifically for ADU construction
• CalHFA ADU Grant: Up to $40,000 for pre-development and construction costs (income-restricted)

Return on Investment

ADUs offer excellent return on investment through multiple channels:

• Rental Income: ADUs in desirable areas can generate $2,000-$4,000+ monthly in rent
• Property Value: Studies show ADUs can increase home values by 20-30%
• Family Flexibility: Housing for aging parents or adult children provides financial and caregiving benefits
• Home Office/Studio: Dedicated work space can increase productivity and work-life balance

Choosing the Right ADU Builder

Selecting an experienced ADU builder is crucial for project success. Look for contractors who:

• Specialize in ADU construction with a proven portfolio
• Are licensed, bonded, and insured
• Handle design, permitting, and construction in-house
• Provide clear, detailed contracts and timelines
• Offer strong warranties on workmanship
• Have positive reviews and references from past clients

At Maison Remodeling, we've completed dozens of ADU projects across the Bay Area. Our full-service approach ensures seamless execution from initial concept to final walkthrough.